View Full Version : CLAMP, CLAMP_TO_EDGE and CLAMP_TO_BORDER
01-18-2002, 03:01 PM
I cant understand the difference betwen this three methods (I've read the opengl 1.3 specification a lot of times), CLAMP uses the border texels, but what happens is I set the border value to 0 at glTexImage2d?
Thanks in advance
01-19-2002, 03:57 AM
If you use GL_CLAMP, which uses the border texels, and the border is 0 (black), then in bilinear (and above) filtering, it will sample the black border, when calculating texel values for the edge of the texture.
If you have a skybox for instance, or a cubemap, this is seen as black lines appearing between textures.
You wont actually see this on GF3, because GF3 treats GL_CLAMP as GL_CLAMP_TO_EDGE.
CL_CLAMP_TO_EDGE, forces it to only sample pixels from the actual texture, not the border, so you dont get this black line sympton.
As far as GL_CLAMP_TO_BORDER goes, I dont know what this extension does.
Hope this helps a bit,
01-19-2002, 06:00 AM
CLAMP_TO_BORDER causes OpenGL to only take the border color at the edge of the texture rather than the average of the border color and texture edge texels. This allows for a perfect border around the texture.
01-19-2002, 08:16 AM
I believe the question he tries to ask is: "what is the color of the border when I specify a 0 sized border"
My guess is it is undefined, though I was too lazy to look it up for real :-)
[This message has been edited by jwatte (edited 01-19-2002).]
01-19-2002, 08:52 AM
I believe when the border parameter is 0, it still samples from a black border, when texels taken for a filter are outside of the current texture.
01-19-2002, 09:03 PM
WRT the CLAMP_TO_EDGE and CLAMP distinction, we have a little test app (http://www.ati.com/na/pages/resource_centre/dev_rel/sdk/rage128sdk/OpenGL/Samples/Rage128EdgeClamp.html) which illustrates this to some degree. Note that many implementations don't get GL_CLAMP right, but this matters little in games since the GL_CLAMP_TO_EDGE behavior is what the overwhelming majority of developers expect when using GL_CLAMP. TRIBES, for example, shipped with this wrong and had to issue a patch using the GL_CLAMP_TO_EDGE extension to eliminate the black grid lines that showed up on their terrain on implementations which did GL_CLAMP right.
On NVIDIA hardware correct clamping needs to be enabled in the registry. It's kind of strange but GL_CLAMP_TO_EDGE does not work correctly unless you set OGL_TexClampBehavior to 1 in the registry.
There was a topic on this some time ago and the NVIDIA guys said that enabling this by default would break a lot of applications.
That is, they said that correct GL_CLAMP behaviour could not be enabled by default but why GL_CLAMP_TO_EDGE isn't handled correctly is strange.
[This message has been edited by PH (edited 01-20-2002).]
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