FX vs 9700 (Doom3)

[Jazzcat]

Hentet ifra web'en:

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Welcome to id Software's Finger Service V1.5!

Name: John Carmack

Email:

Description: Programmer

Project:

Last Updated: 01/29/2003 18:53:43 (Central Standard Time)

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Jan 29, 2003

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NV30 vs R300, current developments, etc

At the moment, the NV30 is slightly faster on most scenes in Doom than the

R300, but I can still find some scenes where the R300 pulls a little bit

ahead. The issue is complicated because of the different ways the cards can

choose to run the game.

The R300 can run Doom in three different modes: ARB (minimum extensions, no

specular highlights, no vertex programs), R200 (full featured, almost always

single pass interaction rendering), ARB2 (floating point fragment shaders,

minor quality improvements, always single pass).

The NV30 can run DOOM in five different modes: ARB, NV10 (full featured, five

rendering passes, no vertex programs), NV20 (full featured, two or three

rendering passes), NV30 ( full featured, single pass), and ARB2.

The R200 path has a slight speed advantage over the ARB2 path on the R300, but

only by a small margin, so it defaults to using the ARB2 path for the quality

improvements. The NV30 runs the ARB2 path MUCH slower than the NV30 path.

Half the speed at the moment. This is unfortunate, because when you do an

exact, apples-to-apples comparison using exactly the same API, the R300 looks

twice as fast, but when you use the vendor-specific paths, the NV30 wins.

The reason for this is that ATI does everything at high precision all the

time, while Nvidia internally supports three different precisions with

different performances. To make it even more complicated, the exact

precision that ATI uses is in between the floating point precisions offered by

Nvidia, so when Nvidia runs fragment programs, they are at a higher precision

than ATI's, which is some justification for the slower speed. Nvidia assures

me that there is a lot of room for improving the fragment program performance

with improved driver compiler technology.

The current NV30 cards do have some other disadvantages: They take up two

slots, and when the cooling fan fires up they are VERY LOUD. I'm not usually

one to care about fan noise, but the NV30 does annoy me.

I am using an NV30 in my primary work system now, largely so I can test more

of the rendering paths on one system, and because I feel Nvidia still has

somewhat better driver quality (ATI continues to improve, though). For a

typical consumer, I don't think the decision is at all clear cut at the

moment.

For developers doing forward looking work, there is a different tradeoff --

the NV30 runs fragment programs much slower, but it has a huge maximum

instruction count. I have bumped into program limits on the R300 already.

As always, better cards are coming soon.

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Doom has dropped support for vendor-specific vertex programs

(NV_vertex_program and EXT_vertex_shader), in favor of using

ARB_vertex_program for all rendering paths. This has been a pleasant thing to

do, and both ATI and Nvidia supported the move. The standardization process

for ARB_vertex_program was pretty drawn out and arduous, but in the end, it is

a just-plain-better API than either of the vendor specific ones that it

replaced. I fretted for a while over whether I should leave in support for

the older APIs for broader driver compatibility, but the final decision was

that we are going to require a modern driver for the game to run in the

advanced modes. Older drivers can still fall back to either the ARB or NV10

paths.

The newly-ratified ARB_vertex_buffer_object extension will probably let me do

the same thing for NV_vertex_array_range and ATI_vertex_array_object.

Reasonable arguments can be made for and against the OpenGL or Direct-X style

of API evolution. With vendor extensions, you get immediate access to new

functionality, but then there is often a period of squabbling about exact

feature support from different vendors before an industry standard settles

down. With central planning, you can have "phasing problems" between

hardware and software releases, and there is a real danger of bad decisions

hampering the entire industry, but enforced commonality does make life easier

for developers. Trying to keep boneheaded-ideas-that-will-haunt-us-for-years

out of Direct-X is the primary reason I have been attending the Windows

Graphics Summit for the past three years, even though I still code for OpenGL.

The most significant functionality in the new crop of cards is the truly

flexible fragment programming, as exposed with ARB_fragment_program. Moving

from the "switches and dials" style of discrete functional graphics

programming to generally flexible programming with indirection and high

precision is what is going to enable the next major step in graphics engines.

It is going to require fairly deep, non-backwards-compatible modifications to

an engine to take real advantage of the new features, but working with

ARB_fragment_program is really a lot of fun, so I have added a few little

tweaks to the current codebase on the ARB2 path:

High dynamic color ranges are supported internally, rather than with

post-blending. This gives a few more bits of color precision in the final

image, but it isn't something that you really notice.

