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Fix bug in MED mapper protection bit implementation
author Michael Pavone <pavone@retrodev.com>
date Sun, 28 Apr 2024 23:33:11 -0700
parents 9f3008f91bec
children
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1 //******************************************************************************
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2 //***************** FOR PARAMETERS TO ADJUST SEE BELOW THIS! *******************
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3 //******************************************************************************
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4 // NTSC composite simulator for BlastEm, now with comb filter
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5 // Shader by Sik, based on BlastEm's default shader
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6 //
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7 // Now with gamma correction (NTSC = 2.5 gamma, sRGB = 2.2 gamma*)
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8 // *sorta, sRGB isn't exactly a gamma curve, but close enough
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9 //
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10 // It works by converting from RGB to YIQ and then encoding it into NTSC, then
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11 // trying to decode it back. The lossy nature of the encoding process results in
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12 // the rainbow effect. It also accounts for the differences between H40 and H32
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13 // mode as it computes the exact colorburst cycle length.
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14 //
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15 // This shader tries to work around the inability to keep track of previous
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16 // pixels by sampling seven points (in 0.25 colorburst cycle intervals), that
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17 // seems to be enough to give decent filtering (four samples are used for
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18 // low-pass filtering, but we need seven because decoding chroma also requires
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19 // four samples so we're filtering over overlapping samples... just see the
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20 // comments in the I/Q code to understand).
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21 //
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22 // The comb filter works by comparing against the previous scanline (which means
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23 // sampling twice). This is done at the composite signal step, i.e. before the
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24 // filtering to decode back YIQ is performed.
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25 //
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26 // Thanks to Tulio Adriano for helping compare against real hardware on a CRT.
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27 //******************************************************************************
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28
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29 // How strong is the comb filter when reducing crosstalk between Y and IQ.
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30 // 0% (0.0) means no separation at all, 100% (1.0) means complete filtering.
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31 // 80% seems to approximate a model 1, while 90% is closer to a model 2 or 32X.
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32 const mediump float comb_strength = 0.8;
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33
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34 // Gamma of the TV to simulate.
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35 const mediump float gamma_correction = 2.5;
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36
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37 //******************************************************************************
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38
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39 // Parameters coming from BlastEm
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40 uniform mediump float width; // Screen width (depends on video mode)
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41 uniform sampler2D textures[2]; // Raw display for each field
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42 uniform mediump vec2 texsize; // Texture size
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43 varying mediump vec2 texcoord; // Texture coordinate of current pixel
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44 uniform int curfield; // Even or odd field?
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45 uniform int scanlines; // Enable scanlines?
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46
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47 // Converts from RGB to YIQ
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48 mediump vec3 rgba2yiq(vec4 rgba)
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49 {
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50 return vec3(
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51 rgba[0] * 0.3 + rgba[1] * 0.59 + rgba[2] * 0.11,
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52 rgba[0] * 0.599 + rgba[1] * -0.2773 + rgba[2] * -0.3217,
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53 rgba[0] * 0.213 + rgba[1] * -0.5251 + rgba[2] * 0.3121
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54 );
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55 }
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56
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57 // Encodes YIQ into composite
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58 mediump float yiq2raw(vec3 yiq, float phase)
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59 {
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60 return yiq[0] + yiq[1] * sin(phase) + yiq[2] * cos(phase);
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61 }
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62
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63 // Converts from YIQ to RGB
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64 mediump vec4 yiq2rgba(vec3 yiq)
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65 {
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66 return vec4(
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67 yiq[0] + yiq[1] * 0.9469 + yiq[2] * 0.6236,
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68 yiq[0] - yiq[1] * 0.2748 - yiq[2] * 0.6357,
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69 yiq[0] - yiq[1] * 1.1 + yiq[2] * 1.7,
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70 1.0
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71 );
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72 }
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73
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74 void main()
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75 {
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76 // The coordinate of the pixel we're supposed to access
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77 // In interlaced mode, the entire screen is shifted down by half a scanline,
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78 // y_offset is used to account for this.
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79 mediump float y_offset = float(curfield) * -0.5 / texsize.y;
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80 mediump float x = texcoord.x;
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81 mediump float y = texcoord.y + y_offset;
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82
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83 // Horizontal distance of half a colorburst cycle
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84 mediump float factorX = (1.0 / texsize.x) / 170.667 * 0.5 * width;
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85
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86 // sRGB approximates a gamma ramp of 2.2 while NTSC has a gamma of 2.5
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87 // Use this value to do gamma correction of every RGB value
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88 mediump float gamma = gamma_correction / 2.2;
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89
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90 // Where we store the sampled pixels.
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91 // [0] = current pixel
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92 // [1] = 1/4 colorburst cycles earlier
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93 // [2] = 2/4 colorburst cycles earlier
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94 // [3] = 3/4 colorburst cycles earlier
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95 // [4] = 1 colorburst cycle earlier
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96 // [5] = 1 1/4 colorburst cycles earlier
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97 // [6] = 1 2/4 colorburst cycles earlier
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98 mediump float phase[7]; // Colorburst phase (in radians)
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99 mediump float raw_y[7]; // Luma isolated from raw composite signal
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100 mediump float raw_iq[7]; // Chroma isolated from raw composite signal
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101
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102 // Sample all the pixels we're going to use
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103 for (int n = 0; n < 7; n++, x -= factorX * 0.5) {
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104 // Compute colorburst phase at this point
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105 phase[n] = x / factorX * 3.1415926;
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106
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107 // Y coordinate one scanline above
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108 // Apparently GLSL doesn't allow a vec2 with a full expression for
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109 // texture samplers? Whatever, putting it here (also makes the code
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110 // below a bit easier to read I guess?)
