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Transparent pixel in overlay colormap

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Q: I work on various SGI machines, mainly Indigo Impact and Octane. My question is: given a colormap in an overlay visual, how do I know which pixel is the transparent one?


    int type; /* type of transparency */
    int transparent; /* index value for transparent */

    glXGetConfig(display, &visuals[i], GLX_TRANSPARENT_TYPE_EXT, &type);
    if (type == GLX_TRANSPARENT_INDEX_EXT) {
        glXGetConfig(display, overlay_visual,
            GLX_TRANSPARENT_INDEX_VALUE_EXT, &transparent);
        printf("index %d is transparent\n", transparent);
    } else {
        printf("visual has no transparent index\n");