Beautiful image.
I am following this thread closely because I have been thinking about the 
same thing since my image <450bcef5@news.povray.org>. I have no solution to 
offer, but the cogs are turning in my brain  ;-)

Thomas

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"Kirk Andrews" <kir### [at] hotmailcom> wrote:
> Ahhh, there's the goal (and I hadn't thought about bird poop, either!).  The
> trick is getting that grime *just* in the recesses.  Samuel Benge has been
> trying to help me figure out how to do that.  If you have any suggestions
> on how that could be done in POV I'd love to hear it.
>
> -- Kirk
Hmm, the way I would do it, is with absorbing media applied to a cylinder
with a radius just a little bit smaller than the column's. Its density
might be (emi-)spherical so the vast majority of the grime would be located
near the bottom.

Nice pic, BTW. I really like the sky, it looks like a "simple" procedural
sky, but simple procedural skies are anything but simple to get right, IMO.

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Alain wrote:
> Kirk Andrews nous apporta ses lumieres en ce 2007/06/24 07:03:
>> By "jitter", do you mean a slight amount of 
>> turbulence and
>> rotation/translation each time, averaged together?
>>
>> Thanks,
>>
>> Kirk
>>
> That jitter means some small to medium, relative to the dimentions of 
> the object, translation of the object pattern, using many instances all 
> averaged together. You need at least 10 averaged patterns for this to 
> work, and usualy over 100 to get good results. As you don't use 
> reflection, it can still be relatively fast, it become painfuly slow if 
> you have any reflection, particularly if you have self reflection.

And the pattern you derive from this will need to be encapsulated in a 
pigment_pattern{} block, in order to use pigment_maps with it:

pigment{
  pigment_pattern{ my_predeclared_jittered_object_pigment }
  pigment_map{
   [0 some_pigment]
   [1 some_other_pigment]
  }
}

~Sam

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Bill Pragnell wrote:

> 
> If you're using an isosurface or other pigment function-based perturbation,
> you should be able to use the same pattern to provide texturing - limit
> your grime textures to the same regions of the colour map as are used for
> the erosion. 

Agreed.  Pretty much the standard approach.


>The other thing you could try is using the un-weathered base
> object to define an object pattern - only areas inside the object could
> then be textured, i.e. just the weathered sections. I don't know if these
> are practical for your purposes...

I have had some success with this approach also, basically I think I 
jittered the object pattern through several minor scale changes.  I used 
that approach on this chalice image.  In the test shot the green shows 
where the grime colour would fall.

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Attachments:
Download 'img.0132.sm.jpg' (106 KB) Download 'img.0210.jpg' (40 KB)

Preview of image 'img.0132.sm.jpg'

Preview of image 'img.0210.jpg'

"Kirk Andrews" <kir### [at] hotmailcom> wrote:
> I'm working on a scene of some ancient Greece-style ruins.  It has proved to
> be very difficult to get everything to look sufficiently weathered and
> "ruined".  But here is one of the columns as it stands at the moment.  If
> anyone has experience or suggestions in ruining things in POV, I'd love to
> hear them.
>
> - Kirk

I once did broken columns by clipping a regular column with an
iso-landscape.  It took some tweaking, but it looked fairly good.

ADB

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Jim Charter <jrc### [at] msncom> wrote:

> I have had some success with this approach also, basically I think I
> jittered the object pattern through several minor scale changes.  I used
> that approach on this chalice image.  In the test shot the green shows
> where the grime colour would fall.


That's a beautiful chalice Jim! Did I miss the original full size image?

Did you use multiple versions scaled differently for the jittered object
pattern, or one version scaled multiple times?

Mark Slone

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Renderdog wrote:

 >
 > That's a beautiful chalice Jim! Did I miss the original full size image?


Thanks Mark.  No, nothing missed.  That test render was as far as I took 
things.  In fact I don't think the render even includes the grunge 
effect, that was going to be for a second cut which would have also had 
a nice fabric for the display surface.  But I lost interest as usual. 
Funny, Jaime and others have put fabric capabilities into our hands in a 
way that it wasn't when I did the model.  Really should finish this one 
off. Actually the other necessary component, lighting for such a model, 
has also been improved by the efforts of others including Jaime, and I 
am learning it on my current project, so maybe I can roll it all 
together and make a good final render of that chalice.

 >
 > Did you use multiple versions scaled differently for the jittered object
 > pattern, or one version scaled multiple times?
 >

That latter I think.

pigment {
   average
   pigment_map {
     #local I=0;#while(I<90)
       [1
         object {
           object {
             k13_texture_
             #local J = rand(S);
             scale <.9+.2*J,1,.9+.2*J>
           }
           pigment { rgb  CHSL2RGB( <70,.12,.29> ) }
           pigment { rgb  CHSL2RGB( <65,.24,.08> ) }
         }
       ]
       [1
         object {
           object {
             k13_texture_
             rotate y*(-10+20*rand(S))
           }
           pigment { rgb  CHSL2RGB( <70,.12,.29> ) }
           pigment { rgb  CHSL2RGB( <65,.24,.08> ) }
         }
       ]
       #local I=I+1;#end
    }
  }

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