Hi Bob,
I agree to what you said regarding fog_type=1, but why do you think the ground
fog (fog_type=2) is not a solution? It's not just ground fog but fog along the
up vector.
So my idea was that with an up vector that points in the same direction as the
vector from the camera location to the 'look_at' point you would get is very,
very weak up to some point where it has a high constant value. My understanding
is, that you could achive this when you set distance so that the fog is
generally very weak; set fog_offset so that the fog hides the protein at the
desired location; set fog_alt to adjust how fast the fog hides the protein. But
then again, I was not able to get the desired results...
Alexander
fog {
distance 1
color rgbft <0,0,0,0.0,0.0>
fog_type 2
up <0,0,1>
fog_offset ZCoordinateOfProteinCenter /* Constant below this */
fog_alt 0.5 /* Decay rate */
}
----- Ursprüngliche Mail -----
> Von: "Robert Hanson" <[email protected]>
> An: [email protected]
> Gesendet: Dienstag, 15. November 2011 16:02:57
> Betreff: Re: [Jmol-users] fog/zShade in POV-Ray
> Well, after a bit of experimentation, I'd say this is probably not
> going to work. POV-Ray assumes that you are IN the fog yourself,
> looking around. So everything in a scene is foggy. There is no way
> around that. Certainly fog_type=2 (ground fog) is not a solution.
>
> Jmol uses a very different idea -- you are looking at the model, and
> it is partially obscurred by fog. This is much more dramatic, and much
> more useful. The key setting is zSlab, which is assumed to be about
> 300 in POV-Ray but is critically important to be around 50 for Jmol.
> (I just checked in a bug fix that was setting zSlab to 0 by default in
> Jmol.)
>
> If you want something that is about the same in both, try this:
>
> in Jmol:
>
> background white
> set zSlab 300
> set zDepth -1000
> set zShadePower 3
> set zShade
>
> Then in POV-Ray use:
>
> fog {
> distance 1500
> color rgb <1,1,1>
> fog_type 1
> }
>
>
>
> Bob
>
>
> ps. For the record....
>
> The algorithm for fog in Jmol is this, applied to each pixel:
>
> f = (zDepth - z) / (zDepth - zSlab) // fraction of depth, from zDepth
> (0) to zSlab(1)
>
> p = f^zShadePower // default: f^3
>
> pixel_color = background_color + p * (object_color - background_color)
>
> = p * object_color + (1 - p) * background_color
>
> Comparing that to POV-Ray:
>
> PIXEL_COLOR = exp(-d/D) * OBJECT_COLOR + (1-exp(-d/D)) * FOG_COLOR
>
> where d is the pixel depth from the camera and D is the "fog distance
> parameter"
>
> Note that these would be identical if
>
> FOG_COLOR = background_color
> p = exp(-d/D)
>
> Of course, that's not what p is. The main difference here is that it
> is quite possible in Jmol to have an object disappear. That is
> intentional. In Jmol, we absolutely allow that, for good reason -- for
> good effect. zShadePower was developed for the infinite crystal view
> in the Epcot Touch-A-Molecule display. We need that, because we only
> have a certain number of unit cells in actuality, and it is very
> effective. In contrast, it's not possible in POV-Ray to get full
> merging with background color, probably because that is more
> realistic, and the POV-Ray formula results in the color being 1/e of
> pixel_color "at depth" (0.368) instead of 0 * pixel_color, as in Jmol.
>
> Both of these algorithms result in a fog that gets stronger further
> from you, allowing objects closer to the viewer to be less affected
> than distant objects. The power of that effect is more adjustable in
> Jmol than in POV-Ray, and the formula is different, because I needed
> full disappearance within a finite distance, which I suppose is not
> realistic so is not modeled by POV-Ray, and I needed to have the
> observer and possibly part of the model outside of the fog.
>
>
>
>
> --
> Robert M. Hanson
> Professor of Chemistry
> St. Olaf College
> 1520 St. Olaf Ave.
> Northfield, MN 55057
> http://www.stolaf.edu/people/hansonr
> phone: 507-786-3107
>
>
> If nature does not answer first what we want,
> it is better to take what answer we get.
>
> -- Josiah Willard Gibbs, Lecture XXX, Monday, February 5, 1900
>
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