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

Triangles, Strips, and Fans

Triangles are made up of collections of three vertices. When separate triangles are rendered, each triangle is independent of all others. A triangle is rendered by projecting each of the three vertices into screen space and forming three edges running between the edges. A sample is considered covered if it lies on the positive side of all of the half spaces formed by the lines between the vertices. If two triangles share an edge (and therefore a pair of vertices), no single sample can be considered inside both triangles. This is important because, although some variation in rasterization algorithm is allowed by the OpenGL specification, the rules governing pixels that lie along a shared edge are quite strict:

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