Smooth Shading: Color Interpolation

Smooth Shading:
Color Interpolation
Andrew G. Zaferakis
UNC Chapel Hill
COMP 236 Spring 2000

Homework 5a - Smooth Shading

Part A: The coding assignment.

The clippers: softgl_homoclip_smooth.cpp, softgl_homoclip_smooth.hpp, softgl_homoclip_flat.cpp softgl_homoclip_flat.hpp
The rasterizers: softgl_trirast_smooth.cpp, softgl_trirast_smooth.hpp, softgl_trirast_flat.cpp, softgl_trirast_flat.hpp

SoftGL: The software renderer

SoftGL is our software version of OpenGL, we intend to supply a subset of the OpenGL extensions, and implement many different features that OpenGL does not have. Each assignment will build on our SoftGL code so check back often. This assignment updated the clipping and rasterization. Grab the new executable below.
Download the SoftGL viewer: (pc, sgi)

For this assignment, we implemented smooth shading, or color interpolation, across the face of a triangle. To achieve the RGB color values, we peformed perspectively correct clipping, but not rasterization. The mathematics behind interpolating color is exactly the same as the depth interpolation described in hw3. What we have found in comparison with various different hardware implementations is interesting. When using a simple plane with different color values for the four vertices, SoftGL matches hardware in all cases when the plane is not clipped. When the plane is clipped, we did not match any hardware. Major errors exist in almost all hardware implementations. Below are screenshots from various graphics archictectures, showing the inaccuracies. nVidia's GeForce 256 does perform perspectively correct color interpolation, and it should be considered correct. What is interesting is that SGI's InfiniteReality2 architecture does not perform color interpolation correctly, yet it will never appear to look incorrect, the colors always smoothly blend together, which is obviously not the case in other cards (see below). Try this out on your architecture and see what results you get. Toggle between SoftGL and OpenGL by pressing the spacebar. The SoftGL controls are shown below.
The images on the left are produced from SoftGL, the images on the right are from various video cards. The first image shows the plane when it is fully in view, which should match perfectly, the second image shows the plane being clipped, it is also rotated so that it is very close to being aligned with the z-axis, which exploits the incorrect perspective color interpolation.

(click on images for larger versions)

SoftGL	nVidia GeForce256


The SoftGL version is not doing perspectively correct rasterization. You can notice this by looking at the lower left corner, the blue is much larger then on the GeForce256.	The GeForce256 is doing perspectively correct color interpolation. You'll notice the upper right corner's red is larger then the upper left's green, because red is closer.

SoftGL	SGI InfiniteReality2


	The IR2 will always smoothly interpolate colors, but it is incorrect, you'll notice that the blending of the blue from the bottom left through the top middle is much different then SoftGL.

SoftGL	SGI PC Cobalt Chipset


	The PC SGI hardware looks to perform the same as the IR2 above.

SoftGL	SGI O2


	The SGI O2 creates a banding effect with the green where the quad is split into triangles. I'm not sure what they are doing here in hardware to achieve this result.

SoftGL	Matrox MGA Millenium II PowerDesk


	The Matrox card is performing similar to the SGI O2. Still not sure how they are achieving these results.

**SoftGL Viewer Controls**
Mouse	Used to Rotate
Left Button	Locks mouse movements to controls
>	Zoom In
<	Zoom Out
ESC	Exit
SPACE	Toggle SoftGL vs OpenGL mode

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