How to detect if AA points are accelerated by hardware?

OpenGL OpenGL: Advanced Coding
1

Hi,

I’m using GL_POINTS and want to glEnable(GL_POINT_SMOOTH) but if and only if the point antialiasing is done in hardware - otherwise this statement will absolutely kill performance in my pointsy application.

One of the hardware vendors I support has this characteristic - nice acceleration in general, but no AA points.

How to detect?

Cheers,

Tom

2

Not sure if you can tell if they’re supported in hardware, other than using empirical evidence. But the simple workaround that I’ve used is just avoiding GL_POINTS altogether. In my case, GL_POINTS were particularly problematic: inconsistent appearance from one OS/hardware platform to the next, and larger points (using ARB_point_sprite, allowing for arbitrary 2D textures to be rendered with GL_POINTS) are noticeably clipped in their entirety as soon as the center of the point sprite falls outside the view (not a problem for tiny sprites).

The simple workaround is to do it yourself, and in most cases you’ll have far fewer headaches than dealing with GL_POINTS. If you want consistent hardware support and consistent appearance, it boils down to:

  • submit each sprite’s geometry as a set of 4 vertices, each of which is positioned at the sprite’s center (for a lot of sprites, use glDrawArrays and pass them in a single call); the geometry must be submitted with an attribute that tags each vertex (an int from 0 to 3, effectively a “corner ID”) so as to distinguish the corners from one another
  • setup a program local parameter array of 4 offsets, corresponding to the area of the sprite
  • process your sprites’ vertices with a vertex program that picks up the vertex’s “corner ID” attribue and offsets it according to a lookup in a program local parameter array (the values in the array are used to offset the initally coincident vertices to the various corners of the sprite)
3

Thanks for the response. Would this be more costly than GL_POINTS?

Cheers,

Tom