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annotate shaders/ntsc.f.glsl @ 2335:c05b7c5e6f11

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author Michael Pavone <pavone@retrodev.com>
date Sat, 26 Aug 2023 20:32:53 -0700
parents 06d5e9b08bdb
children 49bd818ec9d8
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2329
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1 //******************************************************************************
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2 // NTSC composite simulator for BlastEm
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3 // Shader by Sik, based on BlastEm's default shader
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4 //
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5 // It works by converting from RGB to YIQ and then encoding it into NTSC, then
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6 // trying to decode it back. The lossy nature of the encoding process results in
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7 // the rainbow effect. It also accounts for the differences between H40 and H32
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8 // mode as it computes the exact colorburst cycle length.
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9 //
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10 // This shader tries to work around the inability to keep track of previous
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11 // pixels by sampling seven points (in 0.25 colorburst cycle intervals), that
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12 // seems to be enough to give decent filtering (four samples are used for
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13 // low-pass filtering, but we need seven because decoding chroma also requires
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14 // four samples so we're filtering over overlapping samples... just see the
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15 // comments in the I/Q code to understand).
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16 //******************************************************************************
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17
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18 uniform mediump float width;
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19 uniform sampler2D textures[2];
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20 uniform mediump vec2 texsize;
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21 varying mediump vec2 texcoord;
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22
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23 // Converts from RGB to YIQ
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24 mediump vec3 rgba2yiq(vec4 rgba)
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25 {
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26 return vec3(
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27 rgba[0] * 0.3 + rgba[1] * 0.59 + rgba[2] * 0.11,
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28 rgba[0] * 0.599 + rgba[1] * -0.2773 + rgba[2] * -0.3217,
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29 rgba[0] * 0.213 + rgba[1] * -0.5251 + rgba[2] * 0.3121
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30 );
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31 }
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32
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33 // Encodes YIQ into composite
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34 mediump float yiq2raw(vec3 yiq, float phase)
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35 {
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36 return yiq[0] + yiq[1] * sin(phase) + yiq[2] * cos(phase);
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37 }
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38
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39 // Converts from YIQ to RGB
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40 mediump vec4 yiq2rgba(vec3 yiq)
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41 {
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42 return vec4(
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43 yiq[0] + yiq[1] * 0.9469 + yiq[2] * 0.6236,
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44 yiq[0] - yiq[1] * 0.2748 - yiq[2] * 0.6357,
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45 yiq[0] - yiq[1] * 1.1 + yiq[2] * 1.7,
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46 1.0
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47 );
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48 }
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49
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50 void main()
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51 {
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52 // Use first pair of lines for hard line edges
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53 // Use second pair of lines for soft line edges
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54 mediump float modifiedY0 = (floor(texcoord.y * texsize.y + 0.25) + 0.5) / texsize.y;
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55 mediump float modifiedY1 = (floor(texcoord.y * texsize.y - 0.25) + 0.5) / texsize.y;
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56 //mediump float modifiedY0 = (texcoord.y * texsize.y + 0.75) / texsize.y;
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57 //mediump float modifiedY1 = (texcoord.y * texsize.y + 0.25) / texsize.y;
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58
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59 // Used by the mixing when fetching texels, related to the way BlastEm
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60 // handles interlaced mode (nothing to do with composite)
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61 mediump float factorY = (sin(texcoord.y * texsize.y * 6.283185307) + 1.0) * 0.5;
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62
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63 // Horizontal distance of half a colorburst cycle
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64 mediump float factorX = (1.0 / texsize.x) / 170.667 * 0.5 * (width - 27.0);
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65
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66 // Where we store the sampled pixels.
