OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Dual-Source Blending

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Two of the blend factors already described in this chapters are the second source blending factors and are special in that they are driven by a second output in the fragment shader. These factors, GL_SRC1_COLOR and GL_SRC1_ALPHA, are produced in the fragment shader by writing to an output whose index is 1 (rather than the default 0). To create such an output we use the index layout qualifier when declaring it in the fragment shader. Example 4.13 shows an example of such a declaration.

Example 4.13. Layout Qualifiers Specifying the Index of Fragment Shader Outputs

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layout (location = 0, index = 0) out vec4 first_output;layout (location = 0, index = 1) out vec4 second_output;

When calling ...