Using OpenGL instancing for rendering 2D scene with object depths and alpha blending

user_363949 0
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Here's what I'm trying to do: I want to render a 2D scene, consisting of a number of objects (quads), using instancing. Objects with a lower y value (towards the bottom of the screen) need to be rendered in front of the ones with higher y values. And alpha blending also needs to work.

So my first idea was to use the Z value for depth, but I soon realized alpha blending will not work unless the objects are drawn in the right order. But I'm not issuing one call for each quad, but use a single instanced call to render the whole scene. Putting the instance data in the correct sorted order seems to work for me, but I doubt this is something I can rely on, since the GPU is supposed to run those computations in parallel as much as possible.

So the question is, is there a way to make this work? The best thing I can think of right now is to issue an instanced call for each separate y value (and issue those in order, back to front). Is there a better way to do this?

opengl
1 Answers

Instancing is best used for cases where each instance is medium-sized: hundreds or maybe thousands of triangles. Quads are not a good candidate for instancing.

Just build and render a sequence of triangles. There are even ways to efficiently get around the lack of a GL_QUADS primitive type in modern OpenGL.

Putting the instance data in the correct sorted order seems to work for me, but I doubt this is something I can rely on, since the GPU is supposed to run those computations in parallel as much as possible.

That's not how GPUs work.

When you issue a rendering command, what you (eventually) get is a sequence of primitives. Because the vertices that were given to that command are ordered (first to last), and the instances in that command are ordered, and even the draws within a single draw command are ordered, an order can be assigned to every primitive in the draw call with respect to every other primitive based on the order of vertices, instances, and draws.

This defines the primitive order for a drawing command. GPUs guarantee that blending (and logical operations and other visible post-fragment shader operations) will respect the primitive order of a rendering command and between rendering commands. That is, if you draw 2 triangles in a single call, and the first is behind the second (with depth testing turned off), then blending for the second triangle will respect the data written by the first.

Basically, if you give primitives to the GPU in an order, the GPU will respect that order with regard to blending and such.

So again, just build a ordered stream of triangles to represent your quads and render them.

user_7340690
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