OpenGL ES and overlapping triangles with VBO

Question

Some background:

I am very new to OGL. My application concerns itself with 2D only. All objects are normal to the viewing direction, and I use orthographic projection. I find that the performance of the system is limited by the number of draw* calls indicating that I need to batch more.

There is only one object that I need to draw, but it consists of thousands of triangles that potentially overlap. I have the ability to pre-compute geometry in my particular application and order the triangles back to front since they have varying degrees of transparency. The vertex attribute consists of the color (only) including alpha that is used in the fragment program.

What I've done:

All the primitives are triangles and I assign the 3 vertices of each triangle the same color since the color is constant across a face. I put all of the vertices, for all triangles, and their colors into a single VBO (16-bit; there aren't that many vertices). The index buffer orders the triangles back to front and I issue a single draw call. I use alpha blending (SRC_ALPHA, ONE_MINUS_SRC_ALPHA).

Result:

I see that the result is correctly blended and rendered on the only machine that I possess and test on. I have not tried it on others. I've searched for quite some time, but in vain, for some definitive answer. BTW, the only reference is in the VBO extension spec where there is a mention of a "sequence of primitives" but it does not address what happens when the primitives overlap.

Question:

Is this the guaranteed behavior? That is will the result be the same as issuing multiple calls within glBegin(...) and glEnd(...) in immediate mode (which is guaranteed by the standard)?

Note: Depth buffer and stencil buffer are turned off.

Clarifying that even though the triangles are normal to the viewing direction all vertices do have a depth, in case that makes a difference — Vivek Kumar
+1 Whereas you say that you are "very new to OGL" you seem to already have a much more solid understanding of many basic principles than the average ES-beginner asking here on SO. — Christian Rau
It is not so much a question of the VBO extension, but just the vertex array interface it builds upon. In the OpenGL 2.0 spec it explains the glDrawArrays and similar calls in terms of behaving like a single glBegin/glEnd block, so one may deduce that the same restrictions hold. But it would be interresting where you got from that the primitives in a single glBegin/glEnd block are rendered in order (or actually arrive at the framebuffer in order)? — Christian Rau
Unfortunately, there isn't a place I can point you to :(. This information came from a colleague who is knowledgeable about the standard but could not answer the question. — Vivek Kumar

Nicol Bolas Nicol Bolas · Accepted Answer · 2012-05-11T11:53:14

It is guaranteed by the OpenGL specification that primitives will be rendered in the order provided. Each primitive pulled from a glDraw* command will be rendered in the order specified by its component vertices.

So yes: if you put the triangles in an order, that's the order you'll get them out when you render them.

OpenGL ES and overlapping triangles with VBO

1 Answers