Question

An answer to my question suggests that DOT3 lighting can help with OpenGL ES rendering, but I’m having trouble finding a decent definition of what DOT3 lighting is.

Edit 1

iPhone related information is greatly appreciated.

Answer 1

DOT3-lighting is often referred to as per-pixel lighting. With vertex lighting the lighting is calculated at every vertex and the resulting lighting is interpolated over the triangle. In per-pixel lighting, as the name implies, the object is to calculate the lighting at every pixel.

The way this is done on fixed function hardware as the iPhone is with so called register combiners. The name DOT3 comes from this render state:

glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_DOT3_RGB);

Look at this blog entry on Wolfgang Engels blog for more info on exactly how to set this up.

When doing per-pixel lighting it’s popular to also utilize a so called normal map. This means that the normal of every point on an object is stored in a special texture map, a normal map. This was popularized in the game DOOM 3 by ID software where pretty low polygon models where used but with high resolution normal maps. The reason for using this technique is that the eye is more sensitive to variation in lighting than variation in shape.

I saw in your other question that the reason this came up was that you wanted to reduce the memory footprint of the vertex data. This is true, instead of storing three components for a normal in every vertex, you only need to store two components for the texture coordinates to the normal map. Enabling per-pixel lighting will come with a performance cost though so I’m not sure if this will be a net win, as usual the advice is to try and see.

Finally the diffuse lighting intensity in a point is proportional to the cosine of the angle between the surface normal and the direction of the light. For two vector the dot product is defined as:

a dot b = |a||b| cos(theta)

where |a| and |b| is the length of the vectors a and b respectively and theta is the angle between them. If the length is equal to one, |a| and |b| are referred to as unit vectors and the formula simplifies to:

a dot b = cos(theta)

this means that the diffuse lighting intensity is given by the dot product between the surface normal and the direction of the light. This means that all diffuse lighting is a form of DOT3-lighting, even if the name has come to refer to the per-pixel kind.