Game Development Reference
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Another option relies on rasterization and also borrows an idea from ray
tracing. To check the visibility to a light, we can cast a ray to the light. If
the light is local, the length of the ray is short. This means we do not have to
traverse much in the scene; the cost is not as high as the cost of ray casting a
long ray in full ray tracing.
In this subsection, we describe how ray casting can be integrated in Forward+
to add shadows from hundreds of lights and show that a perfect shadow from hun-
dreds of lights can be obtained in real time. After adding this feature, Forward+
is not just an extension of forward-rendering pipeline but a hybrid of forward,
deferred-rendering pipelines and ray tracing.
Implementation. To ray cast against the scene, we need the position and normal
vector of a primary ray hit and the acceleration data structure for ray casting.
The position of a primary ray hit can be reconstructed from the depth buffer
by applying inverse projection. The normal vector of the entire visible surface,
which is used to avoid casting rays to a light that is at the back of the surface and
to offset the ray origin, can be written at the depth prepass. The prepass is no
longer writing only the depth value, and so it is essentially identical to a G-pass
in the deferred-rendering pipeline. The acceleration structure has to be updated
every frame for a dynamic scene; however, this is a more extensive research topic
and we do not explore it in this chapter. Instead, we just assume that the data
structure is built already.
After the prepass, implementing a ray-cast shadow is straightforward. In a
pixel shader, we have access to all the information about lights, which includes
light position. A shadow ray can be created by the light position and surface
location. Then we can cast the ray against the acceleration structure for an
intersection test. If the ray is intersecting, contribution from the light is masked.
Although this naive implementation is easy to implement, it is far from prac-
tical in terms of performance. The issue is a legacy of the forward-rendering
pipeline. The number of rays to be cast for each pixel is not constant, which
means the computational load or time can vary considerably among pixels even
if they belong to the same surface. This results in a poor utilization of the GPU.
An alternative is to separate ray casting from pixel shading for better perfor-
mance. After separating ray casting from pixel shading, the pipeline looks like
light culling,
ray-cast job creation,
ray casting,
final shading.
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