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.

-- 
Mike Williams
Gentleman of Leisure

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