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GenuineXP

OpenGL Avoiding the Painter's Algorithm: Using the Depth Buffer?

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GenuineXP    262
I'm using OpenGL for the backend of my game engine and am wondering if somehow I can use the depth buffer (or some other feature) to control "depth" of drawing operations. Currently, my abstracted framework provides this functionality by basically sorting drawing operations by depth and then sending them to OpenGL. This is rather annoying and becomes hard to deal with when I need to combine things like hierarchical transformations with depth. Does OpenGL have some facility to control the "depth" of drawing operations in 2D? If my projections are setup properly, could I just translate things along the z-axis without any perspective correction to attain this effect? Ultimately, I'd like to be able to perform drawing operations in any order and preserve their depth. Is this possible? Thanks!

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Ezbez    1164
Yes. Use an orthographic projection (or, as you called it, without any perspective), then just draw things with a z-component with depth testing enabled (it isn't by default).

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Gage64    1235
Quote:
Ultimately, I'd like to be able to perform drawing operations in any order and preserve their depth. Is this possible?


I'm not sure I understand your question, as that's exactly what the depth buffer is meant for.

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GenuineXP    262
Quote:
Original post by Gage64
I'm not sure I understand your question, as that's exactly what the depth buffer is meant for.

I've only used OpenGL for very simple 3D programs, and mostly 2D graphics, so my knowledge of the depth buffer is virtually non-existant. :-)

I'll give the idea mentioned above a try. Thanks for the replies!

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Nypyren    12073
You'll still have to use the painter's algorithm for partially transparent things (sprites like ghosts or smoke clouds).

If you draw a partially transparent pixel close to the camera, and then draw another pixel further away, the depth buffer prevents the second pixel from drawing at all, though logically you should be able to see through the first pixel to the second.

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