使用GLKit加载立方体贴图后，iOS上openGL ES 2.0的正确立方体贴图坐标是什么？

Question

In order to debug my shader, I am trying to display the just the front face of the cube map. The cube map is a 125x750 image with the 6 faces on top of each other:

First, I load the cube map with GLKit:

_cubeTexture = [GLKTextureLoader cubeMapWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"uffizi_cube_map_ios" ofType:@"png"] options:kNilOptions error:&error];

Then I load it into the shader:

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_CUBE_MAP, self.cubeTexture.name);
glUniform1i( glGetUniformLocation( self.shaderProgram, "cube"), 0);

Then in the fragment shader:

gl_FragColor = textureCube(cube, vec3(-1.0+2.0*(gl_FragCoord.x/resolution.x),-1.0+2.0*(gl_FragCoord.y/resolution.y),1.0));

This displays a distorted image which seems to be a portion of the top of the cube map:

It shouldn't be distorted, and it should show the right face, not the top face.

I can't find any documentation that describes how the coordinates map to the cube, so what am I doing wrong?

Answer 1

It seems that there is a problem with cubeMapWithContentsOfFile. The cubeMapWithContentsOfFiles method (the one that takes an array of 6 images) works perfectly on the simulator. (There is a different issue with both methods on device).

Answer 2

To visualize how texture coordinates work for cube maps, picture a cube centered at the origin, with the faces at distance 1 from the origin, and with the specified cube map image on each face.

The texture coordinates can then be seen as direction vectors. Starting at the origin, the 3 components define a vector that can point in any direction. The ray defined by the vector will then intersect one of the 6 cube faces at a given point. This is the point where the corresponding cube map image is sampled during texturing.

For example, take a vector that points in a direction that is closest to the positive z axis. The ray defined by this vector intersects the top face of the cube. Therefore, the top ( POSITIVE_Z ) image of the cube map is sampled, at the point where the ray intersects the face.

Equivalent rules applies to all other directions. The face corresponding to the largest absolute value of one of the vector components determines which face is sampled, and the intersection point determines the position within the image.

The exact rules and formula can be found in the spec document. For example in the latest spec (OpenGL 4.5), see Section 8.13 "Cube Map Texture Selection", with the matching table 8.19. But as long as you understand that the texture coordinates define a direction vector, you have the main aspect covered.

How you determine the texture coordinates really depends on what you want to achieve. Common cases include:

• Using normal vector as the cube map texture coordinates. This can for example be used for pre-computed lighting effects, where the content of the cube map image contains pre-computed lighting results for each possible normal direction.
• Using the reflection vector as the cube map texture coordinate. This supports the implementation of environment mapping. The content of the cube map is a picture of the environment.