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

Geometry Shader Functions

These functions are available only in geometry shaders to manage the output data streams created by this stage.

The function EmitStreamVertex specifies that a vertex is completed. A vertex is added to the current output primitive in vertex stream stream using the current values of all output variables associated with stream. These include gl_PointSize, gl_ClipDistance[], gl_Layer, gl_Position, gl_PrimitiveID, and gl_ViewportIndex. The values of all output variables for all output streams are undefined after a call to EmitStreamVertex. If a geometry shader invocation has emitted more vertices than permitted by the output layout qualifier max_vertices, the results of calling EmitStreamVertex are undefined.

The function ...