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By Georges Abou Jaoud, notes for a conference.Computer Graphics International, 1993

The Digital Mock-Up

EPFL - Facult ENAC - Laboratoire dinformatique et de visualisation

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The Digital Mock-Up

EPFL - Facult ENAC - Laboratoire dinformatique et de visualisation (LIV)

By Georges Abou Jaoud. 1993.

Table of Contents

Overview 3

Desingn and the Machine 6

Free Mind Designer 8

Shape, Detail Level, Distance, Scale 10

Sketch or Shape Modelling in 3D space 12

Shapes and sketches as elements of the project 22

Attributes and descriptors 23

Design 27

Visualising 28

Notes 31

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Overview

Usually when architects think computer, the first image they get is of a do it allmachine, disregarding the different needs in a design process.

The response of computer aided design packages was in offering generic or slightlyoriented tools that answered the supposed need of architects.

The state of the art in representation graphics, perspective views, hidden lineremoval and simple shading were quite new and attractive therefore a very strongmarketing criteria.

Let us not forget that it was only in the early 1960s that pioneer researchers incomputer graphics such as Steven Coons and Larry Roberts developed perspec-tive algorithms to be executed by a computer.This was as William Mitchell would say an event as momentous, in its way as,Brunelleschis perspective demonstration 1, or the procedure described by LeonBattista Alberti in the treatise Della Pittura (1436).

Today, thirty years after this event, the research situation in computer graphics isfar too complex to allow a single tool to be efficient in all aspects of the subject.

It might seem contradictory to make such an assessment, and say in the sametime that it is much easier today, to rely on hardware implementation of visuali-sation algorithms, and use very efficient tools to build design applications thatwould allow the concentration of all the effort on the data-base and architecturespecific interface offering the best tools ever.

What is meant here is that even if it is very convenient to lean on existing toolssuch as Xwindows or Motif to design a users interface and use libraries thataccess hardware implemented algorithms to deal with graphical aspects such asperspective, shading, lighting and texturing, it remains very sensible to seekspecific packages to fulfil best the different needed tasks.

In the search for optimal solutions for each phase of the design process, connectingdifferent tools around the same kernel, through compatible data-bases or usingstandard data exchange formats, should be the proper attitude as long as theenvironment and operations remain transparent to the user.

Our experience using multiple platforms and design tools for teaching, showedus to the undeniable disparity in the quality and performance of the differenttasks within the same tool.

Some developers provide us with fantastic rendering engines but their modellingis lousy, others would have an excellent users interface for architects but theirrendering programs will remain far behind what is developed for the advertising

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and movie media.

At a first glance, it might not seem very cost effective to buy, use and maintaindifferent tools in an office environment, unless we consider productivity is givingthe right tool to the appropriate person and task.

Communication between different software makers is another aspect in this atti-tude that we cannot neglect, we shall deal with this matter later on, but we canjust point out that it is widely possible today to transfer information between adata-base format and another at the cost of non negligible effort and some lossof information, drastic or not this will depend on what is transferred to where.

Rather than making this paper sound like a trial to existing software for design,let us try to be constructive and start proposing a philosophy, a methodology anda structure that would emerge out of general considerations of our professionand allow an clean implementation and open to existing leading research and

developments for the visualisation aspects.

It is an opportunity for architects to stop for a moment in an attempt to understandthe conceptual and practical aspects of architecture design and try to clone andmaybe enhance the process with so called electronic tools, in a synergy withscientists who specialise in the development of specific rendering and animationalgorithms.

What tools?

What is design?

What could be our ideal numerical description of architecture?

How do we go from a mental image to a numerical representation?

What is the impact of realistic images on the public?

Can we teach spatial understanding with electronic tools?

Is a video animation a better media than a sketch to express the quality of

the space we design?

Hundreds of questions that arise while using and developing tools for the socalled virtual description of the architectural project.

The more we get involved with the specification and use of the computer thestrongest is the need to go back to traditional representation, back to history andtheory of architecture.

The understanding of such a process is quite simple. Beyond the fascination

generated by the possibilities of the machine, it remains necessary to producethe right procedures and model the electronic tool to respond to our needs.

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Albrecht Drer. Perspective Machine

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Cloning architectural con-ception and productionbecomes the first goal,going beyond traditionalmanner becomes adelight.

To clone, may not be theright word, understandingthe process is moreaccurate and necessary toall software design, butthe not marvellous part isthat it leads us to a betterunderstanding andquestioning of our non

computerised way ofthinking and proceeding.

So we go back to earlytreatise of architecture, to theory of perspective and painting, and find out thatthis is where we can find the most accurate and beautiful description of what wedo or intent to do with computers today.

Some of the major aspects of modelling, rendering and simulation techniqueswill be discussed without getting into exhaustive technical detail, in order toconcentrate on our main purpose, conceptual aspects of building numerical mock-ups for the design process.

Design and the machine

Who said design is a simple, sequential, consistent process ?

In order to start getting into the complexity of architecture design, let us start bygetting a rough schema of the overall process.

