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Thread: Using glOrtho for 2D clipping?

  1. #1
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    Join Date
    Jun 2013

    Using glOrtho for 2D clipping?

    I'm trying to use HOR+ in my game so that the field of view expands at a higher aspect ratio. I'm using glOrtho to make sure that things are cut off equally on both sides by increasing the left parameter as the aspect ratio decreases:

    Code :
    screen_aspect_ratio = double(screen_width) / double(screen_height);
    viewport_aspect_ratio = screen_aspect_ratio;
    if(viewport_aspect_ratio > max_viewport_aspect_ratio) viewport_aspect_ratio = max_viewport_aspect_ratio;
    else if(viewport_aspect_ratio < min_viewport_aspect_ratio) viewport_aspect_ratio = min_viewport_aspect_ratio;
    if(screen_aspect_ratio > viewport_aspect_ratio) viewport_width = screen_width * (viewport_aspect_ratio / screen_aspect_ratio);
    else viewport_width = screen_width;
    if(screen_aspect_ratio < viewport_aspect_ratio) viewport_height = screen_height / (viewport_aspect_ratio / screen_aspect_ratio);
    else viewport_height = screen_height;
    viewport_x = (screen_width - viewport_width) / 2;
    viewport_y = (screen_height - viewport_height) / 2;
    ortho_width = viewport_aspect_ratio * HD_SCALE; //HD_SCALE is 1080
    max_ortho_width = HD_ASPECT_RATIO * HD_SCALE; //HD_ASPECT_RATIO is 16:9
    ortho_x = (max_ortho_width - ortho_width) / 2;
    ortho_width += ortho_x;
    ortho_y = 0;
    ortho_height = HD_SCALE;
    glViewport(viewport_x, viewport_y, viewport_width, viewport_height);
    glOrtho(ortho_x, ortho_width, ortho_height, ortho_y, -1, 1);

    This gets the job done fine, but the trick I have to do with ortho_width kind of confuses me. I'd expect that ortho_width defines the width of the clipping plane and that ortho_x defines where the clipping begins, but I have to provide the wrong value to get the right result. I'd expect this would work instead:

    Code :
    ortho_width = viewport_aspect_ratio * HD_SCALE;
    max_ortho_width = HD_ASPECT_RATIO * HD_SCALE;
    ortho_x = (max_ortho_width - ortho_width) / 2;

    ...but that stretches and moves the image to the right when the aspect ratio is lowered. Adding in ortho_width += ortho_x centers everything again... but then ortho_width is no longer the actual width of the clipping plane. Am I using the wrong function to do this?
    Last edited by WizMystery; 06-05-2013 at 03:48 PM.

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