CSE 872 Dr. Charles B. Owen Advanced Computer Graphics1 Other Rendering Techniques Types of...

CSE 872 Dr. Charles B. OwenAdvanced Computer Graphics1

Other Rendering Techniques

• Types of rendering– Wireframe techniques– Scan-line conversion– Reyes rendering– Ray tracing (done)

• Shadows• Reflections• Depth of field, focus, etc.• Antialiasing


Line only techniques

• Hidden-line removal– Line graphics– Wireframe images– “Halo” line images


Floating Horizon Algorithms


Robert’s Algorithm

• First visible line algorithm• O(N2) complexity• All objects must be convex volumes• Excessively complex


Warnock Algorithm

• Spatial subdivision– Quad-trees


Appel’s Algorithm

• Quantitative invisibility– When a line goes behind something, increment by 1

– When it comes out, increment by -1


Haloed Lines


So much for wireframe…


What are the ways to convert primitives to image?• Scan-line conversion

• Ray tracing

Anything else?


Scan-line conversion

• Each primitive is mapped to 2D and then converted to image– List priority algorithms– Z-buffer techniques– A-buffer techniques


Painters Algorithm

• Sort back to front• Draw back to front– That’s all there is to it

• Does this always work?


Z-buffer algorithms

• Associate a z value with each pixel– Contains z for nearest point drawn so far– OpenGL, Direct3D, etc.

• Where do we get the Z?


A-Buffer Algorithms

• Create a list of polygons for each scan line• Determine x extents for each line• Sort in z order• Draw pixel for nearest line


Spanning Scan Line Algorithms

• Create a list of polygons for each scan line• Determine x extents for each line• Determine spans– All end points and intersections partition spans

• Draw front line for each span


List priority algorithms

• Sort things in depth order– Painter’s algorithm paints back to front– We could just paint front if no overlaps

• Newell-Newell-Sancha Algorithm– Provides way to sort polygons

• Splits any polygons if there is a cyclical relationship


Doing things in a z-buffer world

• Z-buffer: the most common scan-line method

– What’s good about it?

– What’s bad about it?


Shadows?

• Why does OpenGL not show shadows?


Drawing Shadows

• Assume a light at (0,0,0). We want to know were the shadow projects on a plane defined by ax+by+cz+d=0– An occluding point S=(sx,sy,sz,1)

– Anything along line through S would be: (sx,sy,sz,1)

– Line intersection with plane is:– asx+bsy+csz+d=0

– a=-d/(asx+bsy+csz)

– Intersection with plane is: -d(sx,sy,sz)/(asx+bsy+csz)


Using this equation

• -d(sx,sy,sz)/(asx+bsy+csz)

)/(

)/(

)/(

10

000

000

000

czbyaxdz

czbyaxdy

czbyaxdx

czbyax

dz

dy

dx

z

y

x

cba

d

d

d

1. Translate so light is at origin2. Put this matrix on stack (using translated plane)3. Draw object we are casting shadow of in shadow color

???

Similar approach existsFor lights at infinity


How I got this working…

• I display the shadow into the stencil buffer• I then draw the polygon with alpha value that

decreases brightness

• See example program…


Shadow Direction

Image from light 0 viewpoint


What all can we do with this?


Lit Polygon Determination



Partition polygons into lit/not lit sets

Subdivide any that are partially lit


Shadow Texture



Create a texture map for each visible polygon


Shadow Map


Depth map from light 0 viewpoint

The map partitions the space into shadowed/not shadowed. For each point, determine which partition we are in


Deep Shadow Maps

3D texture where each point is how much illumination we receive


Shadow Volumes

• Polygons cast polyhedra of shadow volumes• Intersect polyhedra and object space


Reflections/Transparency?

• How?


Environment Maps

• Uses vertex normals to generate texture coordinates• How can we use this to generate:– Reflections– Transparency with refraction?


Depth of Field?

• What?• Why?• How?


Motion Blur

• What?• Why?• How?

CSE 872 Dr. Charles B. Owen Advanced Computer Graphics1 Other Rendering Techniques Types of...

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