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67 // [0] = current pixel
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68 // [1] = 1/4 colorburst cycles earlier
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69 // [2] = 2/4 colorburst cycles earlier
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70 // [3] = 3/4 colorburst cycles earlier
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71 // [4] = 1 colorburst cycle earlier
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72 // [5] = 1 1/4 colorburst cycles earlier
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73 // [6] = 1 2/4 colorburst cycles earlier
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74 mediump float phase[7]; // Colorburst phase (in radians)
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75 mediump float raw[7]; // Raw encoded composite signal
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76
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77 // Sample all the pixels we're going to use
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78 mediump float x = texcoord.x;
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79 for (int n = 0; n < 7; n++, x -= factorX * 0.5) {
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80 // Compute colorburst phase at this point
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81 phase[n] = x / factorX * 3.1415926;
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82
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83 // Decode RGB into YIQ and then into composite
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84 // Reading two textures is a BlastEm thing :P (the two fields in
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85 // interlaced mode, that's taken as-is from the stock shaders)
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86 raw[n] = yiq2raw(mix(
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87 rgba2yiq(texture2D(textures[1], vec2(x, modifiedY1))),
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88 rgba2yiq(texture2D(textures[0], vec2(x, modifiedY0))),
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89 factorY
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90 ), phase[n]);
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91 }
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92
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93 // Decode Y by averaging over the the whole sampled cycle (effectively
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94 // filtering anything above the colorburst frequency)
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95 mediump float y_mix = (raw[0] + raw[1] + raw[2] + raw[3]) * 0.25;
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96
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97 // Decode I and Q (see page below to understand what's going on)
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98 // https://codeandlife.com/2012/10/09/composite-video-decoding-theory-and-practice/
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99 //
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100 // Retrieving I and Q out of the raw signal is done like this
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101 // (use sin for I and cos for Q):
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102 //
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103 // 0.5 * raw[0] * sin(phase[0]) + 0.5 * raw[1] * sin(phase[1]) +
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104 // 0.5 * raw[2] * sin(phase[2]) + 0.5 * raw[3] * sin(phase[3])
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105 //
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106 // i.e. multiply each of the sampled quarter cycles against the reference
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107 // wave and average them (actually double that because for some reason
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108 // that's needed to get the correct scale, hence 0.5 instead of 0.25)
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109 //
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110 // That turns out to be blocky tho, so we opt to filter down the chroma...
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111 // which requires doing the above *four* times if we do it the same way as
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112 // we did for luminance (note that 0.125 = 1/4 of 0.5):
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113 //
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114 // 0.125 * raw[0] * sin(phase[0]) + 0.125 * raw[1] * sin(phase[1]) +
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115 // 0.125 * raw[2] * sin(phase[2]) + 0.125 * raw[3] * sin(phase[3]) +
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116 // 0.125 * raw[1] * sin(phase[1]) + 0.125 * raw[2] * sin(phase[2]) +
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117 // 0.125 * raw[3] * sin(phase[3]) + 0.125 * raw[4] * sin(phase[4]) +
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118 // 0.125 * raw[2] * sin(phase[2]) + 0.125 * raw[3] * sin(phase[3]) +
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119 // 0.125 * raw[4] * sin(phase[4]) + 0.125 * raw[5] * sin(phase[5]) +
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120 // 0.125 * raw[3] * sin(phase[3]) + 0.125 * raw[4] * sin(phase[4]) +
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121 // 0.125 * raw[5] * sin(phase[5]) + 0.125 * raw[6] * sin(phase[6])
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122 //
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123 // There are a lot of repeated values there that could be merged into one,
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124 // what you see below is the resulting simplification.
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125
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126 mediump float i_mix =
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127 0.125 * raw[0] * sin(phase[0]) +
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128 0.25 * raw[1] * sin(phase[1]) +
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129 0.375 * raw[2] * sin(phase[2]) +
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130 0.5 * raw[3] * sin(phase[3]) +
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131 0.375 * raw[4] * sin(phase[4]) +
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132 0.25 * raw[5] * sin(phase[5]) +
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133 0.125 * raw[6] * sin(phase[6]);
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134
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135 mediump float q_mix =
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136 0.125 * raw[0] * cos(phase[0]) +
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137 0.25 * raw[1] * cos(phase[1]) +
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138 0.375 * raw[2] * cos(phase[2]) +
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139 0.5 * raw[3] * cos(phase[3]) +
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140 0.375 * raw[4] * cos(phase[4]) +
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141 0.25 * raw[5] * cos(phase[5]) +
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142 0.125 * raw[6] * cos(phase[6]);
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143
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144 // Convert YIQ back to RGB and output it
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145 gl_FragColor = yiq2rgba(vec3(y_mix, i_mix, q_mix));
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146
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147 // If you're curious to see what the raw composite signal looks like,
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148 // comment out the above and uncomment the line below instead
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149 //gl_FragColor = vec4(raw[0], raw[0], raw[0], 1.0);
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150 }