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Figure 5.6. Illustration of 2.5D culling. (a) Frustum culling creates a long frustum
for a tile with foreground and background. (b) 2.5D culling—splitting depth into eight
cells—does not capture lights falling between foreground and background.
2.5D Culling
At the light-culling stage of Forward+, light geometries are tested against a frus-
tum of each tile that is clipped by the maximum and minimum depth values of
a tile. This light culling works well if there is little variance in the depth in a
tile. Otherwise, it can create a long frustum for a tile. This results in capturing
a lot of lights for a tile, as we can see at the edge of geometries in Figure 5.3(b) ,
although some lights have no influence on any of the pixels in a tile if they fall
at the void space in the frustum.
As the number of lights reported for a tile increases, the computational cost of
final shading increases. This is critical especially when the shading computation
for a light is expensive. This is often the case because one of the motivations of
employing Forward+ is its ability to use sophisticated BRDFs for shading.
One obvious way to improve the eciency of culling is to cull the lights using
a 3D grid. However, this increases the computation as well as the size of the data
to be exported. It is possible to develop sophisticated and expensive culling, but
it shouldn't be overkill. Our proposed 2.5D culling constructs a nonuniform 3D
grid without adding a lot of computation or stressing the memory bandwidth.
The idea is illustrated in Figure 5.6. This approach first constructs a frustum
for a tile in the same way as the screen-space culling described in Section 5.3.
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