A simplistic approach would lead us to first construct a three dimensional geometricdescription of an object then use this data set to generate views in order tocreate what we know as architecture drawings.

This sequential approach to design is far from being convincing, even if every bitof detail or description of the object is considered and included in the data-base.

This philosophy for computer aided design tools, even if modification is madepossible, even if hierarchy and materials can be modified and introduced at different

William Mitchell. Perspective projection and raytracing.

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stages of the process, does not respond to what can be done and is expectedfrom computers for design.

Major CAD packages and drafting tools have been designed and built to fulfiltasks in specific phases of the design process in an architecture office.

Three dimensional modelling allow architects to design and verify their concepts,and perspective projections or synthetic images can be produced both forverification and means of dialogue with the client. Bill of material, plans, elevationsand sections are also possible and are used in the production phase of the project.

If this is what design is all about, then we can start just comparing CAD systemson a basis of which does what operation better, but since we want to considerthe recursive aspects of design, and the disparity of different elements during acertain phase of the design process, and still be able to produce a final consistentproject, we can start looking for new tools and methodologies.

This is simple to explain since we all knowthat at early phases of design, or whatJohn Archea would name the puzzlemaking : what architects do when no oneis looking,2 some parts of the projectscan be defined to a very high level of detailwhile others remain just outlines.

Sketching allows disparity of its composingelements, puzzle making supposes preciseshape cutting, two major aspects that areinherent to the design process from ideato realisation.

Contradictory at a first glance, but not in-compatible, these two aspects of designwill lead us to a search for a methodologythat allows this intuitive, fuzzy and multi-media description of objects as well as astructured and dimensioned construction.

What Louis Kahn describes as sketchingand note making to crystallise yourthoughts in order to develop the picturein the form of a readable design3becomes the leading idea.

Louis Kahn. page from sketchbook 1969and the room 71

In other words, forget structures that impose constraints where all the describingelements of an object should be known before it is allowed to even exist in thename of accuracy or exact simulation of reality or future reality.

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In search for this freedom that is proper to design by sketches and notes, we canget rid of global design consistency for a while.

Design with a computer becomes copying data, images, models, from the mindinto a heap of hybrid information and links between different descriptors and

components of the project.

Consistency remains only within a specific moment or phase of the design and forcertain rules that belong to the selected representation. Therefore, we will startby having the Design Phase as our highest item, that points to a heap of elementsthat describe the design at this precise moment of the process.

The Project will be a set of pointers, we call Design Phases that simply are instan-ces of hybrid elements, each using a proper structure, and containing its ownpointers to descriptions and rules. All elements are stored in different libraries

that will contain low level descriptions of the components.

Extracting the data will be done through a set of rules and queries to the generaldatabase regardless of the element type, hybrid components will be filtered andcalled by what we will call Extraction filters and means of visualisation orrepresentation.

The Computer Science and Architecture Team at the Department of Architectureof the Federal Institute of Technology, started a project called Free-Mind Desi-gner, with this ambiguity proper to architecture in its kernel.

Free-Mind Designer

Free hand drawing is the shortest way from mind to paper, regardless of whatfilter or representation technique is used by the architect to show or visualisewhat is in his mind.

Sketches can be a representation of a reality we are looking at, or images of ourimagination, all through filters such as perception, representation techniques or

any rules and non-rules we want to use in terms of semantics or grammar.

The goal of Free-Mind Designer is to construct a tool that will allow the architectto transpose existing objects, or imaginary models and images of his design, intoa computer with what we shall call a numerical mock-up.

The strong reference to free-hand drawing we intentionally stress on by namingour project the way we did, does not mean that we will build a bit map free-handeditor, the idea is to use in our tool all the conceptual aspects that emerge whenwe talk about traditional sketching.

Reaching total abstraction or realism in sketching, painting or sculpture can be aresult of confrontation with reality or imagination, a photograph or a cubist pain-

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ting can both have for subject the same existing object, on the other hand, we donot lack of examples where extremely realistic paintings have imaginary scenesfor subject.

The same considerations can be raised when building a numerical data-base todescribe either existing full scale built objects or objects that only exist in our

mind, exporting this data to representation tools will generate abstractrepresentation or synthetic realistic images.

How and where Free-Mind Designer can fit in this recursive process betweenimagination, reality, and the description of both, is what we will keep in mind allthe way through the description of the elements that compose the data-base andthe tools for building or extracting the information it holds.

Design process understanding is what we intend to do while constructing Free-Mind Designer, our goal and motivation are driven by two aspects of design, first

is the non scale and variable level of detail proper to sketching, second is themeasurable model building towards realisation.

Form is what is on the designers mind, Sketch is what is gathered in a hybrideditor regardless of dimension rules and scale, Shape includes all the measurable

descriptive aspects, Design links itall into a virtual numerical space.

Digitising devices, CCD cameras and3D scanners to import the data intoSketch and Shape, movementtracking devices, virtual camerasand rendering techniques will let usinteract in this virtual space andVisualise it, then Materialise byoutputting to peripherals such asprinters, video and numericallycontrolled manufacturing tools.

