On 25-9-2014 13:06, Ralf Jonas wrote:
> Hi,
>
> although I'm not very new to povray (I already used it when it was
> DKB-Trace), I'm very impressed by this draft made by a friend of mine in
> Blender.
> It's a kind of bathroom wall with tiles. Each tile in the top row and
> the second row from bottom was made with a height field.
> My question points to the dirt on the wall. It seems, like the dirt
> drops down from above and fades out on its way down.
> Can you tell me, how to do this in povray?
>
> Thanks in advance
> Ralf
>
The most simple setup would be with a layered texture. The underlying 
one being the tiles; overlying that a simple texture with a gradient 
pigment_map with a strong vertical turbulence would do the trick. The 
most 'difficult' part would be to scale and translate that texture 
correctly but knowing the dimensions of the wall would help. For instance:

texture {tile}
texture {
   gradient y
   pigment_map {
     [0.2 a transparent pigment]
     [0.6 a dirt pigment]
   }
   warp {turbulence <0, 0.6, 0>}
   scale ....
   translate ....
}

Thomas

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