Transparent Objects

xinvar · 2005-09-29T01:30:29

Hi, My objective is to display many objects in the scene, some transparent, some not. I read the NeHe tutorial and searched this forum (and google of course) but wasn't lucky enough. Basically I already know that I would need to display the opaque objects first, then transparent objects with depth test enabled but depth buffer writing disabled. I also need to sort all vertices of transparent objects and display them back-to-front (or front to back) with GL_BLEND enabled and the corresponding BlendFunction. Is my understanding correct? Are there any detailed tutorials on this? I would appreciate any hints.

Graphics and GPU Programming Programming

Started by xinvar September 01, 2005 04:16 PM

22 comments, last by GameDev.net 18 years, 6 months ago

xinvar

133

Author

September 02, 2005 10:03 AM

Quote:you can sort the opaque objects along with the transparent ones; if not, you have to render them all before the transparent ones.

Sneftel means: Option 1: Sort all objects (opaque + transparent) and render them back-to-front (Technically correct, but sorting opaque objects is not necessary, see Option 2). Option 2: First, Render all opaque objects, Second, sort all transparent objects, Third, render all (sorted) transparent object back-to-front. Option 3: Use depth peeling (doesn't require sorting, but requires multiple-pass).

Thanks Sneftel. One more question about sorting faces: Assuming no two faces intersect, sort all the faces based on the average positions of each face's vertices. Is this algorithm correct?

Sneftel

1,788

September 02, 2005 11:44 AM

Yep, that'll work.

xinvar

133

Author

September 02, 2005 11:58 AM

Great, thank you Sneftel. Time to start working now! I also took the time to search the literature (again) and was astonished to find that I didn't know about this wonderful Radix Sort (http://codercorner.com/RadixSortRevisited.htm). I probably won't try depth peeling (don't know vertex/fragment program yet) for now.

mossmoss

326

September 07, 2005 01:37 PM

Save some fill rate by rendering opaque stuff front-to-back with z-test enabled.

xinvar

133

Author

September 07, 2005 01:41 PM

Quote:Original post by mossmoss
Save some fill rate by rendering opaque stuff front-to-back with z-test enabled.

Thanks mossmoss. I know I would need to render opaque stuff with z-test enabled, but would front-to-back be better than simply putting everything into the pipeline? I would need to sort them then.

mossmoss

326

September 12, 2005 03:19 PM

Quote:Original post by xinvar
Quote:Original post by mossmoss
Save some fill rate by rendering opaque stuff front-to-back with z-test enabled.

Thanks mossmoss. I know I would need to render opaque stuff with z-test enabled, but would front-to-back be better than simply putting everything into the pipeline? I would need to sort them then.

No, you don't need to sort them, z-buffer would take care of it.
The idea -- in my slightly obtuse way of explaining -- was that if you're going to sort translucents back-to-front for rendering purposes, make sure to sort ONLY translucent objects. Back-to-front is worst case for overdraw if using the z-buffer, so don't sort opaque back-to-front. Either sort them front-to-back or not at all (so they're in a "random" order).

xinvar

133

Author

September 14, 2005 10:37 AM

I see. Thank you!

zedzeek

529

September 14, 2005 02:45 PM

Quote:No, you don't need to sort them, z-buffer would take care of it.

u contradict what u said earlier, what u mentioned first is correct
sort the opaque meshes in front to back order (from my testing in my game this gives about 1% increase in FPS (millage will vary of course) so its up to u if its worth it)

Captain Courgette, eat more veg

xinvar

133

Author

September 14, 2005 03:03 PM

Thank you, zedzeek. I would sort the transparent objects only.

FReY

424

September 15, 2005 03:45 AM

Quote:
sort the opaque meshes in front to back order (from my testing in my game this gives about 1% increase in FPS (millage will vary of course) so its up to u if its worth it)

xinvar, just to elaborate with regards to performance with sorting opaque meshes front-to-back.

There are several situations here:
1.) you are CPU bound. Sorting anything will aggravate the situation.
2.) you are vertex/memory bandwidth-bound: sorting will have no effect.
3.) you are fill-rate bound (in the case of complex shaders). Sorting front-to-back of your opaque objects will allow early-out in the z test per-pixel which improves performance. If you have a complex scene it may be worth doing a quick z only pass of your opaque objects before rendering your opaque objects.

So, the sorting front-to-back is only good advice if you're fill rate bound on the GPUs that you're targetting.

do unto others... and then run like hell.

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