Sketch editor is a portion of spacewhere any kind of element can be

brought regardless of the scale anddetail rules, a concept that is quitecommon in hybrid editors mixingtext with images and projectionsfrom 3D models or even a windowon an element in 3D space.

Leonarado da Vinci Study for theduomo of Milano

This gathering of components, will allow an intermediate step between what L.Kahn would call Form that is the inner image, with no shape nor dimension andDesign as which puts the thing into being.4

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Shape editor is where putting the thing into being is done in terms of dimen-sion, geometric modelling, drafting, material editing, and scheduling.

Attribute editing tools will link component to graphical attributes such as colour,texture, transparency, and non graphical attributes such as data-sheets and

associated text.

Design manager will allow link edition between the project components, sketchor shape, in terms of phases, general and inherent knowledge.

Visualise is where data extraction rules are created and applied to the Sketch andShape data sets, in order to create a Scene description where the lighting andnatural environment is produced, and where the shapes are imported, then sentto drawing, scheduling, publishing, rendering or animating tools. Results are thensent to peripherals through specific drivers in order to Materialise the object on

printers, plotters, rapid-prototyping as well as to projection tools such as displays,video recorders or head-mount stereoscopic displays.

Shape, Detail Level, Distance, Scale

As long as we deal with concepts and pure geometry, measure and scale, can beleft aside while they become a predominant factor as soon as we start dealingwith full scale built objects.

The mind has a tendency to conceive form, links, interrelation of parts regardlessof material and dimension, and pure geometry lives from this distance to physicalaspects. This pure geometry, true at every scale as presented by Paul Valery in aletter introducing Le Nombre dOr by Matila Ghyka5, can be use by artists up tothe point where any relation to reality, to materiality introduces the problem of similarity between the project or the model and the design, which implies that

what is true at certain scale, is not at another.

Since Architects deal with built objects at full scale, the concept of scale and levelof detail is introduced quite clearly in the drafting process.

Three major aspects have to be considered, first is the scale of drawings as aconventional mean to communicate instructions for building within usual rules ofscale dependent symbols, second is the choice of a particular scale as a tool tocomprehend and communicate different components of the project such as spaceor relation to environment, the third aspect is brought by computers and the non

scale illusion of 3D models.

As far as the choice of a scale dependent symbols for building instructions, the

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content of a drawing at a 1:200 scale is not the same at a 1:50 scale, every oneagrees to this and we can very easily understand why it is totally foolish to startdefending the fact that computer numerical models are built to a 1:1 scale.

Talking about the ability to zoom interactively, and extract the drawing sheets outof the so called 1:1 model at the desired scale, have led most CAD systems and

new users to defend this theory as new approach to design, freeing the architectsfrom scale. The result of such an invention, is a total lack of quality in the soproduced drawings in terms of communication of building instructions, not tomention that for those who still think that a drawing is only a section through ageometrical model, and still have to produce a final drawing including certainsymbols, they have to invent three dimensional objects that do not correspond toany reality in order to generate the expected symbol of the object on the drawing.Quite a tricky procedure, a common example might get this image very clear.Even at a non detail scale such as 1:200 one might want to express the dooropening direction, the usual symbol is to add a triangle or an arc to the opening

in plan, and will end up with a 3D model where every door is a quarter of acylinder whose section will produce the desired effect in plan.

When the plan or elevation considered as a working tool for design and commu-nication, Vittorio Gregotty qualifies it as, not just instructions for building orreduced representations of an object pre-existing in imagination or reality, butfunctions as different calligraphy, with what he calls, different structuringrules an different meaning in relation to the system of orthogonal projection inwhich they are used.6

From there on, we decided to introduce scale dependent drawings or symbols,associated to an object in the project, these symbols are independent of thegeometrical description of the object, and will appear in a drawing if it is producedby a procedure called plan, section or elevation rather than an orthographicmathematical projection.

Only if the symbol of the object does not exist for this particular view and scale,we shall get the information out of the relevant three dimensional model.

Since the information of a scale dependent symbol is not necessarily extractedfrom its geometry, the concept of scale stays different from the concept of detail

perception and distance, and we know that the simplified attitude to increasedetail with scale is not the proper thing to do.

This doesnt mean that information from the geometrical database or a part of asmaller scale symbol can not be imported into the subsequent scale. Therefore,we present drawing symbols for plans, sections and elevation as two-dimensionalinformation, contained in overlapping scale dependent collections of vectors.

Introducing the notion of scale in a 3D modelling process becomes very difficultsince the numerical model we are generating is ruled by full scale dimensions and

the architect builds in a virtual environment in terms of true measures. Introducingthe concept of detail level in this 1:1 model, will allow us to build numericalmock-ups at different level of detail for the same object. Nothing new if we

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consider that distance and detail level of an object can be very easily introducedusing a hierarchical model where the added levels of detail can be simply cut ifirrelevant when viewed.

