cubemap.cpp

unsigned int loadCubemap(const char* textureFile) {
    stbi_set_flip_vertically_on_load(false);

    const unsigned int horizontalChunks = 4;
    const unsigned int verticalChunks   = 3;
    // positions for the layout:
    //    T
    //  L F R B
    //    D
    const unsigned int positions[] = {
        1, 2, // right
        1, 0, // left
        0, 1, // top
        2, 1, // down
        1, 1, // front
        1, 3  // back
    };

    unsigned int textureID;
    glGenTextures(1, &textureID);
    glBindTexture(GL_TEXTURE_CUBE_MAP, textureID);

    // load original file
    int tWidth, tHeight, nrChannels;
    unsigned char* data = stbi_load(textureFile, &tWidth, &tHeight, &nrChannels, 0);
    if (data == NULL) {
        std::cout << "Failed to load skybox texture." << std::endl;
        stbi_image_free(data);
        return 0;
    }

    const unsigned int chunkWidth  = tWidth / horizontalChunks;
    const unsigned int chunkHeight = tHeight / verticalChunks;

    // cut fragments
    for (int i = 0; i < 6; i++) {
        const int len = chunkWidth * chunkHeight * nrChannels;
        unsigned char* chunkData;
        chunkData = (unsigned char*)malloc(len * sizeof(char));

        for (unsigned int line = 0; line < chunkHeight; line++) {
            int readOffset = (tWidth * positions[i * 2] * chunkHeight) + (tWidth * line) +
                             (chunkWidth * positions[i * 2 + 1]);
            int writeOffset = line * chunkWidth;
            // multiply nrChannels in the very end to reduce copy-pasting a bit
            std::copy_n(data + (readOffset * nrChannels), chunkWidth * nrChannels,
                        chunkData + (writeOffset * nrChannels));
        }

        glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X + i, 0, GL_RGB, chunkWidth, chunkHeight, 0,
                     GL_RGB, GL_UNSIGNED_BYTE, chunkData);
        free(chunkData);
    }

    // unload original image data
    stbi_image_free(data);

    // set texture cube global parameters
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    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);
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_R, GL_CLAMP_TO_EDGE);

    return textureID;
}