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

Geometry Shader Inputs and Outputs

The inputs and outputs of the geometry shader are specified using layout qualifiers and the in and out keywords in GLSL. In addition to user-defined inputs and outputs, there are several built-in inputs and outputs that are specific to geometry shaders. These are described in some detail in the following subsections. The in and out keywords are also used in conjunction with layout qualifiers to specify how the geometry shader fits into the pipeline, how it behaves, and how it interacts with adjacent shader stages.