Incremental definition of space is a very useful concept in order to reducecomputing time and control the level of detail, but it will not respond to our

concern in disparity of detail levels among the different components of the sameproject.

When building physical mock-ups to explain a project, one would build a 1:200model in card-board and a 1:1000 model in wood and maybe a 1:1 part of theproject using the real material, all of these objects explaining or representing thesame project, but have different expressions depending on the used material andthe scale.

It is always possible to add miniature details to any of the reduced reproductions,

we will not get into another chapter explaining the difference between realisticminiatures and architecture models, and we will stick to the fact that reducingreality can lead to very kitsch objects that are not of any help explaining theproject.

Building the numerical mock-up follows the same rules, and we could do exactlythe same introducing the mode of representation and detail level, which willallow us to have maximum level of detail using the best techniques of renderingon a part of the building, and in the same time developing an abstraction in termsof detail level such as the one we do for a 1:200 model even if we are working ana so called 1:1 space.

In conclusion, and as far as the vectorial information goes, objects will havedetail level and distance dependent 3D geometric instances, 2D scale dependantsymbols.

The type of query ant method of visualisation will allow the extraction of the rightinformation from the proper component and complete the information if not definedfrom the closest existing component.

Sketch or Shape Modelling in 3D space

If we forget about technical plans or fancy perspectives to convince a client, andthink of the design phase of the project, this superb moment when the mindworks with a tool not only to transmit an idea but enter this iteration betweenmaterialisation and thought to give birth to the project.

At this particular moment of design, we could ask our selves an absurd questionto know if building a mock-up is better than drawing a plan or sketching ?

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If we do think so then we can agree with all this in fashion discourse on howcomputer aided design is better because we build directly in space.

The plan Is the generator, says Le Corbusier, who defends this plan that carries in it self the essence of sensation7 as the ultimate tool for design.

Building an object directly into space, involves the same type of attitude as towardsbuilding a physical mock-up, it is not better or worse than any other method.

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Usually, computer design tools, ( rather than CAD or any CA), allows the architectto work simultaneously on different parallel projections of the object in differentwindows. These projection can be assimilated to plan, elevation, section andperspective. To be precise and since we do not consider a plan nor an elevation assimply a section trough the model, these windows will be referred to as a front,back, or top view.

This access to spatial specification of a point is not so far from traditional designin plan, elevation and section as long as the user draws strictly in one of the plansleaving heights for example to a next step.The difference starts when we want to position directly three dimensional primi-tives in space, giving co-ordinates of an element in a minimum of two plans tocomplete the missing co-ordinate.

Placing 3D primitives directly in space, using snap grids and into a perspective oraxonometric view, or input devices such as pressure sensitive stylus, dial-but-

tons or even a data-glove, involves a certain training and pushes the architectinto a new way of building.

Building is done in a virtual space, free of material restrictions, no glue nor cut-ters and card board, but the same sensation of going directly from a concept inmind to its miniature construction.

This is when virtual modelling become interesting and some times a real chal-lenge to architects, but the loss of contact with the material can become frustrating,and the necessity to go back to pencil and card board remains essential.

But then why bother with the computer ?

We dont need to answer this question, since we are into complementary meansand one does not replace another, the numerical model will give us a thousandpossibilities that a physical mock-up cant do and vice versa.

When we start talking about 3D Modelling, our first intuition is to think directlyvolumes in space as opposed to drawing points, lines, curves, an polygons onpaper, or in a 2D space.

In order to clear this confusing aspect, we will try to extrapolate an old descrip-tion of Leonardo da Vinci:

The first principle of the science of painting is the point; the second is theline; the third is the surface; fourth is the body which is enclosed by thesesurfaces...8

Modelling in tree-dimensional space deals with the same components, zero di-mension for points, one dimension for lines, two dimensions for surfaces andthree dimensions for volumes. Fractal geometry introduced objects with another

dimension, fractal dimension that could fit perfectly every time Leonardo da Vincidescribes elements such as atmosphere or interstices between light and trees.Fractal dimensions will be discussed later when dealing with natural phenomenaand non Euclidean geometry.

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Modeling in space with points, lines, surfaces or solids, have each its advantagesand limits, specially in terms of rendering, and the choice will mainly depend onthe desired visual effect.

The way of constructing geometric elements will mostly depend on whether weare dealing with points, lines, surfaces or volumes. Specific interaction modes

are inherent to the type of elements and the geometric database model.

Graphic editors provide the user with a creation number of modelling tools, suchas pre-defined primitives and operators over these primitives, editing tools toerase cut, or transform. Three dimensional models allow you to go from lines andcurves, used as contours and sweep, revolve or extrude them into surfaces.Applying these operations to surfaces will allow the generation of volumes.

We shall not get into a detailed description of these editing operations unless weneed to point out an important tool for the object of our concern.

On modeling ways, Paul Klee introduces thisnotion of extrusion, a point moves andgenerates a line, the extrusion of the linegenerates a surface and if we had a materialcapable of projecting a surface to generatean effect similar to the line effect, we couldgenerate in space an imaginary volume9.

