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/usr/share/psychtoolbox-3/PsychOpenGL/PsychGLSLShaders/WeightedSumOfFourChannelsTextureShader.vert.txt is in psychtoolbox-3-common 3.0.12.20160126.dfsg1-1ubuntu1.

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/* Shader for weighted summing of input texture layers into
 * a scalar output value which is replicated into all four output
 * components.
 *
 * (c) 2014 by Mario Kleiner. Licensed under MIT license.
 */

/* Attributes passed from Screen(): See the ProceduralShadingAPI.m file for infos: */
attribute vec4 modulateColor;

/* Information passed to the fragment shader: */
varying vec4 mixWeights;

void main()
{
    /* Apply standard geometric transformations to patch: */
    gl_Position = ftransform();
    gl_TexCoord[0] = gl_MultiTexCoord0;
    mixWeights = modulateColor;
}