1 of 29 Interesting Illumination Demo There’s a very nice Java illumination model demo which may...

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Interesting Illumination Demo

There’s a very nice Java illumination model demo which may help you understand the effects of different kinds of reflections available at:http://www.siggraph.org/education/materials/HyperGraph/illumin/vrml/pellucid.html

Try playing with the various parameters and see if you can predict what the sphere will look like

Course Website: http://www.comp.dit.ie/bmacnamee

Computer Graphics 16:Polygon Rendering

Methods

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Contents

Today we will start to look at rendering methods used in computer graphics

– Flat surface rendering– Gouraud surface rendering– Phong surface rendering

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No Surface RenderingIm

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Flat Surface RenderingIm

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No Surface Rendering Vs Flat Surface Rendering

Flat Surface RenderingNo Surface Rendering

Images come from: www.siggraph.org/education/materials/HyperGraph/scanline/shade_models/shading.htm

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Gouraud Surface RenderingIm

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Gouraud Surface RenderingFlat Surface Rendering


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Phong Surface RenderingIm

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Phong Surface RenderingGouraud Surface Rendering


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Flat Surface Rendering

The simplest method for rendering a polygon surface

The same colour is assigned to all surface positions

The illumination at a single point on the surface is calculated and used for the entire surface

Flat surface rendering is extremely fast, but can be unrealistic

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Overcoming Flat Shading Limitations

Just add lots and lots of polygons – however, this is SLOW!

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Gouraud Surface Rendering

Gouraud surface shading was developed in the 1970s by Henri GouraudWorked at the University of Utah along with Ivan Sutherland and David EvansOften also called intensity-interpolation surface renderingIntensity levels are calculated at each vertex and interpolated across the surface

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Gouraud Surface Rendering (cont…)

To render a polygon, Gouraud surface rendering proceeds as follows:

1. Determine the average unit normal vector at each vertex of the polygon

2. Apply an illumination model at each polygon vertex to obtain the light intensity at that position

3. Linearly interpolate the vertex intensities over the projected area of the polygon

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The average unit normal vector at v is given as:

or more generally:


n

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Illumination values are linearly interpolated across each scan-line

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Advantages of Gouraud Surface Rendering

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Gouraud Surface Rendering ExampleIm

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Gouraud Surface Rendering Implementation

Gouraud surfacing rendering can be implemented relatively efficiently using an iterative approach

Typically Grouaud shading is implemented as part of a visible surface detection technique

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Problems With Gouraud Shading

Gouraud shading tends to miss certain highlighting In particular Gouraud shading has a problem with specular reflections

Also, Gouraud shading can introduce anomalies known as Mach bands

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Problems With Gouraud Shading (cont…)

The major problem with Gouraud shading is in handling specular reflections

Gouraud shading Phong shading

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Mach Bands

A psychological phenomenon whereby we see bright bands where two blocks of solid colour meetA good demo is available to experiment with this at:

http://www.nbb.cornell.edu/neurobio/land/OldStudentProjects/cs490-96to97/anson/MachBandingApplet/

Try playing with the various parameters and see if you can predict what the sphere will look like

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Phong Surface Rendering

A more accurate interpolation based approach for rendering a polygon was developed by Phong Bui Tuong

Basically the Phong surface rendering model (or normal-vector interpolation rendering) interpolates normal vectors instead of intensity values

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Phong Surface Rendering (cont…)

To render a polygon, Phong surface rendering proceeds as follows:

1. Determine the average unit normal vector at each vertex of the polygon

2. Linearly interpolate the vertex normals over the projected area of the polygon

3. Apply an illumination model at positions along scan lines to calculate pixel intensities using the interpolated normal vectors

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Phong Surface Rendering (cont…)

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Phong Surface Rendering Implementation

Phong shading is much slower than Gouraud shading as the lighting model is revaluated so many times

However, there are fast Phong surface rendering approaches that can be implemented iteratively

Typically Phong shading is implemented as part of a visible surface detection technique

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Phong Shading ExamplesIm

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Phong Shading ExamplesIm

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Summary

For realistic rendering of polygons we need interpolation methods to determine lighting positions

Flat shading is fast, but unrealistic

Gouraud shading is better, but does not handle specular reflections very well

Phong shading is better still, but can be slow

1 of 29 Interesting Illumination Demo There’s a very nice Java illumination model demo which may...

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