Volumetric Light Effects in Killzone: Shadow Fall - GPU Pro: Advanced Rendering Techniques - page 138

float GetLookupDepth ( float inViewSpaceDepth )
{ float depth_range

= 128.0; // < Depth range R

float inv_curve_power = 1.0/2.0; // < Inv. power (1.0 / C)

return pow (( inViewSpaceDepth / depth_range ), inv_curve_power );

}

Listing 3.1. Calculating the 3D texture lookup depth coordinate for a given view-space
depth.

We render the particles as traditional camera-facing billboards. The particle's
alpha channel will be used to define the scattering amount. To calculate the
influence a particle has on each depth slice, we use the code shown in Listing 3.2.
In the function we first calculate the 3D texture lookup depth coordinate. We
then scale the value up to a 0.0 to 15.0 range to map it within the range of our
depth slice indices. The resulting value particle_slice_pos defines the position
of the particle between the slices. The influence a particle has on each depth
slice is based on the absolute value of the difference between particle_slice_pos
and each slice index. We refer to this value as distance because it describes the
distance of the particle to the depth slice in texture space.
If the distance is 0.0, the particle is located at the same depth as the depth
slice and the particle influence should be 1.0. A distance equal to or greater
than 1.0 means the particle is more than one slice away and the influence will
be 0.0. For distances ranging between 0.0 and 1.0, the influence will be equal to
the reversed value of distance . For any slice index the influence can be easily
calculated by clamping the distance value to a 0.0 to 1.0 range and reversing the
result to a 1.0 to 0.0 range using f ( x )=1 − x .

float GetAmountValue ( float inParticleDepth ,
float inScatteringAmount ,
int inSliceIndex )

{

int num_depth_slices = 16; // < Number of depth slices
float particle_slice_pos = GetLookupDepth ( inParticleDepth ) ￿
( num_depth_slices − 1) ;

float distance = abs ( particle_slice_pos −

inSliceIndex );

float influence =1.0 −

saturate ( distance );

return inScatteringAmount ￿ influence ;

}

Listing 3.2. Calculating the particle's scattering amount value for a single depth slice.