HueFilter.java

package com.marshalchen.common.uimodule.imageprocessing.filter.colour;

import com.marshalchen.common.uimodule.imageprocessing.filter.BasicFilter;
import android.opengl.GLES20;

/**
 * A hue adjusting filter implementation of BasicFilter.
 * This filter rotates the hue, in radians, by a set amount.
 * Value should be in [0,6.28]; however, all values should produce acceptable results.
 * @author Chris Batt
 */
public class HueFilter extends BasicFilter {
	private static final String UNIFORM_HUEADJUST = "u_HueAdjust";
	private float hueAdjust;
	private int hueAdjustHandle;
	
	/**
	 * Creates a HueFilter that adjusts the hue of input image by a given number of radians.
	 * @param hueAdjust
	 * The amount the hue should be adjusted in radians.
	 */
	public HueFilter(float hueAdjust) {
		this.hueAdjust = hueAdjust;
	}
	
	// Adapted from http://stackoverflow.com/questions/9234724/how-to-change-hue-of-a-texture-with-glsl - see for code and discussion
	@Override
	protected String getFragmentShader() {
		return 
				 "precision mediump float;\n" 
				+"uniform sampler2D "+UNIFORM_TEXTURE0+";\n"  
				+"varying vec2 "+VARYING_TEXCOORD+";\n"	
				+"uniform float "+UNIFORM_HUEADJUST+";\n"
				+"const vec4  kRGBToYPrime = vec4 (0.299, 0.587, 0.114, 0.0);\n"
				+"const vec4  kRGBToI = vec4 (0.595716, -0.274453, -0.321263, 0.0);\n"
				+"const vec4  kRGBToQ = vec4 (0.211456, -0.522591, 0.31135, 0.0);\n"
				+"const vec4  kYIQToR = vec4 (1.0, 0.9563, 0.6210, 0.0);\n"
				+"const vec4  kYIQToG = vec4 (1.0, -0.2721, -0.6474, 0.0);\n"
				+"const vec4  kYIQToB = vec4 (1.0, -1.1070, 1.7046, 0.0);\n"
				
		  		+"void main() {\n"
		  		+"   vec4 color = texture2D("+UNIFORM_TEXTURE0+","+VARYING_TEXCOORD+");\n"
		  		+"   float YPrime = dot(color, kRGBToYPrime);\n"
		  		+"   float I = dot(color, kRGBToI);\n"
		  		+"   float Q = dot(color, kRGBToQ);\n"
		  		+"   float hue = atan(Q, I);\n"
				+"   float chroma = sqrt(I * I + Q * Q);\n"
		  		+"   hue += (-"+UNIFORM_HUEADJUST+");\n"
		  		+"   Q = chroma * sin(hue);"
				+"   I = chroma * cos(hue);" 
		  		+"   vec4 yIQ = vec4(YPrime, I, Q, 0.0);\n"
		  		+"   color.r = dot(yIQ, kYIQToR);\n"
		  		+"   color.g = dot(yIQ, kYIQToG);\n"
		  		+"   color.b = dot(yIQ, kYIQToB);\n"
				+"   gl_FragColor = color;\n"
		  		+"}\n";
	}
	
	@Override
	protected void initShaderHandles() {
		super.initShaderHandles();
		hueAdjustHandle = GLES20.glGetUniformLocation(programHandle, UNIFORM_HUEADJUST);
	} 
	
	@Override
	protected void passShaderValues() {
		super.passShaderValues();
		GLES20.glUniform1f(hueAdjustHandle, hueAdjust);
	}
}