Per-pixel environment mapping, rather than per-vertex. This fixes a pet-peeve

of mine, which is large panes of environment mapped glass that aren't

tessellated enough, giving that awful warping-around-the-triangulation effect

as you move past them.

Light and view vectors normalized with math, rather than a cube map. On

future hardware this will likely be a performance improvement due to the

decrease in bandwidth, but current hardware has the computation and bandwidth

balanced such that it is pretty much a wash. What it does (in conjunction

with floating point math) give you is a perfectly smooth specular highlight,

instead of the pixelish blob that we get on older generations of cards.

There are some more things I am playing around with, that will probably remain

in the engine as novelties, but not supported features:

Per-pixel reflection vector calculations for specular, instead of an

interpolated half-angle. The only remaining effect that has any visual

dependency on the underlying geometry is the shape of the specular highlight.

Ideally, you want the same final image for a surface regardless of if it is

two giant triangles, or a mesh of 1024 triangles. This will not be true if

any calculation done at a vertex involves anything other than linear math

operations. The specular half-angle calculation involves normalizations, so

the interpolation across triangles on a surface will be dependent on exactly

where the vertexes are located. The most visible end result of this is that

on large, flat, shiny surfaces where you expect a clean highlight circle

moving across it, you wind up with a highlight that distorts into an L shape

around the triangulation line.

The extra instructions to implement this did have a noticeable performance

hit, and I was a little surprised to see that the highlights not only

stabilized in shape, but also sharpened up quite a bit, changing the scene

more than I expected. This probably isn't a good tradeoff today for a gamer,

but it is nice for any kind of high-fidelity rendering.

Renormalization of surface normal map samples makes significant quality

improvements in magnified textures, turning tight, blurred corners into shiny,

smooth pockets, but it introduces a huge amount of aliasing on minimized

textures. Blending between the cases is possible with fragment programs, but

the performance overhead does start piling up, and it may require stashing

some information in the normal map alpha channel that varies with mip level.

Doing good filtering of a specularly lit normal map texture is a fairly

interesting problem, with lots of subtle issues.

Bump mapped ambient lighting will give much better looking outdoor and

well-lit scenes. This only became possible with dependent texture reads, and

it requires new designer and tool-chain support to implement well, so it isn't

easy to test globally with the current Doom datasets, but isolated demos are

promising.

The future is in floating point framebuffers. One of the most noticeable

thing this will get you without fundamental algorithm changes is the ability

to use a correct display gamma ramp without destroying the dark color

precision. Unfortunately, using a floating point framebuffer on the current

generation of cards is pretty difficult, because no blending operations are

supported, and the primary thing we need to do is add light contributions

together in the framebuffer. The workaround is to copy the part of the

framebuffer you are going to reference to a texture, and have your fragment

program explicitly add that texture, instead of having the separate blend unit

do it. This is intrusive enough that I probably won't hack up the current

codebase, instead playing around on a forked version.

Floating point framebuffers and complex fragment shaders will also allow much

better volumetric effects, like volumetric illumination of fogged areas with

shadows and additive/subtractive eddy currents.

John Carmack

[muffe]

Kan noen gi meg en kortversjon?

[Biera]

R300 bra. NV30 bra men bråkete. Å kode er gøy.

[Alastor]

R300 bra. NV30 bra men bråkete. Å kode er gøy.

 

Som jeg skulle sagt det selv

[Deidei]

Hmm...DOOM 3 ble jo vist frem med et R300 kort på E3, som jeg tror kjørte ganske bra. R300 er også (som vi ser i testene) ofte bedre med masse effekter på, men NV30 har mer "raw power".

Går ikke an å si før DOOM 3 kommer.

[Robin B]

Hvist FX hadde mye "RAW POWER" så hadde det vært slik Nvidia lovet,ingen ytelses nedgang med AA og AF.Men på grunn av manglende båndbredde vil det få problemer med å kjøre alt på max i spill.

 

 

Helt klart så vil de klare å lage nye raskere drivere men allerede nå ser man ut fra tester at det møter veggen noe nye drivere ikke kan rette opp.

Det er slutt med den tiden at man trenger RAW hastighet,Fps er allerede høy nok og det er et fåtall som bryr seg om 200fps i spill eller 400 fps.

Det vi trenger er grafikk kort som klarer å kjøre alt på max uten at fps faller under hva som er spillbart.