Lighting is also pure math. Here you try to find out how much light falls on an object when you know how many lights there are and where they are. These mathematics make use of the direction of the light and the distance between the light source and the vertex of the triangle. The light intensity is only calculated at the vertices of the triangles that form the objects. Now light calculations aren't that simple, because you have light sources and light sources. There are 4 different types of light. First of all you have a global light. A global light is a bit of a trick, being a base light intensity level that is used to make sure that every object is somewhat visible. So it's a bit like saying everything has a light intensity of 10 to start with. This kind of light is very handy if you are building up a scene, just add a simple global light and everything will be visible. Naturally, this kind of light is very boring since everything will look the same. To change that we add uniform lights. Uniform lights emit light equally in all directions, so every object can be lit by them as long as they are near enough. The light intensity only depends on the distance between the vertex and the light source. So all objects that are at the same distance from the uniform light will get the same amount of light, whether they are above, below or right doesn't matter- the direction is irrelevant. It's pretty obvious that the math involved with this kind of lights are more expensive than a simple global light source. But let's go a step further and add directional lights. Directional lights will light objects based on distance and direction. Let me illustrate this: assume you have a flashlight and you shine it onto a side of a cube. In this case you will see a very bright surface. The direction of the light is perpendicular to the surface. But now if you look at the sides of the cube, where the direction of the light source is parallel to the surface (same direction), you will notice that those sides are rather dark compared to the first side. This effect is achieved by directional lights. Most light sources in the real world are a mix between uniform and directional lights. Pure forms are very rare but usually the directional light source delivers better, more realistic results. But again, we have to note that the math become more intense because now we need to look at the distance, the direction of the light source and the orientation of the surface. But we don't stop here: lets add spots. A spot is even more complex since a very small zone will be lit very intense while further away from the center of the spot the light intensity drops very fast. I'll not go into the math but it's again obvious that it's more workâ€¦

Now while the operations for the Transformations were identical for every vertex of every triangle of every object, the operations for the light sources can be very different. In one scene you might have a couple of directional lights while in another scene you might have a uniform light with a couple of spots. On top of all this, the math involved is more intense for different kinds of light and differences in math can result in differences in clock cycles. So if a scene has a uniform light and a spot light it might very well be that the hardware light engine requires 1 cycle to find the light intensity caused by the uniform light while it requires 10 for the spot. This imposes a risk for performance claims, if a company tells you that the hardware light engine can handle 8 lights within x clockcycles, it will probably mean 8 uniform lights. However, if you use spots or directional lights the performance will drop considerably. Exactly how good the light engine is remains thus to be seenâ€¦ just be aware that there are lights and there are lights.

This difference in clock cycles can give more problems as the hardware transform and lighting engine might have completely different timings. Assume that a transform takes 2 cycles, while a lighting takes between 1 and 10 cycles, depending on the type of lights. This would introduce a bottleneck between the two units. One of the two units will be idle waiting for the other to finish. Buffers can help a bit but they can always suffer from under-runs and overflows, so again timing can influence the actual throughput of these engines.

Another question I ask myself is: How useful is hardware lighting in games? Hardware lighting is per vertex lighting. The light intensity is only calculated at the vertices of the triangles forming an object. In between the intensity is interpolated between the values at the vertices. Now assume I want to place a wall in my 3D world. A wall is just a rectangle and a rectangle can be formed by two large triangles. No problem so far, but now I want to add a torch casting a typical flickering light spot on the wall. Now per vertex lighting only calculates light at the vertices, which would be top left and right, and bottom left and right, in between you will get a uniform interpolation. If we assume that the torch is right in the middle of the wall then the distance to all corners is equal and thus the light intensity will be the same for all vertices resulting in a completely uniform lit wall. Not really what we wanted.