Transfer GL_TEXTURE_2D Image to GL_TEXTURE_CUBE_MAP

I am attempting to implement a Skybox into my C++ OpenGL project, and I want to use 6 x preloaded GL_TEXTURE_2D's retrieved from my TextureLibrary which loads all textures into the application as GL_TEXTURE_2D.

The issue I am running into is that I cannot use a GL_TEXTURE_2D for each face of a GL_TEXTURE_CUBE_MAP (that I know of). I have tried a variety of different approaches, however it appears that I cannot refer to or copy the image data from a GL_TEXTURE_2D to a GL_TEXTURE_CUBE_MAP face such as GL_TEXTURE_CUBE_MAP_POSITIVE_X on the GPU side.

To get this working currently the texture is being loaded on the CPU into my TextureLibrary as a GL_TEXTURE_2D, then it is being loaded again on the CPU in my Skybox class as a target of a GL_CUBE_MAP face, just as a temporary and inefficient way around this issue.

Is there any way to have the face of a GL_CUBE_MAP refer to or copy the data of a GL_TEXTURE_2D?

I plan to utilise these textures as GL_TEXTURE_2D's throughout the application in the future so I want to have them loaded on the CPU side once into my TextureLibrary, and only when they are used by a Skybox, the data is referenced or copied on the GPU side into a GL_TEXTURE_CUBE_MAP.

Alternatively, I could scrap the idea of using a GL_TEXTURE_CUBE_MAP, and pass my skybox shader 6 x Sampler2D's; however, I would like to know if there is any way I can do this per the above - OR any other ideas.

Solution

You can try this:

//generate cubemap texture object name
glGenTextures(1, &my_cube_map_texture);
//make it a cubemap texture
glBindTexture(GL_TEXTURE_CUBE_MAP, my_cube_map_texture);

//for each cubemap face
for (int i=0; i < 6; ++i) {

    //create face
    //where i
    //0: GL_TEXTURE_CUBE_MAP_POSITIVE_X
    //1: GL_TEXTURE_CUBE_MAP_NEGATIVE_X
    //2: GL_TEXTURE_CUBE_MAP_POSITIVE_Y
    //3: GL_TEXTURE_CUBE_MAP_NEGATIVE_Y
    //4: GL_TEXTURE_CUBE_MAP_POSITIVE_Z
    //5: GL_TEXTURE_CUBE_MAP_NEGATIVE_Z
    glTexImage2D(
        GL_TEXTURE_CUBE_MAP_POSITIVE_X + i, //target
        0,                //level
        iformat,          //internalformat
        width,            //width
        height,           //height
        0,                //border
        format,           //data format
        GL_UNSIGNED_BYTE, //data type
        NULL              //no copy, allocate only
    );

    //copy data from existent 2d texture into the specific face
    glCopyImageSubData(
        my_texture_2d[i],    //srcName
        GL_TEXTURE_2D,       //srcTarget
        0,                   //srcLevel
        0,                   //srcX
        0,                   //srcY
        0,                   //srcZ
        my_cube_map_texture, //dstName
        GL_TEXTURE_CUBE_MAP, //dstTarget
        0,                   //dstLevel
        0,                   //dstX
        0,                   //dstY
        i,                   //dstZ
        width,               //srcWidth
        height,              //srcHeight
        1                    //srcDepth
    );

}

Prerequisites:

OpenGL 4.3+
the textures have to be square images (width == height)
the cubemap faces must have all the same sizes, therefore the textures have to be of the same size
the format must match, otherwise copy will produce error

Reference:

There is an alternative solution, follow the links for more information:

If everything else fails:

bind a buffer to GL_PIXEL_PACK_BUFFER target
copy texture data into buffer via glGetTexImage
bind buffer to GL_PIXEL_UNPACK_BUFFER target
create cubemap face

No client memory involved, all on the GPU side.