Paul Klee. From point to line to surface to

volume.

Painting in virtual space gives us this material to continue the process and ex-trude surfaces in space, and realise what Klee considered as an unfortunateutopia.

Point or Voxels this is what Georges Seurat would have chosen, the model wouldbe a 3D matrix of cells, the larger the cell, the closest to impressionism.

At high spatial resolution, the cell becomes a zero dimension point with a very

high level of detail, this is the type of models generated by MRI scanners.

This type of modelling can be very heavy and inefficient but remains veryinteresting when a complex object is digitised using for example a laser 3D scan-ner. The scanner transmits xyz co-ordinates and RGB colour for each point of theobject into a matrix of void and coloured contour. These objects can be directlybrought from the real world to the numerical space to be shaded and rendered.For natural objects or free form elements such as clouds, fire, geological forma-tions, voxel models appear to be very well suited. Bringing the information downto a collection of points each described by its spatial co-ordinates, collar andtransparency.

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The cell could take anyshape and if the cellbecomes a cube, thenumerical mock-upcould resemble amodel made out of

cubic wooden blocksfor example. Com-bining 3D matrixes ofcubes, allows the des-cription of a buildingsshape with justmatrixes and Booleanoperations or withspatial-occupancyenumeration, as in the

following example forthe Seagram Buildingof Mies van der Rohein New York.10

Lines projected on a plane were the tool for perspective engravers, wireframemodels commonly regarded as old technology because they are useless in caseshading and rendering is needed. Wireframe perspectives can still be a beautifuland useful way of expression, drawing lines in space can also be a very quick andinteresting way of verification and a stimulus for the imagination.

Lines in space can ex-press non material re-lation between objectsor simple a nondimensioned structureas well as a physicalmodel built out ofstraight or curved wiressoldered or glued.

Not to forget the impor-tance of curves inspace, and the flexibilityallowed by parametriccubic curves, gives thearchitect interactiveaccess to the shape andtension of a curvetrough its control

points.

Facets , not to be

Seagram Building in terms of Boolean operators andvoxels

Sebastiano Serlio 1611

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confused with surfaces, are planar objects bounded with lines an curves, whichcan bound void to give the illusion of volumes. Facet based models can approximatesurfaces, and can be given colour, texture and other material properties for shadingand rendering, and as well as lines, remain very economical regarding storage ofinformation and computer resources.

Card board and glue, is the proper image of what can be done with facet basedmodels.

Wireframe and facets, Sebastiano Serlio, Fivebooks of architecture, 1611

Surfaces were the obsession of renaissancepainters, and very useful to architects formodelling freeform objects. The generalisation ofparametric cubic curves leads to parametric

bicubic surfaces, rational curves and surfaces havethe advantage of being able to define preciselyconic sections on one hand, and remain invariantunder rotation, scaling, transformation and pers-pective.

Shading and rendering techniques can be appliedto surfaces as well as to facets as long as thenormal to these objects is known. The normalpermits also the differentiation of two sides andthe mathematical surface can be efficiently usedto represent solid objects that have a nonsignificant thickness.

Drawing by Leonardo daVinci and a renderedNURBS based surface .

Moving control pointsinteractively in space,cutting and trimming sur-

faces, opens a freedomthat remains very difficultto obtain by traditionalplanar representation.Architects who want todeal with free form curvesand surfaces, can dealwith geometric objectthat can be dimensioned,scaled and therefore

transported intorealisation drawingswhich is far from being

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the case with free-hand drawing.

Non Uniform Beta Spline based surface models also called NURBS can be veryattractive because of the flexibility they offer, but remain less efficient for usualarchitectural objects that are mostly planar and require less computational effortand storage space to be described.

Antonio Gaudi used wire and fabric onto which he suspended weights to study hiswild but rational shapes. The wire he used would be the curves representing themesh, the surface is just the fabric, this structure is then controlled by the hangingstrings and weights that in our numerical space would be the control points andhulls.

The following two examples illustrate a parallel approach to design with materialand numerical mock-ups

Solid models, or collections of solids in space for the cubists, and for Paul C-zanne who treated nature in terms of the cylinder, the sphere, the cone, all inperspective.

A collection of 3D planes and surfaces do not necessarily bound a closed volume,solid models introduce the notion of volume well as the notion of inside andoutside. Bound with facets or surfaces, this elements will allow us to computeproperties of objects that depend on these notions.

As long as we are dealing with the bounds of volumes, we remain with the samepossibilities offered by surfaces in terms of rendering, solid notions become anecessity introducing refractive transparency. Solid models are the closest wecan get in terms of geometric description of architecture physical components.

Building in solid will allow cutting into a volume for cross section purposes, aswell as volumetric calculation for scheduling or physical properties.

