#extension GL_ARB_texture_rectangle : enable uniform sampler2DRect MyTex; uniform sampler2DRect MyTex1; varying vec2 texcoord1; varying vec2 texcoord2; void main (void) { vec4 color = texture2DRect(MyTex, texcoord1); vec4 color2 = texture2DRect(MyTex1, texcoord1); // texcoord2 does not work. gl_FragColor = (color + color2) / 2.; }