8.1.1 Phong Highlights with Texture

One of the problems with the OpenGL lighting model is that specular reflectance is computed before textures are applied in the normal pipeline sequence. To achieve more realistic looking results, specular highlights should be computed and added to image after the texture has been applied. This can be accomplished by breaking the shading process into two passes. In the first pass diffuse reflectance is computed for each surface and then modulated by the texture colors to be applied to the surface and the result written to the color buffer. In the second pass the specular highlight is computed for each polygon and added to the image in the framebuffer using a blending function which sums 100% of the source fragment and 100destination pixels. For this particular example we will use an infinite light and a local viewer. The steps to produce the image are as follows:

1. Define the material with appropriate diffuse and ambient reflectance and zero for the specular reflectance coefficients.
2. Define and enable lights.
3. Define and enable texture to be combined with diffuse lighting.
4. Define modulate texture environment.
5. Draw lit, textured object into the color buffer with the vertex colors set to 1.0.
6. Define new material with appropriate specular and shininess and zero for diffuse and ambient reflectance.
7. Disable texturing, enable blending, set the blend function to GL_ONE, GL_ONE.
8. Draw the specular-lit, non-textured geometry.
9. Disable blending.

This implements the basic algorithm, but the Gouraud shaded specular highlight still leaves something to be desired. We can improve on the specular highlight by using environment mapping to generate a higher quality highlight. We generate a sphere map consisting only of a Phong highlight [37] and then use the GL_SPHERE_MAP texture coordinate generation mode to generate texture coordinates which index this map. For each polygon in the object, the reflection vector is computed at each vertex. Since the coordinates of the vector are interpolated across the polygon and used to lookup the highlight, a much more accurate sampling of the highlight is achieved compared to interpolation of the highlight value itself. The sphere map image for the texture map of the highlight can be computed by rendering a highly tessellated sphere lit with only a specular highlight using the regular OpenGL pipeline. Since the light position is effectively encoded in the texture map, the texture map needs to be recomputed whenever the light position is changed.

The nine step method outlined above needs minor modifications to incorporate the new lighting method:

5
1. disable lighting.
2. load the sphere map texture, enable the sphere map texgen function.
3. enable blending, set the blend function to GL_ONE, GL_ONE.
4. draw the unlit, textured geometry with vertex colors set to 1.0.
5. disable texgen, disable blending.

With a little work the technique can be extended to handle multiple light sources.