PovRay gives a warning when Patch objects are used in a Difference or 
Intersection. (Ver 3.6 & 3.7)
Parse Warning: Patch objects not allowed in intersection.

3.4.2  Finite Patch Primitives
There are six totally thin, finite objects which have no well-defined 
inside. They are bicubic patch, disc, smooth triangle, triangle, polygon 
and mesh / mesh2. They may be combined in CSG union but cannot be used 
in other types of CSG (or inside a clipped_by statement).


except for differencing a patch from a primitive when the use of the 
inverse keyword in the CSG helps.


relevant?

-- 
Regards
     Stephen

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Stephen <mcavoys_at@aoldotcom> wrote:
> PovRay gives a warning when Patch objects are used in a Difference or 
> Intersection. (Ver 3.6 & 3.7)
> Parse Warning: Patch objects not allowed in intersection.

> 3.4.2  Finite Patch Primitives
> There are six totally thin, finite objects which have no well-defined 
> inside. They are bicubic patch, disc, smooth triangle, triangle, polygon 
> and mesh / mesh2. They may be combined in CSG union but cannot be used 
> in other types of CSG (or inside a clipped_by statement).

> I???ve found that all four operations work well with the Bicubic_Patch, 
> except for differencing a patch from a primitive when the use of the 
> inverse keyword in the CSG helps.

  I don't understand how it could work ok with a bicubic patch, because
a bicubic patch has no well-defined interior.

  (Meshes don't have one either, by default, but an interior calculation
for closed meshes can be turned on with the 'inside_vector' feature.
IIRC discs work like planes in that they divide the space into two parts:
the outside and the inside. However, because the visible surface does not
cover the entire partition, they will cause odd behavior when used in CSG.)

-- 
                                                          - Warp

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On 10/12/2010 10:23 AM, Warp wrote:
>    I don't understand how it could work ok with a bicubic patch, because
> a bicubic patch has no well-defined interior.
>

That is my understanding too but I did a couple of tests before posting. 
In the cases I tried it gave me the results I expected. (Assuming that a 
patch can be used in a difference or intersection.)

>    (Meshes don't have one either, by default, but an interior calculation
> for closed meshes can be turned on with the 'inside_vector' feature.
> IIRC discs work like planes in that they divide the space into two parts:
> the outside and the inside. However, because the visible surface does not
> cover the entire partition, they will cause odd behavior when used in CSG.)
>

That is true, discs do cause odd behavior when used in CSG.
-- 
Regards
     Stephen

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> On 10/12/2010 10:23 AM, Warp wrote:
>> I don't understand how it could work ok with a bicubic patch, because
>> a bicubic patch has no well-defined interior.
>>
>
> That is my understanding too but I did a couple of tests before posting.
> In the cases I tried it gave me the results I expected. (Assuming that a
> patch can be used in a difference or intersection.)
>
>> (Meshes don't have one either, by default, but an interior calculation
>> for closed meshes can be turned on with the 'inside_vector' feature.
>> IIRC discs work like planes in that they divide the space into two parts:
>> the outside and the inside. However, because the visible surface does not
>> cover the entire partition, they will cause odd behavior when used in
>> CSG.)
>>
>
> That is true, discs do cause odd behavior when used in CSG.

You may difference FROM a patch. It will only cut the parts of the patch 
that are not inside the solid object.
An intersection should give the same result: Only the part of the patch 
that intersects the solid object if the patch is the first object.

The message is mostly a warning as the result may not be what you expect.


Alain

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On 10/12/2010 4:27 PM, Alain wrote:
> You may difference FROM a patch. It will only cut the parts of the patch
> that are not inside the solid object.
> An intersection should give the same result: Only the part of the patch
> that intersects the solid object if the patch is the first object.
>

I think that Jim H should update the documentation. If he has nothing 
else to do. :-P

> The message is mostly a warning as the result may not be what you expect.
>

Maybe the warning should say something like:



-- 
Regards
     Stephen

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On 12/11/2010 10:01 AM, Stephen wrote:
> Maybe the warning should say something like:



howz zat ...
http://wiki.povray.org/content/Documentation:Reference_Section_4.1#Finite_Patch_Primitives

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On 11/12/2010 5:31 PM, Jim Holsenback wrote:
> On 12/11/2010 10:01 AM, Stephen wrote:
>> Maybe the warning should say something like:


>
> howz zat ...
>
http://wiki.povray.org/content/Documentation:Reference_Section_4.1#Finite_Patch_Primitives
>

That was quick, Jim. :-D

I actually thought that, that message should be the Pov-Ray warning.
What I think the documentation should say is along the lines of.

Solids may be differenced from bicubic patches with the output giving 
the expected results. That is the solid object cuts a hole in the patch.
Differencing a bicubic patch from a solid may give unexpected results, 
especially if the inverse keyword is used.

Intersecting a solid and a bicubic patch will give the expected results. 
That is the parts of the patch that intersect the solid object will be 
visible.

Merging a solid and a bicubic patch will remove the parts of the bicubic 
patch that intersect the solid.

<end>

It is a bit wordy and could use some sub-editing.



-- 
Regards
     Stephen

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On 12/11/2010 02:21 PM, Stephen wrote:
> It is a bit wordy and could use some sub-editing

more better?
http://wiki.povray.org/content/Documentation:Reference_Section_4.1#Finite_Patch_Primitives

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On 11/12/2010 7:33 PM, Jim Holsenback wrote:
> On 12/11/2010 02:21 PM, Stephen wrote:
>> It is a bit wordy and could use some sub-editing
>
> more better?
>
http://wiki.povray.org/content/Documentation:Reference_Section_4.1#Finite_Patch_Primitives
>

Much more gooder, actualy :-)

I just hope I'm right. :lol:

-- 
Regards
     Stephen

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