// In geometry shaders
out int gl_ViewportIndex ;
// In fragment shaders
in int gl_ViewportIndex ;
In the geometry language, gl_ViewportIndex is used to specify the index of the viewport to which the next primitive emitted from the shader should be drawn. Primitives generated by the geometry shader will undergo viewport transformation and scissor testing using the viewport transformation and scissor rectangle selected by the value of gl_ViewportIndex. The viewport index used will come from one of the vertices in the primitive being shaded. However, which vertex the viewport index comes from is implementation-defined, and so it is recommended to use the same viewport index for all vertices of a primitive. If a geometry shader does not assign a value to gl_ViewportIndex, viewport transform and scissor rectangle zero will be used. If a geometry shader statically assigns a value to gl_ViewportIndex and there is a path through the shader that does not assign a value to gl_ViewportIndex, the value of gl_ViewportIndex is undefined for executions of the shader that take that path.
In the fragment language, gl_ViewportIndex will have the same value that was written to the output variable gl_ViewportIndex in the geometry stage. If the geometry stage does not dynamically assign to gl_ViewportIndex, the value of gl_ViewportIndex in the fragment shader will be undefined. If the geometry stage makes no static assignment to gl_ViewportIndex, the fragment stage will read zero. Otherwise, the fragment stage will read the same value written by the geometry stage, even if that value is out of range. If a fragment shader contains a static access to gl_ViewportIndex, it will count against the implementation defined limit for the maximum number of inputs to the fragment stage.
OpenGL Shading Language Version
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|gl_ViewportIndex (fragment stage)||-||-||-||-||-||-||-||-||-||✔||✔||
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