3 #include "effect_util.h"
4 #include "gamma_compression_effect.h"
11 GammaCompressionEffect::GammaCompressionEffect()
12 : destination_curve(GAMMA_LINEAR)
14 register_int("destination_curve", (int *)&destination_curve);
15 register_uniform_float("linear_scale", &uniform_linear_scale);
16 register_uniform_float("c0", &uniform_c0);
17 register_uniform_float("c1", &uniform_c1);
18 register_uniform_float("c2", &uniform_c2);
19 register_uniform_float("c3", &uniform_c3);
20 register_uniform_float("c4", &uniform_c4);
21 register_uniform_float("beta", &uniform_beta);
24 string GammaCompressionEffect::output_fragment_shader()
26 if (destination_curve == GAMMA_LINEAR) {
27 return read_file("identity.frag");
29 if (destination_curve == GAMMA_sRGB ||
30 destination_curve == GAMMA_REC_709 || // Also includes Rec. 601, and 10-bit Rec. 2020.
31 destination_curve == GAMMA_REC_2020_12_BIT) {
32 return read_file("gamma_compression_effect.frag");
37 void GammaCompressionEffect::set_gl_state(GLuint glsl_program_num, const string &prefix, unsigned *sampler_num)
39 Effect::set_gl_state(glsl_program_num, prefix, sampler_num);
41 // See GammaExpansionEffect for more details about the approximations in use;
42 // we will primarily deal with the differences here.
44 // Like in expansion, we have a piecewise curve that for very low values
45 // (up to some β) are linear. Above β, we have a power curve that looks
48 // y = ɑ x^ɣ - (ɑ - 1)
50 // Like in expansion, we want to approximate this by some minimax polynomial
51 // in the range β..1. However, in this case, ɣ is typically around 0.4, and
52 // x^0.4 is actually very hard to approximate accurately in this range.
53 // We do a little trick by instead asking for a polynomial of s=sqrt(x),
54 // which means we instead need something like s^0.8, which is much easier.
55 // This warps the input space a bit as seen by the minimax algorithm,
56 // but since we are optimizing for _maximum_ error and not _average_,
57 // we should not add any extra weighting factors.
59 // However, since we have problems reaching the desired accuracy (~25%
60 // of a pixel level), especially for sRGB, we modify w(x) from
61 // GammaExpansionEffect to remove the special handling of the area
62 // around β; it is not really as useful when the next step is just a
63 // dither and round anyway. We keep it around 1, though, since that
64 // seems to hurt less.
66 // The Maple commands this time around become (again using sRGB as an example):
69 // > beta := 0.0031308;
70 // > gamma_ := 1.0/2.4;
71 // > w := x -> piecewise(x > 0.999, 10, 1);
72 // > numapprox[minimax](alpha * (x^2)^gamma_ - (alpha - 1), x=sqrt(beta)..1, [4,0], w(x^2), 'maxerror');
74 // Since the error here is possible to interpret on a uniform scale,
75 // we also show it as a value relative to a 8-, 10- or 12-bit pixel level,
78 if (destination_curve == GAMMA_sRGB) {
79 // From the Wikipedia article on sRGB; ɑ (called a+1 there) = 1.055,
80 // β = 0.0031308, ɣ = 1/2.4.
81 // maxerror = 0.000785 = 0.200 * 255
82 // error at 1.0 = 0.000078 = 0.020 * 255
83 uniform_linear_scale = 12.92;
84 uniform_c0 = -0.03679675939;
85 uniform_c1 = 1.443803073;
86 uniform_c2 = -0.9239780987;
87 uniform_c3 = 0.8060491596;
88 uniform_c4 = -0.2891558568;
89 uniform_beta = 0.0031308;
91 if (destination_curve == GAMMA_REC_709) { // Also includes Rec. 601, and 10-bit Rec. 2020.
92 // Rec. 2020, page 3; ɑ = 1.099, β = 0.018, ɣ = 0.45.
93 // maxerror = 0.000131 = 0.033 * 255 = 0.134 * 1023
94 // error at 1.0 = 0.000013 = 0.003 * 255 = 0.013 * 1023
95 uniform_linear_scale = 4.5;
96 uniform_c0 = -0.08541688528;
97 uniform_c1 = 1.292793370;
98 uniform_c2 = -0.4070417645;
99 uniform_c3 = 0.2923891828;
100 uniform_c4 = -0.09273699351;
101 uniform_beta = 0.018;
103 if (destination_curve == GAMMA_REC_2020_12_BIT) {
104 // Rec. 2020, page 3; ɑ = 1.0993, β = 0.0181, ɣ = 0.45.
105 // maxerror = 0.000130 = 0.533 * 4095
106 // error at 1.0 = 0.000013 = 0.053 * 4095
108 // Note that this error is above one half of a pixel level,
109 // which means that a few values will actually be off in the lowest
110 // bit. (Removing the constraint for x=1 will only take this down
111 // from 0.553 to 0.501; adding a fifth order can get it down to
112 // 0.167, although this assumes working in fp64 and not fp32.)
113 uniform_linear_scale = 4.5;
114 uniform_c0 = -0.08569685663;
115 uniform_c1 = 1.293000900;
116 uniform_c2 = -0.4067291321;
117 uniform_c3 = 0.2919741179;
118 uniform_c4 = -0.09256205770;
119 uniform_beta = 0.0181;