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GPUImageHueFilter.m
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GPUImageHueFilter.m
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#import "GPUImageHueFilter.h"
// Adapted from http://stackoverflow.com/questions/9234724/how-to-change-hue-of-a-texture-with-glsl - see for code and discussion
NSString *const kGPUImageHueFragmentShaderString = SHADER_STRING
(
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;
}
);
@implementation GPUImageHueFilter
@synthesize hue;
- (id)init
{
if(! (self = [super initWithFragmentShaderFromString:kGPUImageHueFragmentShaderString]) )
{
return nil;
}
hueAdjustUniform = [filterProgram uniformIndex:@"hueAdjust"];
self.hue = 90;
return self;
}
- (void)setHue:(CGFloat)newHue
{
// Convert degrees to radians for hue rotation
hue = fmodf(newHue, 360.0) * M_PI/180;
[self setFloat:hue forUniform:hueAdjustUniform program:filterProgram];
}
@end