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

Special Geometry Shader Primitives

Special attention should be paid to the adjacency primitive types available to geometry shaders (lines_adjacency and triangles_adjacency). These primitives have four and six vertices, respectively, and allow adjacency information—information about adjacent primitives or edges—to be passed into the geometry shader. Lines with adjacency information are generated by using the GL_LINES_ADJACENCY or GL_LINE_STRIP_ADJACENCY primitive mode in a draw command such as glDrawArrays(). Likewise, triangles with adjacency information are produced by using the GL_TRIANGLES_ADJACENCY or GL_TRIANGLE_STRIP_ADJACENCY primitive types. These primitive types can be used without a geometry shader present and will be interpreted as ...