Graphics Reference
In-Depth Information
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vec3 shade(
const
in vec3 P,
const
in vec3 n) {
vec3 radiance
= vec3(0.0);
// Assume only one light
vec3 offset = lightPosition - P;
float
distanceToLight = length(offset);
vec3 w_i
= offset / distanceToLight;
vec3 w_o
= -normalize(P);
if
(visible(P, w_i, distanceToLight, shadowMap)) {
vec3 L_i = lightPower / (4
*
PI
*
distanceToLight
*
distanceToLight);
// Scatter the light.
radiance +=
L_i
*
evaluateFiniteScatteringDensity(w_i, w_o)
*
max(0.0, dot(w_i, n));
}
return
radiance;
}
void
main() {
vec3 P = Pinterp;
radiance = shade(P, n);
}
Listing 15.34: Helper functions for the pixel shader.
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#
define
PI 3.1415927
bool
visible(
const
in vec3 P,
const
in vec3 w_i,
const
in
float
distanceToLight,
sampler2DShadow shadowMap) {
return true
;
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}
/
**
Returns f(wi, wo). Same as BSDF::evaluateFiniteScatteringDensity
from the ray tracer.
*
/
vec3 evaluateFiniteScatteringDensity(
const
in vec3 w_i,
const
in vec3 w_o) {
vec3 w_h = normalize(w_i + w_o);
return
(k_L +
k_G
*
((s + 8.0)
*
pow(max(0.0, dot(w_h, n)), s) / 8.0)) / PI;
}
However, there is one exception. The software renderers iterated over all the
lights in the scene for each point to be shaded. The pixel shader is hardcoded
to accept a single light source. That is because processing a variable number
of arguments is challenging at the hardware level. For performance, the inputs
to shaders are typically passed through registers, not heap memory. Register
allocation is generally a major factor in optimization. Therefore, most shading
compilers require the number of registers consumed to be known at compile
time, which precludes passing variable length arrays. Programmers have devel-
oped three
forward-rendering
design patterns for working within this limitation.
These use a single framebuffer and thus limit the total space required by the
algorithm. A fourth and currently popular
deferred-rendering
method requires
additional space.
