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  1. #1
    Oct 1999

    Sobre DOOM3/GF

    Olha oq eu achei: (bluesnews)

    ame: John Carmack
    Description: Programmer
    Jan 29, 2003
    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

    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

    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

    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

  2. # Publicidade

  3. #2
    Autor do tópico
    Oct 1999

    Vejam este review...
    pqp..a nova FX ta mto animal..

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