...
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.
