Citation :
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.
-
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
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