Send shader compile log to stderr instead of stdout.

[movit] / gamma_compression_effect.cpp

#include <assert.h>

#include "effect_util.h"
#include "gamma_compression_effect.h"
#include "util.h"

using namespace std;

GammaCompressionEffect::GammaCompressionEffect()
        : destination_curve(GAMMA_LINEAR)
{
        register_int("destination_curve", (int *)&destination_curve);
}

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);
}

void GammaCompressionEffect::set_gl_state(GLuint glsl_program_num, const string &prefix, unsigned *sampler_num)
{
        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
                set_uniform_float(glsl_program_num, prefix, "linear_scale", 12.92);
                set_uniform_float(glsl_program_num, prefix, "c0", -0.03679675939);
                set_uniform_float(glsl_program_num, prefix, "c1", 1.443803073);
                set_uniform_float(glsl_program_num, prefix, "c2", -0.9239780987);
                set_uniform_float(glsl_program_num, prefix, "c3", 0.8060491596);
                set_uniform_float(glsl_program_num, prefix, "c4", -0.2891558568);
                set_uniform_float(glsl_program_num, prefix, "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
                set_uniform_float(glsl_program_num, prefix, "linear_scale", 4.5);
                set_uniform_float(glsl_program_num, prefix, "c0", -0.08541688528);
                set_uniform_float(glsl_program_num, prefix, "c1", 1.292793370);
                set_uniform_float(glsl_program_num, prefix, "c2", -0.4070417645);
                set_uniform_float(glsl_program_num, prefix, "c3", 0.2923891828);
                set_uniform_float(glsl_program_num, prefix, "c4", -0.09273699351);
                set_uniform_float(glsl_program_num, prefix, "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.)
                set_uniform_float(glsl_program_num, prefix, "linear_scale", 4.5);
                set_uniform_float(glsl_program_num, prefix, "c0", -0.08569685663);
                set_uniform_float(glsl_program_num, prefix, "c1", 1.293000900);
                set_uniform_float(glsl_program_num, prefix, "c2", -0.4067291321);
                set_uniform_float(glsl_program_num, prefix, "c3", 0.2919741179);
                set_uniform_float(glsl_program_num, prefix, "c4", -0.09256205770);
                set_uniform_float(glsl_program_num, prefix, "beta", 0.0181);
        }
}