precision highp float; varying highp vec2 textureCoordinate; uniform sampler2D inputImageTexture; uniform mediump float hueAdjust; const highp vec4 kRGBToYPrime = vec4 (0.299, 0.587, 0.114, 0.0); const highp vec4 kRGBToI = vec4 (0.595716, -0.274453, -0.321263, 0.0); const highp vec4 kRGBToQ = vec4 (0.211456, -0.522591, 0.31135, 0.0); const highp vec4 kYIQToR = vec4 (1.0, 0.9563, 0.6210, 0.0); const highp vec4 kYIQToG = vec4 (1.0, -0.2721, -0.6474, 0.0); const highp vec4 kYIQToB = vec4 (1.0, -1.1070, 1.7046, 0.0); void main () { // Sample the input pixel highp vec4 color = texture2D(inputImageTexture, textureCoordinate); // Convert to YIQ highp float YPrime = dot (color, kRGBToYPrime); highp float I = dot (color, kRGBToI); highp float Q = dot (color, kRGBToQ); // Calculate the hue and chroma highp float hue = atan (Q, I); highp float chroma = sqrt (I * I + Q * Q); // Make the user's adjustments hue += (-hueAdjust); //why negative rotation? // Convert back to YIQ Q = chroma * sin (hue); I = chroma * cos (hue); // Convert back to RGB highp vec4 yIQ = vec4 (YPrime, I, Q, 0.0); color.r = dot (yIQ, kYIQToR); color.g = dot (yIQ, kYIQToG); color.b = dot (yIQ, kYIQToB); // Save the result gl_FragColor = color; }