From this I conclude that when rendering geometry which is all seams or mostly seams, when using GL_TRIANGLE_STRIP or _FAN, then I should never use indexed arrays, and should instead always use gl[Multi]DrawArrays.
No, and the reason is quite simple.
Your conclusion is based on the fact you have analysed a single quad composed by two triangles. These two triangles drawn using triangle fan/strip cannot be simplified using indexed arrays.
But try to think about a large terrain geometry. Each terrain block is drawn as a quad, using triangle fan/strip primitive. For example:
Each triangle strip in the figure has in common all vertices with adjacent triangle strips, and using indices allow to compress the geometry definition, instead of repeating vertices for each triangle strip.
Basically, drawing primitives (triangles, fans and strips) using indices are usefull whenever you can share most of vertices of a single primitive with another one.
Sharing the information allow to save information transmission bandwidth, but it is not the only advantage. Actually indexed arrays allow:
- Avoid the synchronization of the information belonging to the same “conceptual” vertex, specified many times
- Allow to perform the same shader operation on a single vertex instead executing many times, one for each vertex duplication.
- Furthermore, combining the use of triangle strips/fans and indices allow the application to compress the indices buffer, since the strip/fan specification requires less indices (a triangle requires always 3 indices for each face).
Indexed array cannot be used, as you have specified, whenever a vertex cannot share every information associated with it (color, texture coords and so on) with another coincident vertex.
Just for the sake of completeness, the size of the information necessary for the geometry specification is not the only factor wich determine the optimal rendering operation.
Infact, another fundamental factor for primitive rendering is the cache localization of the data. Badly specified geometry data (non interleaved buffer objects, long triangle strips…) causes a lot of cache misses, degrading graphic card performance.
In order to optimize the rendering operation, the vertex specification shall be reordered in a way to reuse previously specified vertices, with the most probability. In such way, the graphic card cache line can reuse previously specified vertices without fetching them from memory.