Skybox texture artifact on edge

Question

I have strange problem with drawing skybox texture on Mac. On iPhone everything is going fine. I have tried to change near and far planes value with no success.

It is a skybox of six textures, and for every texture I set this:

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

What could be the problem?

Screenshot:

EDIT:

I have tried to set background color to red, to ensure if it is bleeding through the texture

I have also tried to change coords of the texture to this:

float err_corr = 0.5 / TEXTURE_SIZE;

GLfloat vertices[24] = {    
        -1.0f - err_corr, -1.0f - err_corr,  1.0f + err_corr,
    1.0f + err_corr, -1.0f - err_corr,  1.0f + err_corr,
    -1.0f - err_corr,  1.0f + err_corr,  1.0f + err_corr,
    1.0f + err_corr,  1.0f + err_corr,  1.0f + err_corr,
    -1.0f - err_corr, -1.0f - err_corr, -1.0f - err_corr,
    1.0f + err_corr, -1.0f - err_corr, -1.0f - err_corr,
    -1.0f - err_corr,  1.0f + err_corr, -1.0f - err_corr,
    1.0f + err_corr,  1.0f + err_corr, -1.0f - err_corr,
    };

And the box is drawn in this order:
GLubyte indices[14] = {0, 1, 2, 3, 7, 1, 5, 4, 7, 6, 2, 4, 0, 1};

SOLUTION:

The solution is to set GL_CLAMP_TO_EDGE for the cubemap itself!

glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

Answer 1

Are you positive that you've set GL_CLAMP_TO_EDGE for each individual texture? This is a part of individual texture state and it defaults to GL_REPEAT, so this needs to be done after binding with glBindTexture. I realize in the original post you literally say that you set it for each texture, but I'm wondering what the code actually looks like?

For example:

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

glBindTexture(GL_TEXTURE_2D, tex1);
//upload texture, etc.

glBindTexture(GL_TEXTURE_2D, tex2);
//upload texture, etc.

This is not the same as:

glBindTexture(GL_TEXTURE_2D, tex1);
//upload texture, etc.
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

glBindTexture(GL_TEXTURE_2D, tex2);
//upload texture, etc.
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

Answer 2

inset '0.5f / texture_width' from the left and right side of the texture and '0.5f / texture_height' from top and bottom. You're seeing sampling artefacts - texture samplers usually sample from the centre of texels so on the left and right, top and bottom (at 0 and 1 uv) you're getting 50% of whatever texel your sampler gets when it's out of bounds. That's either the other side of the texture or a default sampling colour.

Answer 3

My best guess is that the difference between MAC and iPhone is caused by antialiasing on the MAC. My 3D programming is a bit rusty, but I guess you would have to make all the squares part of the same mesh or something like that.

Dirty hack solution would be to make all the squares a bit too big so they overlap and clip each other and adjust UV coordinates accordingly.

Answer 4

This is usual problem with cubemaps on older HW (or iphones maybe). Check for the

GL_ARB_seamless_cube_map

extension if it is available on your hw. If so. Problem will be propably solved by using it.

Answer 5

Are the six textures set up, specifically, as a cube map or are each of the skybox faces rendered separately with their own texture? I heard some early PC systems had rather poor filtering when it came to handling cubemaps in that they didn't necessarily filter across the from one face texture to the next when you sampled near the cube edges. AFAIK the SGX (iphone GPU) should filter across edges correctly.

It did occur to me that the bottom of your side textures looks to be a similar shade of blue to the artefact . Could you try, say, making the bottom row (or a few rows) of pixels a hideous shade of magenta and see if that changes the artefact colour? That might give some more idea of what may or may not be happening.