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Efficient translucency...

Started by GenPFault Mar 18, 2007 at 12:03 AM 5 replies 800+ views

Original Post

Mar 18, 2007 at 12:03 AM

I'm trying to create a simple OBJ renderer and hit a snag with per-layer/group translucency. Right now I'm just tossing each layer/group into its own display list for performance reasons, but rendering them in no particular order rarely looks correct. Models are quite large, on the order of 10000+ triangles, and are usually entirely in-frame. Apparently to use the built-in blending modes correctly you have to sort your translucent geometry from back to front before rendering. Unfortunately the only way I've read to do this is to use a BSP tree for my geometry, and rendering them requires a whole bunch of immediate-mode OpenGL calls, one for each polygon (unless I'm misunderstanding something). Is there some easy way to do correct translucency in OpenGL without sorting? If not, is there an easier/quicker way than using BSP trees?

dpadam450

523

dpadam450

Mar 18, 2007 at 3:48 AM

I was just going to ask, Is there a way to map an alpha of > .9 to not do any depth buffer writing. That way if something is clear, I dont have to draw it last.

GenPFault

Author

GenPFault

Mar 18, 2007 at 10:27 AM

Quote:
Original post by dpadam450
I was just going to ask, Is there a way to map an alpha of > .9 to not do any depth buffer writing. That way if something is clear, I dont have to draw it last.

This maybe? Under 'Another Good Trick':
http://www.sjbaker.org/steve/omniv/alpha_sorting.html

MaliciousDigit

Mar 18, 2007 at 11:29 AM

i have had to handle the same situation. I simply sorted my groups so that transparent groups where drawn last. I also turn on back face culling fro transparent groups. You can also disable drawing to the color buffer the first pass, then set your depth function to equal and do another pass. Sorting polygons is almost never the most efficient way to do transparency IMHO.

GenPFault

Author

GenPFault

Mar 18, 2007 at 1:11 PM

Quote:
Original post by MaliciousDigit
i have had to handle the same situation. I simply sorted my groups so that transparent groups where drawn last. I also turn on back face culling fro transparent groups. You can also disable drawing to the color buffer the first pass, then set your depth function to equal and do another pass. Sorting polygons is almost never the most efficient way to do transparency IMHO.

I would sort groups but they tend to be non-convex and like to wrap around each other is novel ways.

MaliciousDigit

Mar 18, 2007 at 2:44 PM

Do depth peeling then. It's simular to what I described with the depth buffer, here's a paper on it.

GenPFault

Author

GenPFault

Mar 18, 2007 at 3:48 PM

Quote:
Original post by MaliciousDigit
Do depth peeling then. It's simular to what I described with the depth buffer, here's a paper on it.

Thanks for the link, but it looks like that method won't work on low-end and non-Nvidia hardware. The java3d scenegraph solves this somehow, I'll have to go back and look at it. It may just be sorting the layers like you said, in which case I'll have to give it a try.

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