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PowerVR serious problem
01-12-2015, 02:03 AM
Post: #25
RE: PowerVR serious problem
If you're talking about alpha testing, the problem is that it does something the PowerVR chip doesn't like. It's the operation in the first place that is bad. Actually, alpha testing in general is sorta "deprecated" and most modern games won't be using it; but it worked fine on the PSP so it's used all the time by a ton of games.

The basic idea is this: let's say I have a texture where some of the pixels are partially transparent. Like a shadow or something. Alpha testing allows it to say "only draw the parts that are AT LEAST this opaque." So it might only draw the darkest parts of the shadow, skipping the lighter parts of the shadow entirely.

That particular example is probably one where you'd say "it's not so bad if we disable the effect entirely." There are other much more complicated things that are done with alpha testing (that are more necessary for graphics to look even close to right), though. This is just an easy to understand example.

Anyway, the problem is that the GPU just wants to DRAW. It doesn't want someone constantly grabbing it's arm and saying "hey wait, not that one. You have to skip that pixel." This makes PowerVR especially very slow, because it's not built to stop-and-go like that (and, as I mentioned, it's somewhat of an antiquated practice to make the gpu do that.)

If someone were to redo the way the game draws things, and make it use more modern techniques, the problem would go away. That wouldn't be emulation though, that would be a port or a remake.

Aside from that though, the PowerVR SGX540 and 544 have not terribly good precision in shaders. One thing was fixed recently by replacing a modulus (remainder of division) math operator with a function that did _exactly_ what the spec _requires_ the modulus operator to be equivalent to. So some of the mathematics on the gpu (or its drivers, hard to tell) don't actually have correct results. That's kinda scary, and makes it hard to make things come out right, especially when a lot of PSP games do really neat tricks to get 150% out of the PSP hardware, and we have to translate these neat tricks into things that work on OpenGL.

We may be able to find ways to workaround the bad math and precision in that GPU, but ultimately it's just going to make things slower, not faster, because we will have to do more complicated things or do them in more steps or in multiple ways to get the precision necessary for things to actually work and show up.

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Messages In This Thread
PowerVR serious problem - Laska - 06-06-2014, 07:17 AM
RE: PowerVR serious problem - barbanium - 06-06-2014, 02:10 PM
RE: PowerVR serious problem - Laska - 06-06-2014, 02:34 PM
RE: PowerVR serious problem - barbanium - 06-06-2014, 02:49 PM
RE: PowerVR serious problem - Laska - 06-06-2014, 02:55 PM
RE: PowerVR serious problem - Laska - 06-06-2014, 05:22 PM
RE: PowerVR serious problem - [Unknown] - 06-06-2014, 05:52 PM
RE: PowerVR serious problem - Laska - 06-06-2014, 06:03 PM
RE: PowerVR serious problem - nc50lc - 06-07-2014, 05:20 AM
RE: PowerVR serious problem - Laska - 06-07-2014, 12:21 PM
RE: PowerVR serious problem - Laska - 06-08-2014, 10:20 AM
RE: PowerVR serious problem - Ritori - 06-27-2014, 09:42 PM
RE: PowerVR serious problem - Laska - 06-27-2014, 10:18 PM
RE: PowerVR serious problem - emualador - 07-24-2014, 02:55 PM
RE: PowerVR serious problem - Laska - 07-24-2014, 03:44 PM
RE: PowerVR serious problem - Laska - 09-05-2014, 06:05 AM
RE: PowerVR serious problem - Laska - 09-22-2014, 07:37 AM
RE: PowerVR serious problem - Laska - 12-30-2014, 09:02 AM
RE: PowerVR serious problem - [Unknown] - 12-30-2014, 05:25 PM
RE: PowerVR serious problem - Laska - 12-30-2014, 06:14 PM
RE: PowerVR serious problem - [Unknown] - 12-30-2014, 07:00 PM
RE: PowerVR serious problem - Laska - 01-11-2015, 11:17 PM
RE: PowerVR serious problem - [Unknown] - 01-12-2015 02:03 AM
RE: PowerVR serious problem - Laska - 01-12-2015, 11:57 AM
RE: PowerVR serious problem - fivefeet8 - 02-27-2015, 02:30 PM
RE: PowerVR serious problem - The Phoenix - 01-12-2015, 11:16 AM
RE: PowerVR serious problem - Laska - 02-26-2015, 09:41 PM
RE: PowerVR serious problem - [Unknown] - 02-27-2015, 03:49 PM
RE: PowerVR serious problem - Laska - 02-27-2015, 04:21 PM
RE: PowerVR serious problem - [Unknown] - 02-27-2015, 05:26 PM
RE: PowerVR serious problem - Laska - 02-28-2015, 12:40 AM
RE: PowerVR serious problem - The Phoenix - 02-28-2015, 02:05 AM
RE: PowerVR serious problem - Laska - 02-28-2015, 05:27 PM

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