Can glDrawElements be used independently from polygon type?

Question

Is the only solution grouping vertices into separate glDrawElements(GL_TRIANGLES, ...) and glDrawElements(GL_QUADS, ...) draw calls or is there a way of sending data describing no of polygon sides into geometry shader and sorting out type of polygon inside geometry shader? https://i.stack.imgur.com/4Ee4e.jpg What you see as my output console is output of my mesh structure. I have:

vector <float> vert_data;
unsigned int faces_no;
vector <unsigned int> indices_on_face;
vector <unsigned int> indices;

First is basicly exactly what is send to open-gl buffer: coordinates, normals, colors etc. Second one says how many faces are described in this data. Third says number of verticles that are in the polygon. It goes in order. Forth one are indices(glBufferData(GL_ELEMENT_ARRAY_BUFFER, ...)). So basicly I am looking for a way to send third one into geometry shader. I know it is possible to order faces types using flag while importing from assimp but that would loose the faces order. And still wouldnt let me draw everything with single draw call, so I would have to create bunch of draw functions for every type of polygon:( Maybe there would be possible something like: first change every indices_on_face[i] by adding all previous ones to it. Set first verticle number that is drawn inside geometry shader during draw call. Inside geometry shader compare current number of verticle with indices_on_face[i] that would tell when to generate polygon out of vertices. Does gl_VertexID hold number dependent from count of passed vertex, independent from indices? How can I formulate a draw call that will fit it?

Answer 1

No, there is no way to do what you want. A primitive includes the count of vertices for each base primitive, and that doesn't get to change within a rendering command.

Also, a geometry shader would almost certainly slow down your rendering rather than speed it up, particularly compared to the standard solution: just send triangles. Break up any non-triangular polygons into triangles, so you can just send them all in a single draw call. I mean, the hardware is going to do that for you regardless, so you may as well do it yourself.

Most mesh exporters will have an option to do this for you. And in those cases where they don't, Open Asset Importer can do it for you at load time by passing the aiProcess_Triangulate flag to your loading function.