Am 10.09.2017 um 21:22 schrieb jr:
> hi,
> 
> On 10/09/2017 16:03, clipka wrote:
>> Am 10.09.2017 um 16:01 schrieb Stephen:
>>> On 10/09/2017 14:27, clipka wrote:
>>>> Am 10.09.2017 um 15:13 schrieb Stephen:
>>>>> If I trace an object. How can I get the colour of the intersection of
>>>>> the ray and object. As far as I can see trace() only returns the normal.
>>>> You'll need to know the pigment of the object, and invoke `eval_pigment`
>>>> on the intersection point you get...
>>> Thanks, I had a blank in my memory.
>>>> ... and hope that the pigment isn't UV-mapped,
>>> Hope is not involved because there is a good chance I would want to do
>>> that. Is it at all possible?
>> No; not with the current versions of POV-Ray. The `trace` function would
>> have to be extended to report the UV coordinate of the intersection, so
>> that you could then feed it into `eval_pigment` instead of the 3D
>> coordinate.
> 
> do I understand this correctly?  if I trace some object with a regular
> pigment it will work, but the trace will fail if the object's
> pigment/colour is "not simple"?  seems .. strange.

No, the `trace` function works perfectly fine for what it was designed
for: It gives you the point in 3D space where a given ray intersects a
given geometric object. In and of itself, it has nothing to do with
pigments whatsoever.

Likewise, the `eval_pigment` function works perfectly fine for what it
was designed for: It gives you the colour of a given pigment for a given
point in 3D space. In and of itself, it has nothing to do with geometry
whatsoever.

And that's where the problem with UV-mapped textures arises: Since
`trace` knows nothing about textures, and `eval_pigment` knows nothing
about geometry, the two can only be combined for good effect if there is
a direct mapping between geometry and pigment via 3D XYZ space. But with
UV-mapping that's not the case, and hence the /combo/ of the two
functions breaks down there.

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