Sign in to follow this  
tobspr

Why does GLSL use integers for texture fetches?

Recommended Posts

After reading AMD's presentation about the GCN architecture, which states that integer divisions cost multiple cycles, and unsigned integer divisions are much faster, I changed my data types to "uint" where appropriate, since I do not use negative values in most cases anyways.

 

However, all variants of texelFetch only accept ivec2/ivec3. What is the reason for this? From what I can see, unsigned integers would be much more appropriate. 

 

I do not know about any extensions which has a special handling for negative texture integer coordinate values,

so, is there any reason the texture coordinates are not unsigned?

 

 

 

 

 

Share this post


Link to post
Share on other sites

TBH I thought it was a bad call. And I still think it is.
 
However I found one instance where the fetch being an int was useful instead of being an uint: Clamp to edge emulation.
I needed my fetches to clamp to edge; so typical code would look like this:

ivec2 xy = some_value - another_value;
xy = clamp( xy, 0, textureResolution.xy );
float val = texelFetch( myTex, xy ).x;

This code would not work as intended if "xy" were to be uvec2, because values below 0 would wrap, and hence clamped to textureResolution (the other edge!) instead of clamping to 0. It would be the same as doing xy = min( xy, textureResolution.xy );

However, I'm like Hodgman: I prefer unsigned integers because we're addressing memory here, and negative memory makes no sense, and I prefer assert( x < elemSize ) over assert( x >= 0 && x < elemSize );
This case I talk about (clamp to edge) can simply be solved through explicit casts. IMO ints here have more trouble than benefits.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this