Building with blocks can be simulated with parametric shapes such as spheres,cubes, cones or pyramids, but Boolean operations applied to volumes is a muchpowerful tool than block building alone or spatial-occupancy descriptions. In otherwords, constructive solid geometry (CGS) where simple primitives combined by

means of regularised Boolean set operators that are included directly in therepresentation.11

Ordinary Boolean set operators, defined as union, intersection and subtraction,applied to two objects, do not necessarily yield a solid. Regularised Boolean setoperators that restrict any operation to generate a solid, were the retained choicefor most solid models.

We shall not get into an exhaustive description of solid models nor Booleanoperators, but since we intend to deal with hybrid data sets it is important to

point ordinary set operators. Allowing the generation of points, lines, surfacesand void will permit designing space in terms of void or facets as an interfacebetween two voids etc.

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A concise description of volume or space in just a few lines such as the generationof the volumetric model of the Maison Minimum by Le Corbusier in terms of solidsand Boolean operators does not lack of interest.

Boolean operations can be transparent to the user if rooms and doorways carry

the attribute void, and shall be directly extracted from the solid described interms of a solid primitive, a set of voxels or a CGS tree.

Maison minimun Geometry in terms of void andsolid, in plan

Maison minimun Geometry in terms of void and

solid, axonometric view

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Shapes and sketches as elements of theproject

Up to this point, we have described graphic entities in terms of their geometry. In

order to generate these entities and use them in what we call Design environment,we have to give access to the geometric data base both for procedural generationand extraction of information through identification data.

Procedural elements, are an entry access to the geometric data base, a higherlevel generation of parametric elements that allow to introduce architectural con-cepts such as walls, openings, stairs or doors.

Introducing such concepts, we allow the architect to interact in terms of cons-tructive concepts that carry their inherent rules and react one to the other. In

presence of a window, the wall shall react by providing an opening and insertingthe shape of window in this opening.

External procedures for computing and generating models for natural phenomenasimulation such as clouds, water, fire, rocks or trees, can also be considered asprocedural elements.

Element Identification data, is the set of alphanumeric data that allows to link theelements to others or to further attributes or descriptions, and give access to theelements from external procedures.

In this ID set, an element is provided a name, a type in terms of its geometricdescription, a scale or detail level, date of creation, a category and a list ofkeywords and pointers to other descriptors.

Procedural elements an identification sheets, provide a powerful tool to thegeneration of geometric elements issued from expert systems or from algorithmsinvolving geometric construction rules.

Geometry issued from rules whether in a constructivist or deconstructivist way,could use such tools to describe the generation of form in terms of solid, void,

movement, transformation and trace concepts.

Full access to the elements for generation, interaction and feed back leads us tointroducing Knowledge-based design tools and models as proposed by YehudaKalay 12, where an object carries inherent descriptive and functionalknowledge to construct an abstract data type, rules applied to this knowledgecould then generate links between individual objects and transform them into anetwork of interrelated parts of one whole.

Designing in terms of language, vocabulary, relations and grammar as presented

by Gerhard Shmitt as a high level of abstraction to describe and representreality13

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Artificial Intelligence and expert systems for computable design was seriouslyrejected by architects when presented as systems to generate designautomatically, and is contrary to any architectural ethics, but remain veryinteresting if approached from Kallays or Mitchells point of view as tools forarchitects.

When you are dealing or designing in brick, you must ask brick what it wants, orwhat it can do, a sentence from Louis Kahn 14, that can resume the idea behindwhat we call Knowledge-based design.

Attributes and descriptors

Now that elements are linked to an identification sheet, they can point to lists of

attributes, non graphical description sheets, or graphical attributes for surfacedescription.

Non graphical attributes or description sheets, allow the link of the object to atext describing it, an image, a data-sheet containing its price, physicalcharacteristics etc.

Graphical attributes, describe the elements surface in terms of colour, specularity,reflection, refraction, transparency, diffraction and texture.

Since no substance can be perceived without shadow and light. Shadow andlight belong to light15, visual attributes are dependant on the light model usedand on the rendering procedure. A surface has more than one colour identifier, itcan have an intrinsic colour, a colour for its highlights, one for the incandescenceand light it produces, or a mapped matrix.

Whether we have a mapped matrix that gives a colour to each point of the sur-face or if we are dealing with intrinsic uniform colours, the resulting image willdepend on the surface characteristics to light.

Colour encoding for computer graphics, deals generally with an additive system

as long as we are sending images to displays, an additive mixture of Red, Greenand Blue.

In such a system, a colour is defined in terms of a value for each of these primaryadditive colours, white is the maximum of the three, black is zero for the threevalues. This encoding system, RGB, is satisfactory for most computing and displayprocedures, but remains very difficult to use by a designer. Reaching the rightcolour, even interactively in terms of red green and blue is not an easy task,therefore other encoding systems might be useful to introduce such as the HLS,Hue Lightness and Saturation system.

The HLS system allows a much better access to choosing colour without gettinginto primary mixing, since the basic hue can be displayed and the variation in

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terms of saturation and lightness can be used to alter the chosen colour.From a computational point of view, HLS encoding is more efficient than RGB fora modification of an images range of hue, keeping the same lightness and satu-ration.

