From: Tristan Croll (tristan.croll_at_qut.edu.au)
Date: Thu Aug 21 2014 - 18:33:44 CDT

Hi John,

I'll reply off-list with a couple of example images, but I don't think it's a problem with any particular switches as such - the same effect arises when I use Tachyon directly from the VMD render dialog. I think what's going on has to do with how the depth cueing fog and transparency is calculated... The fog is done by simply fading the object colours towards the background colour with distance, so the net effect is essentially a washing-out/loss of contrast, correct? Then, when a distant object is viewed through a transparent surface it should be darkened according to the surface's opacity, but it should still maintain that same level of contrast - just as when looking at objects in a real fog, putting on sunglasses doesn't make them any clearer. Instead, in Tachyon a transparent foreground object in front of a "fogged" background object increases its contrast, and the effect is cumulative: a background object whose ray has passed through 2-3 (or more) transparent surfaces ends up looking more-or-less compl
etely unfogged. I'm trying to visualise local structures in the context of an X-ray map, so the combination of this effect and the complex surface becomes very confusing.

Cheers,

Tristan

-----Original Message-----
From: John Stone [mailto:johns_at_ks.uiuc.edu]
Sent: Thursday, 21 August 2014 11:35 PM
To: Tristan Croll
Cc: Norman Geist; VMD Mailing List
Subject: Re: AW: vmd-l: Tachyon transparency and depth cueing

Tristan,
  If you're running Tachyon external to VMD, you need to be sure
to use the -trans_vmd flag, otherwise it uses a different shading
algorithm for transparent objects. It wasn't clear from your email
how you were running it, but it would be helpful if you completely
specified what you had previously tried (all flags, etc). Also,
which version of VMD are you running?

Cheers,
  John Stone
  vmd_at_ks.uiuc.edu

On Thu, Aug 21, 2014 at 10:15:30AM +0000, Tristan Croll wrote:
> After a bit more tinkering, it seems that setting the depth cueing mode to
> Exp2 does the trick. For the other modes, (most) objects viewed through a
> transparent surface are shaded as if they're in the foreground.
>
>
>
> Tristan Croll
> Lecturer
> Faculty of Science and Technology
> Institute of Health and Biomedical Engineering
> Queensland University of Technology
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> On 21 Aug 2014, at 7:43 pm, "Norman Geist"
> <[1]norman.geist_at_uni-greifswald.de> wrote:
>
> The problem is that depth cueing depends on the background or?
>
>
>
> Norman Geist.
>
>
>
> Von: [2]owner-vmd-l_at_ks.uiuc.edu [[3]mailto:owner-vmd-l_at_ks.uiuc.edu] Im
> Auftrag von Tristan Croll
> Gesendet: Donnerstag, 21. August 2014 09:15
> An: [4]vmd-l_at_ks.uiuc.edu
> Betreff: vmd-l: Tachyon transparency and depth cueing
>
>
>
> Hi,
>
>
>
> Just wondering: is there a "magic" command-line switch in Tachyon which
> gets depth cueing working correctly with transparency? Every time I
> attempt to render this combination using the standard switches, anything
> viewed through a transparent surface loses its "fog". I find that the
> trans_raster3d helps somewhat, but is still not infallible.
>
>
>
> Thanks,
>
>
>
> Tristan
>
>
>
>
>
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