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 Features
Volumetric
Rendering in Realtime
Camera in the Fog
There is now one more neat trick to perform - allowing the camera to enter
the fog. Actually, the fog clipping plane and the geometry clipping plane
are aligned - then the trivial case will already work At some point -
parts of the fog volume will be culled against the near clipping plane.
Since the front plane is by default cleared with 0s (indicating that those
pixels are 0 depth from the camera) than when the clipping of the front
volume begins to occur - the pixel's being rendered on those polygons
would have been 0 anyway.
There is one more problem that crops up. To accommodate an object moving
through the fog - two steps were added, one of which acted as the front
side of the fog. But if the camera is inside the fog volume, then a key
assumption has been broken. Not all of the fog volume is actually rendered
since part of the fog volume is clipped away. This means that Step 4 in
the above algorithm now becomes a major problem - as it becomes the effective
front side of the fog. The polygons of the fog volume can no longer replace
those pixels set by the scene since the fog volume polygons have been
(at least partially) culled away.
The solution to this is simple. Step 4 was added specifically to allow
objects that were only partially obscured by fog to render correctly,
since any pixel rendered in step 4 would be replaced by step 5 if it were
in the fog. Obviously, if the camera is inside the fog - then all parts
of an object are partially obscured by fog. Thus, step 4 should be disabled
completely. The following is a complete and general implementation of
the rendering of uniform density, convex fog hulls.
- Clear
the buffer(s)
- Render
the scene into an off-screen buffer A, encoding each pixel's w depth
as its alpha value- Z Buffering enabled.
- Render
the backside of the fog volume into off-screen buffer A, encoding each
pixel's w depth.
- If the
camera is not inside fog, render the scene into an off-screen buffer
B (or copy it from buffer A before step 2 takes place), using the w
depth alpha encoding. Otherwise, skip this step.
- Render
the front side of the fog volume into off-screen buffer B with w alpha
encoding. If step 4 was executed, the fog volume should be in front
of parts of the scene that are obscured by fog, it will replace them
at those pixels. If step 4 was not executed, then some of these polygons
were culled away.
- Subtract
the two buffers in screen space using the alpha value to blend on a
fog mask.
Further Optimizations
and Improvements
Cleary,
this is a simple foundation for fog - there are numerous improvements
and enhancements that can be made. Perhaps highest on the list is a precision
issue. Most hardware allows only 8 bit alpha formats. Because so much
is dependent on the w depth - 8 bits can be a real constraint. Imagine
a typical application of a volumetric fog - a large sheet of fog along
the ground. No matter what function used to take the depth and render
it into fog - there remains a virtual far and near clipping plane for
the fog. Expanding these planes means either less dense, or less precise
fog, while keeping them contracted means adjusting the fog clipping planes
for each fog volume rendered.
On new and upcoming hardware, however, there is a trick with the pixel
shaders. Why not keep some more bits of precision in one of the color
channels, and use the pixel shader to perform a carry operation? At first
glance it appears that 16 bit math easily be accomplished on parts designed
to operate at only 8. However, there is one nasty limiting factor - on
a triangle basis - the color interpolators work at only 8 bits. Texture
coordinates, on the other hand, typically operate at much higher precision,
usually at least 16 bits. Although texture coordinates can be loaded into
color registers, the lower bits of precision are lost . An alternative
is to create a 1D step function filled texture, with each texel representing
a higher precision value embedded in the alpha and color channels. Unfortunately,
the precision here is usually limited to the size of a texture.
Once the issue of higher precision is addressed, it is possible to render
concave volumes even with limited 8-bit hardware. This must be accomplished
by either rendering concave fog volumes as a collection on its convex
parts, or by summing the multiple entry points of fog and subtracting
away the multiple exit points. Unfortunatly, the high precision trick
will not work for the latter approach since there is no way to both read
and write the render target in the pixel shader. Although a system of
swapping between multiple buffers carefully segmented to avoid overlap
might work, this latter approach will probally not be feasible until hardware
allows rendering into 16 bit formats (i.e. a 16 bit alpha format).
Finally, there are many artistic enhancements that can be made on this
kind of volumetric effect. To make volumetric light, for instance, the
alpha blends modes can be changed to additive rather then blend, thereby
adding light to the scene. Decay constants can also be modeled in this
way, to accomplish some surface variations of fog density.
Additionally, fog volumes can be fitted with textures on top of them that
operate much like bump maps do - varying the height of the fog at that
point without changing the actual geometry. To create an animated set
of ripples in fog, for instance, one can take a ripple depth texture and
move it along the surface of the fog volumes, and adding it to the w depth.
Other texture tricks are possible as well - noise environment maps can
be coupled to fog volumes to allow primitive dust effects.
And of course, it can be quite fun to draw the fog mask without actually
drawing the object - creating an invisible object moving through the scene.
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