In an essay on mixing of colour by Newton16, we can read Painters express this

plus or minus in the perfection of a colour of the same type (hue), use terms suchas pure or altered, simple or broken, and from a practical point of view, dealingwith electronic colour, we can imagine using simultaneously RGB and HLS colourencoding in order to select a colour then brake it, alter it with hue and saturation.As far as printing goes, colour has to be transformed into a subtractive system interms of Cyan Magenta and Yellow, Black is added since we know that inks do notgive a satisfactory black or grey in an equal amount of the three primary subtractivecolours.

This codification of colour called CMYK is used for colour printing device, but

since colour calibration depend on the used inks and the technology, it remainsnecessary to work with specific calibration programs and encode the colour infor-mation simultaneously for the display and the printers.Now that the object is linked to a colour or a colour map, we shall not enter thedifferent types of shading and rendering algorithm and stick to surface descrip-tion.

This is sufficient if constant shading will be used, producing realistic images willinvolve reaction of materials to light.

Diffuse reflection parameters will generate mat materials, specular reflection willallow introducing polished surfaces with highlights and inter-object reflection oftenintroducing a third element called roughness and controls the size of specularhighlights.

With simple Phong shading algorithms, an interpolation of surface normal, slightlymodified to take account of the above parameters, reach a fair touch of realismbut makes objects look like made of plastic.

The reason for this effect is that plastic does not shift the colour of highlights,more sophisticated algorithms allow the control of how much colour shifting is

performed and this is the reason why we introduced previously the notion ofspecular colour. As an example, we said plastic will leave the highlight whitewhile gold will shift white light in its highlights to yellow.

Transparency can be added to objects, ranging from opacity to invisible glasswhich will only be seen because of its reflection and if implemented, refractiveindex of transparent material will lead to more realistic effects.

Texture was almost introduced when we applied a colour map to the object, butwe believe that even if colour matrix mapping to surfaces can simulate texture to

a certain extent. We shall call texture the perturbation of a surface and its nor-mal by bump mapping, or intersecting the surface with solid patterns generatedby procedural texture synthesis17. Hyper-texture, an extension of procedural

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solid texture synthesis, models phenomena intermediate between shape andtexture by using space filling applicative functions to modulate density18. Hyper-texture can be used for hair, fur, glass and specially erosion effects.

Colour mapping

Light sources do not really belong to surface description unless we considerincandescence of a surface. Light source parameters such as colour, attenuationparameters, spread and intensity can be pointed at by a geometrical object andbehave as an attribute.

We have described elements in terms of their geometrical attributes and attachedthem to an identification data sheet, we do the same thing with surface descrip-tion in order to introduce pre-defined materials such as concrete, glass, plastic,or any construction material.

Associating each colour, surface attributes and texture to an identification sheet,with a name, and pointers to description components such as descriptive text,example images or associated material properties for thermal, structure, andcost calculation.

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Bump mapping (above) ; Museum lighting simulation (below)

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Johannes Itten Elements of geomtry and contrast, Bauhaus course.

Design

2D graphic elements, 3D descriptions, sketches, images, materials, text are storedin libraries, each with an identification sheet, linked one to the other by inherentpointers.

The project, or one phase of the project, is a network of these hybrid elementsput together, linked together in a geometrical space we shall call a scene and alogical space we call a partition.

Most CAD packages, place elements in space and call layers the logical subdivi-sion of space. Layers and sheets belong to two-dimensional concepts, thereforewe introduce the concept of elements belonging to logical partitions, dependingon their categories or any data base query applied to the fields of their identifica-tion sheets.Partitions are portions of space with no boundaries and just their own

co-ordinate system and can be placed in a scene at a relative position to thescenes co-ordinate system.

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The general concept here would be the following:

Objects contain elements that contain other elements, the highest element inthe hierarchical tree is called an object.

Apartition has an associated data-extraction sheet that contains its identifier

and rules such as the categories of elements allowed, or any selection rules.Partitions contain a selection of objects or only certain elements, if animationhas to be considered, a partition will belong to a time frame and will containmotion properties of objects.( we just situate where we want motion to beintroduced and we shall not get into movement and kinetics at this point).

The scene is associated to an identification sheet, and contains partitions andis considered as a network of elements at a certain moment of the project.

Theproject is a collection of scenes that can be associated to phases of design

or logical combination of scenes.

Visualising

During the building of the mock-up in this numerical world, operation were doneinteracting with the objects through parallel projections, and perspective views.Extracting the data or visualising scenes of the project involves data filtering,projection and rendering rules.

Scheduling is part of the visualisation even if graphical aspects are not involved,we consider alphanumeric description of objects as a means to visualise andexplain the project.

Schedules are then exported to text processing, data base managers or anyprocessing program.

Drafting generates sheets, that are scale dependent and category dependenttherefore belonging to a specific partition or scene.

A sheet is not an image, it is a two-dimensional collection of lines and curves wecall plans, elevations, sections, perspective or axonometric projections.

