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// GLSL debanding shader, use as: source-shader=path/to/deband.glsl // (Loosely based on flash3kyuu_deband, but expanded to multiple iterations)
//------------ Configuration section ------------ // The threshold of difference below which a pixel is considered to be part of // a gradient. Higher = more debanding, but setting it too high diminishes image // details. #define THRESHOLD 64
// The range (in source pixels) at which to sample for neighbours. Higher values // will find more gradients, but lower values will deband more aggressively. #define RANGE 8
// The number of debanding iterations to perform. Each iteration samples from // random positions, so increasing the number of iterations is likely to // increase the debanding quality. Conversely, it slows the shader down. // (Each iteration will use a multiple of the configured RANGE, and a // successively lower THRESHOLD - so setting it much higher has little effect) #define ITERATIONS 4
// (Optional) Add some extra noise to the image. This significantly helps cover // up remaining banding and blocking artifacts, at comparatively little visual // quality. Higher = more grain. Setting it to 0 disables the effect. #define GRAIN 48
// Note: If performance is too slow, try eg. RANGE=16 ITERATIONS=2. In general, // an increase in the number of ITERATIONS should roughly correspond to a // decrease in RANGE and perhaps an increase in THRESHOLD. //------------ End of configuration ------------
// Wide usage friendly PRNG, shamelessly stolen from a GLSL tricks forum post float mod289(float x) { return x - floor(x / 289.0) * 289.0; } float permute(float x) { return mod289((34.0*x + 1.0) * x); } float rand(float x) { return fract(x / 41.0); }
// Helper: Calculate a stochastic approximation of the avg color around a pixel vec4 average(sampler2D tex, vec2 pos, float range, inout float h) { // Compute a random rangle and distance float dist = rand(h) * range; h = permute(h); float dir = rand(h) * 6.2831853; h = permute(h);
vec2 pt = dist / image_size; vec2 o = vec2(cos(dir), sin(dir));
// Sample at quarter-turn intervals around the source pixel vec4 ref[4]; ref[0] = texture(tex, pos + pt * vec2( o.x, o.y)); ref[1] = texture(tex, pos + pt * vec2(-o.y, o.x)); ref[2] = texture(tex, pos + pt * vec2(-o.x, -o.y)); ref[3] = texture(tex, pos + pt * vec2( o.y, -o.x));
// Return the (normalized) average return cmul*(ref[0] + ref[1] + ref[2] + ref[3])/4.0; }
vec4 sample(sampler2D tex, vec2 pos, vec2 tex_size) { float h; // Initialize the PRNG by hashing the position + a random uniform vec3 m = vec3(pos, random) + vec3(1.0); h = permute(permute(permute(m.x)+m.y)+m.z);
// Sample the source pixel vec4 col = cmul*texture(tex, pos);
for (int i = 1; i <= ITERATIONS; i++) { // Use the average instead if the difference is below the threshold vec4 avg = average(tex, pos, i*RANGE, h); vec4 diff = abs(col - avg); col = mix(avg, col, greaterThan(diff, vec4(THRESHOLD/(i*16384.0)))); }
// Add some random noise to the output vec3 noise; noise.x = rand(h); h = permute(h); noise.y = rand(h); h = permute(h); noise.z = rand(h); h = permute(h); col.rgb += (GRAIN/8192.0) * (noise - vec3(0.5));
return col; }
// vim: set ft=glsl:
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