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don't use hard-coded values for Builder::radiance()

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Instead we use constexpr expressions of the values,
which makes it much easier to remember where they
come from.
This commit is contained in:
Mathias Agopian
2019-08-26 16:47:07 -07:00
committed by Mathias Agopian
parent f71fea4ef5
commit 417ba87fcb
1 changed files with 56 additions and 14 deletions
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70
filament/src/IndirectLight.cpp
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@@ -72,21 +72,63 @@ IndirectLight::Builder& IndirectLight::Builder::irradiance(uint8_t bands, float3
}
IndirectLight::Builder& IndirectLight::Builder::radiance(uint8_t bands, float3 const* sh) noexcept {
float3 irradiance[9];
if (bands >= 1) {
irradiance[0] = sh[0] * 0.282095f;
if (bands >= 2) {
irradiance[1] = sh[1] * -0.325735f;
irradiance[2] = sh[2] * 0.325735f;
irradiance[3] = sh[3] * -0.325735f;
if (bands >= 3) {
irradiance[4] = sh[4] * 0.273137f;
irradiance[5] = sh[5] * -0.273137f;
irradiance[6] = sh[6] * 0.078848f;
irradiance[7] = sh[7] * -0.273137f;
irradiance[8] = sh[8] * 0.136569f;
}
// Coefficient for the polynomial form of the SH functions -- these were taken from
// "Stupid Spherical Harmonics (SH)" by Peter-Pike Sloan
// They simply come for expanding the computation of each SH function.
//
// To render spherical harmonics we can use the polynomial form, like this:
// c += sh[0] * A[0];
// c += sh[1] * A[1] * s.y;
// c += sh[2] * A[2] * s.z;
// c += sh[3] * A[3] * s.x;
// c += sh[4] * A[4] * s.y * s.x;
// c += sh[5] * A[5] * s.y * s.z;
// c += sh[6] * A[6] * (3 * s.z * s.z - 1);
// c += sh[7] * A[7] * s.z * s.x;
// c += sh[8] * A[8] * (s.x * s.x - s.y * s.y);
//
// To save math in the shader, we pre-multiply our SH coefficient by the A[i] factors.
// Additionally, we include the lambertian diffuse BRDF 1/pi and truncated cos.
constexpr float M_SQRT_PI = 1.7724538509f;
constexpr float M_SQRT_3 = 1.7320508076f;
constexpr float M_SQRT_5 = 2.2360679775f;
constexpr float M_SQRT_15 = 3.8729833462f;
constexpr float C[] = { M_PI, 2.0943951f, 0.785398f }; // <cos>
constexpr float A[] = {
1.0f / (2.0f * M_SQRT_PI) * C[0] * M_1_PI, // 0 0
-M_SQRT_3 / (2.0f * M_SQRT_PI) * C[1] * M_1_PI, // 1 -1
M_SQRT_3 / (2.0f * M_SQRT_PI) * C[1] * M_1_PI, // 1 0
-M_SQRT_3 / (2.0f * M_SQRT_PI) * C[1] * M_1_PI, // 1 1
M_SQRT_15 / (2.0f * M_SQRT_PI) * C[2] * M_1_PI, // 2 -2
-M_SQRT_15 / (2.0f * M_SQRT_PI) * C[2] * M_1_PI, // 3 -1
M_SQRT_5 / (4.0f * M_SQRT_PI) * C[2] * M_1_PI, // 3 0
-M_SQRT_15 / (2.0f * M_SQRT_PI) * C[2] * M_1_PI, // 3 1
M_SQRT_15 / (4.0f * M_SQRT_PI) * C[2] * M_1_PI // 3 2
};
// this is a way to "document" the actual value of these coefficients and at the same
// time make sure the expression and values are always in sync.
struct Debug {
static constexpr bool almost(float a, float b) {
constexpr float e = 1e-6f;
return (a > b - e) && (a < b + e);
}
};
static_assert(Debug::almost(A[0], 0.282095f), "coefficient mismatch");
static_assert(Debug::almost(A[1], -0.325735f), "coefficient mismatch");
static_assert(Debug::almost(A[2], 0.325735f), "coefficient mismatch");
static_assert(Debug::almost(A[3], -0.325735f), "coefficient mismatch");
static_assert(Debug::almost(A[4], 0.273137f), "coefficient mismatch");
static_assert(Debug::almost(A[5], -0.273137f), "coefficient mismatch");
static_assert(Debug::almost(A[6], 0.078848f), "coefficient mismatch");
static_assert(Debug::almost(A[7], -0.273137f), "coefficient mismatch");
static_assert(Debug::almost(A[8], 0.136569f), "coefficient mismatch");
float3 irradiance[9];
bands = std::min(bands, uint8_t(3));
for (size_t i = 0, c = bands * bands; i<c; ++i) {
irradiance[i] = sh[i] * A[i];
}
return this->irradiance(bands, irradiance);
}