#include <assert.h>
+#include "effect_util.h"
#include "gamma_compression_effect.h"
#include "util.h"
+using namespace std;
+
+namespace movit {
+
GammaCompressionEffect::GammaCompressionEffect()
: destination_curve(GAMMA_LINEAR)
{
register_int("destination_curve", (int *)&destination_curve);
+ register_uniform_float("linear_scale", &uniform_linear_scale);
+ register_uniform_float_array("c", uniform_c, 5);
+ register_uniform_float("beta", &uniform_beta);
+}
+
+string GammaCompressionEffect::output_fragment_shader()
+{
+ if (destination_curve == GAMMA_LINEAR) {
+ return read_file("identity.frag");
+ }
+ if (destination_curve == GAMMA_sRGB ||
+ destination_curve == GAMMA_REC_709 || // Also includes Rec. 601, and 10-bit Rec. 2020.
+ destination_curve == GAMMA_REC_2020_12_BIT) {
+ return read_file("gamma_compression_effect.frag");
+ }
+ assert(false);
}
-std::string GammaCompressionEffect::output_fragment_shader()
+void GammaCompressionEffect::set_gl_state(GLuint glsl_program_num, const string &prefix, unsigned *sampler_num)
{
- switch (destination_curve) {
- case GAMMA_LINEAR:
- return read_file("identity.glsl");
- case GAMMA_sRGB:
- return read_file("gamma_compression_effect_srgb.glsl");
- case GAMMA_REC_709: // and GAMMA_REC_601
- return read_file("gamma_compression_effect_rec709.glsl");
- default:
- assert(false);
+ Effect::set_gl_state(glsl_program_num, prefix, sampler_num);
+
+ // See GammaExpansionEffect for more details about the approximations in use;
+ // we will primarily deal with the differences here.
+ //
+ // Like in expansion, we have a piecewise curve that for very low values
+ // (up to some β) are linear. Above β, we have a power curve that looks
+ // like this:
+ //
+ // y = ɑ x^ɣ - (ɑ - 1)
+ //
+ // Like in expansion, we want to approximate this by some minimax polynomial
+ // in the range β..1. However, in this case, ɣ is typically around 0.4, and
+ // x^0.4 is actually very hard to approximate accurately in this range.
+ // We do a little trick by instead asking for a polynomial of s=sqrt(x),
+ // which means we instead need something like s^0.8, which is much easier.
+ // This warps the input space a bit as seen by the minimax algorithm,
+ // but since we are optimizing for _maximum_ error and not _average_,
+ // we should not add any extra weighting factors.
+ //
+ // However, since we have problems reaching the desired accuracy (~25%
+ // of a pixel level), especially for sRGB, we modify w(x) from
+ // GammaExpansionEffect to remove the special handling of the area
+ // around β; it is not really as useful when the next step is just a
+ // dither and round anyway. We keep it around 1, though, since that
+ // seems to hurt less.
+ //
+ // The Maple commands this time around become (again using sRGB as an example):
+ //
+ // > alpha := 1.055;
+ // > beta := 0.0031308;
+ // > gamma_ := 1.0/2.4;
+ // > w := x -> piecewise(x > 0.999, 10, 1);
+ // > numapprox[minimax](alpha * (x^2)^gamma_ - (alpha - 1), x=sqrt(beta)..1, [4,0], w(x^2), 'maxerror');
+ //
+ // Since the error here is possible to interpret on a uniform scale,
+ // we also show it as a value relative to a 8-, 10- or 12-bit pixel level,
+ // as appropriate.
+
+ if (destination_curve == GAMMA_sRGB) {
+ // From the Wikipedia article on sRGB; ɑ (called a+1 there) = 1.055,
+ // β = 0.0031308, ɣ = 1/2.4.
+ // maxerror = 0.000785 = 0.200 * 255
+ // error at 1.0 = 0.000078 = 0.020 * 255
+ uniform_linear_scale = 12.92;
+ uniform_c[0] = -0.03679675939;
+ uniform_c[1] = 1.443803073;
+ uniform_c[2] = -0.9239780987;
+ uniform_c[3] = 0.8060491596;
+ uniform_c[4] = -0.2891558568;
+ uniform_beta = 0.0031308;
+ }
+ if (destination_curve == GAMMA_REC_709) { // Also includes Rec. 601, and 10-bit Rec. 2020.
+ // Rec. 2020, page 3; ɑ = 1.099, β = 0.018, ɣ = 0.45.
+ // maxerror = 0.000131 = 0.033 * 255 = 0.134 * 1023
+ // error at 1.0 = 0.000013 = 0.003 * 255 = 0.013 * 1023
+ uniform_linear_scale = 4.5;
+ uniform_c[0] = -0.08541688528;
+ uniform_c[1] = 1.292793370;
+ uniform_c[2] = -0.4070417645;
+ uniform_c[3] = 0.2923891828;
+ uniform_c[4] = -0.09273699351;
+ uniform_beta = 0.018;
+ }
+ if (destination_curve == GAMMA_REC_2020_12_BIT) {
+ // Rec. 2020, page 3; ɑ = 1.0993, β = 0.0181, ɣ = 0.45.
+ // maxerror = 0.000130 = 0.533 * 4095
+ // error at 1.0 = 0.000013 = 0.053 * 4095
+ //
+ // Note that this error is above one half of a pixel level,
+ // which means that a few values will actually be off in the lowest
+ // bit. (Removing the constraint for x=1 will only take this down
+ // from 0.553 to 0.501; adding a fifth order can get it down to
+ // 0.167, although this assumes working in fp64 and not fp32.)
+ uniform_linear_scale = 4.5;
+ uniform_c[0] = -0.08569685663;
+ uniform_c[1] = 1.293000900;
+ uniform_c[2] = -0.4067291321;
+ uniform_c[3] = 0.2919741179;
+ uniform_c[4] = -0.09256205770;
+ uniform_beta = 0.0181;
}
}
+
+} // namespace movit