GLScissors: what is faster/better?

Question

I'm having a small dilemma.

I'm working with Android (not that it is relevant) and I noticed one thing on some phones and not others: the scissor prevents the call glClear(GL_COLOR_BUFFER_BIT) from working correctly.

Therefore I'm wondering if it is better to do:

gl.glDisable(GL10.GL_SCISSOR_TEST);
gl.glClear(GL10.GL_COLOR_BUFFER_BIT);
gl.glEnable(GL10.GL_SCISSOR_TEST);

or

gl.glScissors(0,0,800,480);
gl.glClear(GL10.GL_COLOR_BUFFER_BIT);

Basically, is it better to change to scissors test zone before the clear or is it better to disable/enable?

Answer 1

This is the specification of glScissors , since a glClear is considered in OpenGL as a drawing command. So the behavior you see is perfectly normal (D3D works similarly) but buggy on the other phones where funnily it seems for you to work!

About which solution to choose, I don't know, both are valid. It's a matter of taste I would say. I prefer the first one because it's more easy to figure out what happens.

Now if on your OpenGL implementation the second solution turns out to be faster than the first one, I would picked the second one. Benchmark!

Under the glHood:

Let me share of what I know on desktop GPUs and speculate a bit (don't take it too seriously!). A glClear command actually results in a draw of a full-screen quad, since drawing triangles is the "fast path" on GPUs. This is probably more efficient than DMAs or fixed hardware, since it's parallel (all shader cores clear its portion of the screen, color, z and stencil) and if you do that you can avoid the cost of fixed hardware.

About glScissor , I heard it's implemented via stencil buffering (the same mechanism than usual OpenGL stencil buffer), so only fragments that fall into the scissor zone can participate to depth dest, fragment shading, blending, etc (this is done for same reason than glClear , avoid dedicated hardware). It could be also implemented as a fragment discard+dynamic branching on modern GPUs.

Now you can see why it works that way. Only fragments of the full screen quad that lies within the scissor zone can "shade" the color buffer and clear it!