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Lutz

OpenGL Strange texture artifact

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Lutz    462
Hey! Yesterday I wanted to fix the texture transitions in my planet rendering program. I knew it's a bit tricky, so for debugging reasons I gave my textures a red border (so really the image has this border, it's not an opengl texture border). When I map the textures to the planet, it looks like that: Note the blocky artifacts near the red lines (which is the texture transition zones). (No, these are no JPEG-artifacts;-) ) And my texture image definitely does not have this artifacts. It still keeps happening when I disable mipmapping, linear max-filtering AND texture compression. If I take a texture which is completely white with a red border, there are NO artifacts. However, to me it looks like texture compression artifacts. But even if I haven't specified texture compression in my program it still happens, so I find it quite strange. So I have some questions: 1) Could it be texture compression artifacts? 2) Can OpenGL enable texture compression although I didn't tell it to? 3) How can I ask OpenGL whether the textures are compressed? Any suggestions? I have an Athlon 1400Mhz with an ATI 9800Pro (128MB), 256 MB RAM, the screen is 800x600x32BPP, the textures are 256x256x24BPP.

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Dybbuk    158
Maybe something like glEnable(GL_CLAMP); may solve the problem.

OpenGL 'loops' textures, unless you tell it shouldn't.

Still, your screenshot shows quite large artifacts, while not clamping only produces small ones. So, maybe is something else..

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Lutz    462
It's not the clamping.
My texture coordinates are in the range 1/(2N)...1-(1/2N), where N=256 is the texture size, so that pixels on the very boundary of the polygon map to the center of the boundary texel (i.e. the red border texel).

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mikeman    2942
Quote:
Original post by Lutz
It still keeps happening when I disable mipmapping, linear max-filtering AND texture compression.

If I take a texture which is completely white with a red border, there are NO artifacts.


This is very strange.How do you load your textures?

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Guest Anonymous Poster   
Guest Anonymous Poster
Depending on video card settings, the texture may be compressed even if you don't explicitly ask for it to be programatically. I know my radeon 8500 has a 'texture quality' setting which seems to lower the size and/or compress textures more than programs request. Such a driver setting may be the cause of your problem

-Extrarius

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Lutz    462
Extrarius:
Ah, interesting, that's what I expected.
How can you ask OpenGL whether a texture is compressed?
And how can you make it NOT compress textures?

okonomiyaki:
Yes, the red line is just one pixel.

I tried linear max filters and nearest neighbour max filters, both showed artifacts, with and without mipmapping.

Before I put the red lines in, I figured out how to make the texture transitions continuous (even when zooming in). The solution (I believe) is to use the same border texels for the involved textures, i.e.

|--------------| |--------------|
| a| |a |
| Texture 1 b| |b Texture 2 |
| c| |c |
|--------------| |--------------|

(a,b,c are some border colors)
Then, if you draw triangles with textures 1 and 2 side by side, the transition should be continuous (if you don't mess with the texture coordinates...).

However, the transition was NOT continuous and I figured out that the problem must be that the border colors do not coincide (indeed, you saw that). So I just set the border colors to red (to make sure that the problem was not my texture generation routine making the border colors different), so I KNEW the border colors are the same (at least in the BMP file) and I saw those artifacts.

So the bottom line: Without the red line you don't really see such blocky artifacts but the transition is not continuous since the border colors don't coincide. I guess the reason is a compression algorithm which does not create visible artifacts without the red line, but changes the colors slightly so that the transition is no longer continuous.

Texture compression would explain why you really see those artifacts only you put the red line in: The texture just can't be compressed very well anymore, so the compression becomes lossy. Like when you try to compress a fractal image with JPEG, you'll get huge artifacts. When you JPEG a smooth image, you don't see artifacts.


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