🔒 Locked

Shaders, one per object

Started by Decrius Mar 25, 2009 at 3:28 AM 4 replies 1.2k views

Original Post

Mar 25, 2009 at 3:28 AM

So, I've been reading up on shaders, and even ran a simple test program with shaders. I know how to write one (the basics), how to load the shaders and use them. However, the shaders apply to everything you draw. That is my problem. I want in my library people to be able to set a shader per object. I would probably make a shader class, holding the pointers to the shader in VRAM. However, how would I switch a shader on and off? It would only be used for one object at a time, and the next object could be an entirely different shader. So I should put a shader on, draw the object, and switch it off. How would I do this? What functions are used for this? And how bad is the overhead of switching (like with switching textures?)? Thanks!

Lord_Evil

Mar 25, 2009 at 3:37 AM

You switch shaders off by calling glUseProgramObject(0).

The cost of switching shaders is very high. I'm not sure but it might be even higher that switching textures. So you should group your objects by shader and only call glUseProgramObject if the shader changes ('no shader' might be considered a special shader object).

Decrius

Mar 25, 2009 at 6:13 AM

Quote:
Original post by Lord_Evil
You switch shaders off by calling glUseProgramObject(0).

The cost of switching shaders is very high. I'm not sure but it might be even higher that switching textures. So you should group your objects by shader and only call glUseProgramObject if the shader changes ('no shader' might be considered a special shader object).

Thanks!

I guessed it would be expensive, but grouping by shader is near impossible in my situation...hmm. Thanks for the tip though :)

swiftcoder

Mar 25, 2009 at 6:26 AM

Quote:
Original post by Decrius
I guessed it would be expensive, but grouping by shader is near impossible in my situation...hmm. Thanks for the tip though :)

If you explain your situation, we may be able to suggest ways to improve it. This is a problem that has been solved many, many times.

Decrius

Mar 25, 2009 at 7:10 AM

Okay. I am writing a GUI library and want to be able to attach shaders to GUI objects.

Since the GUI elements are ordered back to front and grouped by window, it's rather difficult to add another groupings mechanism. I expect only a few GUI elements to be using Shaders though, but this is entirely up to the user (developer) using this library.

swiftcoder

Mar 25, 2009 at 7:20 AM

For GUI objects, the cost of switching shaders should not be a problem - I assume there shouldn't be more than say 10-20 on screen at once.

Topic Locked

This topic has been locked by a moderator. New replies are not allowed.