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/usr/share/psychtoolbox-3/PsychOpenGL/PsychGLSLShaders/LinearCombinationOfTwoImagesShader.frag.txt is in psychtoolbox-3-common 3.0.11.20131230.dfsg1-1build1.

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/* LinearCombinationOfTwoImagesShader.frag.txt
 * A fragment shader that computes a pixel-wise weighted linear combination
 * of two input image textures bound to Image1 and Image2:
 * outimage(x,y) = (Image1Weight * Image1(x,y)) + (Image2Weight * Image2(x,y));
 *
 * This one is e.g., used by the new moglmorpher.m for fast shape morphing
 * on the GPU.
 *
 * (c) 2007 by Mario Kleiner, licensed to you under MIT license.
 */

#extension GL_ARB_texture_rectangle : enable

uniform sampler2DRect Image1;
uniform sampler2DRect Image2;
uniform float Image1Weight;
uniform float Image2Weight;

void main()
{
    vec4 img1 = texture2DRect(Image1, gl_TexCoord[0].st) * Image1Weight;
    vec4 img2 = texture2DRect(Image2, gl_TexCoord[0].st) * Image2Weight;
    gl_FragColor = img1 + img2;
}