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111 mediump float y_above = y - texcoord.y / texsize.y;
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112
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113 // Get the raw composite data for this scanline
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114 mediump float raw1 = yiq2raw(rgba2yiq(
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115 texture2D(textures[curfield], vec2(x, y))
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116 ), phase[n]);
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117
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118 // Get the raw composite data for scanline above
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119 // Note that the colorburst phase is shifted 180°
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120 mediump float raw2 = yiq2raw(rgba2yiq(
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121 texture2D(textures[curfield], vec2(x, y_above))
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122 ), phase[n] + 3.1415926);
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123
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124 // Comb filter: isolate Y and IQ using the above two.
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125 // Luma is obtained by adding the two scanlines, chroma will cancel out
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126 // because chroma will be on opposite phases.
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127 // Chroma is then obtained by cancelling this scanline from the luma
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128 // to reduce the crosstalk. We don't cancel it entirely though since the
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129 // filtering isn't perfect (which is why the rainbow leaks a bit).
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130 raw_y[n] = (raw1 + raw2) * 0.5;
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131 raw_iq[n] = raw1 - (raw1 + raw2) * (comb_strength * 0.5);
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132 }
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133
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134 // Decode Y by averaging over the last whole sampled cycle (effectively
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135 // filtering anything above the colorburst frequency)
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136 mediump float y_mix = (raw_y[0] + raw_y[1] + raw_y[2] + raw_y[3]) * 0.25;
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137
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138 // Decode I and Q (see page below to understand what's going on)
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139 // https://codeandlife.com/2012/10/09/composite-video-decoding-theory-and-practice/
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140 //
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141 // Retrieving I and Q out of the raw signal is done like this
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142 // (use sin for I and cos for Q):
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143 //
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144 // 0.5 * raw[0] * sin(phase[0]) + 0.5 * raw[1] * sin(phase[1]) +
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145 // 0.5 * raw[2] * sin(phase[2]) + 0.5 * raw[3] * sin(phase[3])
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146 //
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147 // i.e. multiply each of the sampled quarter cycles against the reference
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148 // wave and average them (actually double that because for some reason
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149 // that's needed to get the correct scale, hence 0.5 instead of 0.25)
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150 //
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151 // That turns out to be blocky tho, so we opt to filter down the chroma...
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152 // which requires doing the above *four* times if we do it the same way as
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153 // we did for luminance (note that 0.125 = 1/4 of 0.5):
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154 //
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155 // 0.125 * raw[0] * sin(phase[0]) + 0.125 * raw[1] * sin(phase[1]) +
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156 // 0.125 * raw[2] * sin(phase[2]) + 0.125 * raw[3] * sin(phase[3]) +
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157 // 0.125 * raw[1] * sin(phase[1]) + 0.125 * raw[2] * sin(phase[2]) +
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158 // 0.125 * raw[3] * sin(phase[3]) + 0.125 * raw[4] * sin(phase[4]) +
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159 // 0.125 * raw[2] * sin(phase[2]) + 0.125 * raw[3] * sin(phase[3]) +
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160 // 0.125 * raw[4] * sin(phase[4]) + 0.125 * raw[5] * sin(phase[5]) +
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161 // 0.125 * raw[3] * sin(phase[3]) + 0.125 * raw[4] * sin(phase[4]) +
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162 // 0.125 * raw[5] * sin(phase[5]) + 0.125 * raw[6] * sin(phase[6])
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163 //
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164 // There are a lot of repeated values there that could be merged into one,
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165 // what you see below is the resulting simplification.
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166
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167 mediump float i_mix =
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168 0.125 * raw_iq[0] * sin(phase[0]) +
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169 0.25 * raw_iq[1] * sin(phase[1]) +
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170 0.375 * raw_iq[2] * sin(phase[2]) +
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171 0.5 * raw_iq[3] * sin(phase[3]) +
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172 0.375 * raw_iq[4] * sin(phase[4]) +
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173 0.25 * raw_iq[5] * sin(phase[5]) +
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174 0.125 * raw_iq[6] * sin(phase[6]);
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175
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176 mediump float q_mix =
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177 0.125 * raw_iq[0] * cos(phase[0]) +
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178 0.25 * raw_iq[1] * cos(phase[1]) +
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179 0.375 * raw_iq[2] * cos(phase[2]) +
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180 0.5 * raw_iq[3] * cos(phase[3]) +
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181 0.375 * raw_iq[4] * cos(phase[4]) +
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182 0.25 * raw_iq[5] * cos(phase[5]) +
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183 0.125 * raw_iq[6] * cos(phase[6]);
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184
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185 // Convert YIQ back to RGB and output it
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186 gl_FragColor = pow(yiq2rgba(vec3(y_mix, i_mix, q_mix)),
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187 vec4(gamma, gamma, gamma, 1.0));
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188
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189 // If you're curious to see what the raw composite signal looks like,
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190 // comment out the above and uncomment the line below instead
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191 //gl_FragColor = vec4(raw[0], raw[0], raw[0], 1.0);
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192
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193 // Basic scanlines effect. This is done by multiplying the color against a
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194 // "half sine" wave so the center is brighter and a narrow outer area in
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195 // each scanline is noticeable darker.
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196 // The weird constant in the middle line is 1-sqrt(2)/4 and it's used to
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197 // make sure that the average multiplied value across the whole screen is 1
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198 // to preserve the original brightness.
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199 if (scanlines != 0) {
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200 mediump float mult = sin(y * texsize.y * 3.1415926);
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201 mult = abs(mult) * 0.5 + 0.646446609;
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202 gl_FragColor *= vec4(mult, mult, mult, 1.0);
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203 }
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204 }