/*
 * File: PremultipliedAlphaTextureShader.frag.txt
 *
 * Shader for drawing of textures where the alpha channel is looked up
 * separately from the RGB channels, i.e. RGB texel lookup uses texture
 * coordinates offset by given optional shift parameters.
 * The shader premultiplies the alpha value to the RGB channels, instead of
 * outputting it to the alpha channel.
 *
 * (c) 2009 by Mario Kleiner, licensed under MIT license.
 *		 
 */

#extension GL_ARB_texture_rectangle : enable

uniform sampler2DRect Image;
uniform vec4 Offset;

varying vec4 baseColor;

void main()
{
    /* Read alpha channel value from standard texture coordinate set: */
    float alpha = texture2DRect(Image, gl_TexCoord[0].st).a;

    /* Read RGB color values from secondary texture coordinate set: */
    vec3 rgb    = texture2DRect(Image, gl_TexCoord[1].st).rgb;

    /* Assemble back into RGBA quadruple with premultiplied alpha: */
    vec4 rgba   = vec4(rgb * alpha, 1.0);

    /* ... and output to framebuffer, with proper modulation and offset applied: */
    gl_FragColor = (baseColor * rgba) + Offset;
}