The data extracted, is in priority the objects that have 2D scale dependent symbolsfor plan, elevation or section, and the missing geometry is extracted directlyfrom the 3D model as a parallel projection.

A drawing is a collection of sheets, that will be sent to a 2D vectorial editor to becompleted, to add text and dimension.

Rendering involves applying to scene, a virtual camera or projection algorithmsas well as rendering procedures. Parallel projection for architecture is widelyused, this is one of the major problem architects have to face when using generic

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rendering tools that mostly use perspective projections and disregard axonometricor oblique views.

Parallel projections from above the model, will generate a top view similar to asite plan, and controlling the position of near and far clipping planes we cangenerate sections in the numerical mock-up. Top, front, back, right or left views

are called so to avoid confusion with plans, elevations and sections.

Perspective view or simulation of a virtual camera with focal control and depth offield allows proper realistic rendering. Simulating a virtual camera is the choice ofmost rendering system in search for realism, but we just have to mention howambiguous perspective can be and how important other projections are toarchitects

The advantages of perspective in rendering should not lead us to dismiss otherattitudes such as cubism, naive representations or the converging axis19 of

the ancients that lack of consistency but so rich and interesting.

Vanishin line perspective, fresque from Boscoreale, 1st century, museum of Na-ples

Or allow concepts such as the displacement of the vanishing point and opticalangle of Paul Klee20, where three people looking at the same scene or thesubjective path.

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The whole rationale of the process is illustrated with masterly simplicity in Drersfamous woodcut21, says Gombrich, but It also follows from Drersdemonstration that any number of objects can be constructed that will result inthe identical aspect from the peep hole.

This does not neglect perspective in the comprehension of scenes andapprenticeship of space as long as the eye is trained as described by Rodshenko:To teach people to see things under different angles, it is necessary to photographfamiliar objects, objects they know very well under unusual angles; new objectshave to be photographed from many points of views in order to give a completerepresentation.

Rendering these projections using shading algorithms, ray tracing or radiositywill allow the generation of quasi photographic images.

These images are sent to displays, printers, video recorders or to image editorsto be modified, combined with other images or photographs, entering the digitalpost-photographic era, but this is another subject.

All of the above results, can be sent to sketch hybrid editor, that can managetext, bit-map images and vector drawings.

Representation can also be seen as a pipeline of operations simultaneously sendingto the screen multiple rendered images, drawings, animation and even windowson the model where interacting remains possible, this would be electronic sketchbook.

Interacting directly with the virtual mock-up is a new interesting aspect, using ahead-mount stereoscopic display, an image is sent to each eye, we enter themodel. A head tracking system follows head movement and eye tracking cameraswill send the correct images to each eye in real time. A data suite and glovestracks our movements and allow interaction with this virtual space just like in fullscale modelling laboratories with colour and texture.

On front page :

Marcel Duchamp. Nu descendant un escalier and a numeric nude descendantlescalier.

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Notes

1 William J. Mitchell The Reconfigured Eye , Boston, The MIT Press 1992

2 John Archea Puzzle making, Computability of design symposium Dec 1986 ,State university of New York Buffalo

3 Alessandra Latour, Louis I. Kahn, Writings Lectures Interviews, New York,Rizzoli 1991,p10-15

4 Alessandra Latour, Louis I. Kahn, Writings Lectures Interviews, On Form andDesign, New York, Rizzoli 1991,p105

5 Matila C. Ghyka, le nombre dor, Paris, Gallimard, 1931, 1959

6 Vittorio Gregotti, Scale della representazione, Casabella 504, Italy August

1984

7 Le Corbusier, Vers une Architecture, Crs, Paris 1923

8 Louis Servicien, Les carnets de Lonard de Vinci, Paris , Gallimard, 1942

9 Paul Klee, translated to French by Sylvie Girard, La pense cratrice, Paris1980,Dessain et Tolra, p103-6

10 Lionel March abd Philip Steadman, The geometry of environment, London1971,RIBA Publications Limited, p 126-43

11 Foley van Dam Feiner Hughes, Computer Graphics principles and practice1990,Addison-Wesley, p 557

12 Computability of design symposium Dec 1986 , State university of New YorkBuffalo

13 Gerhard Shmitt, Micro Computer Aide Design, New York, John Wiely & Sons,1988

14 Louis Kahn in a speech given at the International Design Conference, in AspenColrado 1972

15 Louis Servicien, Les carnets de Lonard de Vinci, Paris , Gallimard1942, p265

16 trad Pre Antoine Rivoire, Un ssai sur le Mlange des Couleurs par Newton,Amsterdam, Westein 1757

17 J.P.Lewis, Algorithms for solid noise sythesis,Computer Graphics 89, ACMp263

18 Ken Perlin, Hypertexture, Computer Graphics 89, ACM p253

19 Erwin Panofsky, La perspective comme forme symbolique, Paris, Editions deminuit, 1975

20 Paul Klee, translated to french by Sylvie Girard, La pense cratrice, Paris

1980,Dessain et Tolra, p173

21 E H G b i h A t d Ill i N Y k P th B k 1960