Archive for August, 2007

Reviving GLSL

Friday, August 10th, 2007

The other day, looking at planet Gnome, I found one more interesting that the others. This was writen by MDK, and its titled Vector drawing: OpenGL shaders and cairo.

This post continues, in some way, a post written by Tim Janick on his blog, about OpenGL for Gdk/Gtk+, that summarizes some of problems about using OpenGL with Gdk/Gtk+.

MDK made some tests to compare one an another, I had a little curiosity and check the code. The shader version was written in Cg, and as there are a lot of time that I doesn’t write nothing related to shaders, I decided to waste the little free time I have making the GLSL version of this test this afternoon.

I same the sames tests, except by environment:

  • AMD 1800+ 784MB RAM
  • Geforce 5900FX 128 MB RAM
  • modified source code (you’ll dont require Cg Toolkit, instead you’ll require Glew)

And the visual results:

Cairo bezier

Cg bezier

GLSL bezier

But the most curious was about the times (sorry, not chart this time);

  • cairo: 0,18494 s
  • cg: 0.01927 s
  • GLSL: 0.00155 s

GLSL better than cg? I suppose that this is the easy answer but:

  • First at all: its too much difference, it has no sense
  • AFAIK GLSL driver compiler its almost the Cg compiler (they use a Unified Arquitecture), you only need to see the extension EXT_Cg_Shader (use Cg code to define a shader object)
  • OpenGL works asynchronously
  • The profile: Cg implementation uses a concrete profile (CG_PROFILE_ARBVP1), GLSL hasn’t got this concept, it tries allways to search the bext vertex/fragment program extension to use.
  • So: the time of only draw some curves its not a accurate (we knowed it before) measure, it is only a guide

Conclusion: it is good to revive some time GLSL, I need to do this more often, thanks to MDK to give us some code to play !