Function BlackbodyTemperatureAsRgb

Hydrogent/src/HnLight.cpp:144–182

static float3 BlackbodyTemperatureAsRgb(float temp)
{
    // Catmull-Rom interpolation of kBblackbodyRGB
    constexpr int numKnots = kBblackbodyRGB.size();

    // Parametric distance along spline
    const float u_spline = clamp((temp - kMinBlackbodyTemp) / (kMaxBlackbodyTemp - kMinBlackbodyTemp), 0.0f, 1.0f);

    // Last 4 knots represent a trailing segment starting at u_spline==1.0,
    // to simplify boundary behavior
    constexpr int numSegs = (numKnots - 4);
    const float   x       = u_spline * numSegs;
    const int     seg     = static_cast<int>(std::floor(x));
    const float   u_seg   = x - seg; // Parameter within segment

    // Knot values for this segment
    float3 k0 = kBblackbodyRGB[seg + 0];
    float3 k1 = kBblackbodyRGB[seg + 1];
    float3 k2 = kBblackbodyRGB[seg + 2];
    float3 k3 = kBblackbodyRGB[seg + 3];

    // Compute cubic coefficients.
    float3 a = kCRBasis[0][0] * k0 + kCRBasis[0][1] * k1 + kCRBasis[0][2] * k2 + kCRBasis[0][3] * k3;
    float3 b = kCRBasis[1][0] * k0 + kCRBasis[1][1] * k1 + kCRBasis[1][2] * k2 + kCRBasis[1][3] * k3;
    float3 c = kCRBasis[2][0] * k0 + kCRBasis[2][1] * k1 + kCRBasis[2][2] * k2 + kCRBasis[2][3] * k3;
    float3 d = kCRBasis[3][0] * k0 + kCRBasis[3][1] * k1 + kCRBasis[3][2] * k2 + kCRBasis[3][3] * k3;

    // Eval cubic polynomial.
    float3 rgb = ((a * u_seg + b) * u_seg + c) * u_seg + d;

    // Clamp at zero, since the spline can produce small negative values,
    // e.g. in the blue component at 1300k.
    rgb = std::max(rgb, float3{0, 0, 0});

    // Normalize to the same luminance as (1, 1, 1)
    rgb /= Rec709RgbToLuma(rgb);

    return rgb;
}

static float3 GetLightColor(pxr::HdSceneDelegate& SceneDelegate, const pxr::SdfPath& Id)
{