Hartmut Wagener wrote:

> Mike Williams wrote:
> 
>> Wasn't it Hartmut Wagener who wrote:
>>
>>> Mike Williams wrote:
>>>
>>>> Wasn't it Hartmut Wagener who wrote:
>>>>
>>>>
>>>>> Hi,
>>>>>
>>>>> i have written a converter to convert from the stl-format to 
>>>>> povray. The first version of this converter is now some years old, 
>>>>> the last one is ready now written in java.
>>>>>
>>>>> But i have some problems with rendering the scenes. In 
>>>>> binary.images you can see two images, the one showing the result of 
>>>>> the rendering, the other showing the calculated norm-vectors that 
>>>>> really look as they should be for a smooth scene.
>>>>>
>>>>> When gouraud-shading a 3d-scene there might be a error called 
>>>>> mach-banding, but with phon-shading by interpolating the 
>>>>> norm-vectors in stead of the shading-values this mach-banding 
>>>>> should dissapear.
>>>>>
>>>>> But you can see that some triangles are not calculated smooth. I 
>>>>> can post the stl-file (shortened) and the java-prog for 
>>>>> examination, perhaps it is my fault and i am not able to see it ...
>>>>>
>>>>> The problems appear in all of my stl2pov-versions, not only in the 
>>>>> new java-version ...
>>>>
>>>>
>>>>
>>>> Although the displayed calculated normal vectors look as if they're
>>>> pointing in roughly the correct directions, I suspect that they might
>>>> not be exactly correct.
>>>>
>>>> In particular, I suspect that some of the vertices have slightly
>>>> different normals in each of the smooth triangles they are used in. 
>>>> This
>>>> sort of thing can happen if you're calculating the normals one triangle
>>>> at a time. When I do this sort of thing, I now calculate the normals 
>>>> for
>>>> each of the vertices first and store them in an array.
>>>>
>>>
>>> The problem is the thing Slime mentioned in the p.b.images-section.
>>> The CAD-software creates stl-files containing triangles. In the past 
>>> the software creates almost triangles and some quads. Now these quads 
>>> are splitted in to two triangles.
>>> There are two possible solutions to split a "normal" quad in to two 
>>> triangles. In most cases you don't have to care what solution is used 
>>> to split. But in some cases, the one solution produces two triangles 
>>> of almost the same size and the other solution creates two triangles, 
>>> one of them very small and the other very big.
>>> The last solution is not good for smooth shading.
>>> To avoid this i have to find these triangles and transform them to 
>>> the other splitting-solution. Hugh, that is much work to do ...
>>
>>
>>
>> It shouldn't be necessary to change the triangulation, but what you need
>> to do is change the way you calculate the normals.
>>
>> I guess that you currently calculate the normals by looking at the
>> positions of some nearby vertices. In the case of the thin triangles,
>> the closeness of one of those vertices is causing the problem. You end
>> up with most of the curvature happening in the thin triangles and the
>> normal triangles end up being too flat.
>>
>> What you might be able to do is to add a check to see if any of the
>> points you are using to calculate the normal is within a certain minimum
>> distance from the vertex you are calculating the normal for. If it is
>> too close, then use another point further away. This might lead to the
>> thin triangles becoming a little too flat, but that's going to look a
>> lot better than having the large triangles being too flat.
>>
> 
> I am not calcualating the normals for the triangles, they are given by 
> the stl-output. I only normalize them (length 1) if neccessary. What i 
> do is to interpolate the normals for the endpoints of the triangles.
> Perhaps i can use a weighting depending on the distance of the 
> endpoints. This weighting should only be used if the distances of points 
> differ a lot. Hugh, thats hard ...
> But i think that the small triangles and the given normals are bad 
> calculated by the cad-software.
> I will have some checks on that by calculating the normals by my own and 
> comparing them with the given normals ...
> 
> Thanks
> 
> Hartmut

See if PoseRay may be of help to you. It can calculate normals by direct 
averaging and area weighted. If the software you are using exports in 
3DS, OBJ, DXF, RAW or LWO PoseRay may be able to read it. You will also 
have control on the threshold angle and what regions to smooth.

PoseRay home:
http://user.txcyber.com/~sgalls/

later,

FlyerX

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