next gen and alpha
polycount lvl 666
seems that although the technology is getting more advanced stuff like alpha has a taken a backward step.
In some next gen engines, alpha os so expensive as to render it unuseable.
In some next gen engines, alpha os so expensive as to render it unuseable.
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what some do is render it as solid, then save result into texture and blend this texture back on framebuffer...
if you can manage to do all work in a single pass, it is same as now.
but because we now have much more postprocessing it is trickier to handle transparent fx, cause you would need different info for the "pixel below"...
so it gets complex if you want it with same "detail" as solid stuff, but often that is not required
think about it, you only have your framebuffer, the pixels you see. now when you look thru something transparent, you know that there are 2 surfaces (the transparent, and whatever is behind) but on framebuffer it is just 1 pixel, which got mixed out of two color values, each represented different surfaces.
this information now got lost, your brain makes it up again, cause it interprets the similar colored stuff as one surface and so on.
so you have just pixels that contain only one piece of information a time, but transparent stuff has 1 information per surface. even when you have more than one buffer, like depthbuffer, normalbuffer... you still have one information per pixel,you cant easily come around that...
if there is just one surface (all solid), you have all necessary data you need to do fancy fx, but what happens when there is 2 or 3 or many, what if they overlap, intersect...
this is not raytracing, where you just shoot from the eye until you hit a surface,compute color, then go along the ray if transparent...
but in realtime engines, you render each object (triangle soup) after another. each one has no clue about the others...
seems like a backward step thats all.
There are ways to do it, but to be honest a lot of the nice work you see in challenges etc that use multi layered alpha simply would not work in many engines.
However - I'm sure it's possible... just takes time, effort and tears.
next gen woes...
more on those z-fight like things, when sorting triangles, it can happen that they intersect in such a fashion that they are partially in front of each other. now to do the transparency effect accurately, you would need to split the triangles along their intersection line. but that would be really cpu heavy. the main problem is that the blend used for transparency the "lerping" (decal..) is not associative. so ((a lerp b) lerp c) is not the same as (a lerp (b lerp c)), you must not change order.
thats why particle systems often use additive blends, saves sorting time. (order of adding values doesnt change result, cept some rounding problems), but results into that ugly "glowing smoke", as many particles with dark grey on top of each other sum up to white quickly.
Why can't alpha be as simple as the DS/N64?
Also a lot of the new stuff can often be emulated on cpu, just like old drivers would perform vertex-shaders on cpu with SIMD. while I'm no expert I think neither xbox2 nor the ps3 have so called geometry shaders (the only real big new thing about dx10, although a friend told me xbox2 has something similar, and well ps3 has enough cells to do it one).
it's definetely going to be cool to see what comes out of the new tech, but until stuff becomes mainstream it surely will take a while.
as for the alpha problem... hehe in doom 3 u have to chose between either having alpha or normal maps... hehee uber ghey, but thats how it is. one of the reasons why alot of the next gen games have alot of chunky solid characters. I'm sure this will get fixed eventually.
One trick we've used to get soft-alpha to sort in the proper order is to divide the mesh up into chunks by material, so that even though the surface is a single entity (head of hair, or whole tree), it gets sent to the vertex cache in separate chunks, so then each can be sorted against the others, for better layering.
Problem is, this can be very slow, since (as I understand it) you can only have one chunk at a time in the cache, clearing it out and loading the next one takes time. Then there's the fill-rate issue on top of that, rendering each chunk in its entirety before rendering the next one, etc.
Alpha-test seems to avoid all that, so it's super fast, you can just send a single chunk for the whole tree (or attach multiple trees together as one massive chunk, so it's even faster). The clear texels are simply not drawn, so you don't have to render as many of those underlying never-seen pixels.
Or at least that's the way I understand it so far, with my puny artist's brain.
BTW, we have the term "alpha-test" for 1-bit alpha... anybody know the analogous term for soft transparency? Is it just "alpha-sort"? I can't remember.
in opengl alpha-test is used for the 1bit stuff, and blend is used for blending (decal,additive... whatever) of a surface on top of the framebuffer.
alphatest is fast because it can replace the pixel below, whilst alphablend will need the previous drawn pixel and mix it with the new one
Can anyone shed any light (pardon the pun) on this?
So many shaders never enough time to memorise what does what.
TGZ: I think it should be doable to fit multiple binaries for different OSes on one DVD (as has been done with e.g. UT2004). Multiple versions of any product are economically ineffective, it's cheapest to have one unified product that everyone buys.
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Blizzard games use PC/MAC CD's but they are bloody slow for some reason (atleast WoW is).
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TGZ: I think it should be doable to fit multiple binaries for different OSes on one DVD (as has been done with e.g. UT2004). Multiple versions of any product are economically ineffective, it's cheapest to have one unified product that everyone buys.
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Blizzard games use PC/MAC CD's but they are bloody slow for some reason (atleast WoW is).
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Err, you're implying an extra couple files on a CD can make it slower? CD's use a generic filesystem readable from any OS (ISO 9660) and DvDs are similarly compatable. If Blizzazrd's CD's actually are 'slow' on a large scale, it's not because they're PC/Mac compatible.