On 05/18/2018 10:16 PM, clipka wrote:
> Am 18.05.2018 um 19:00 schrieb William F Pokorny:
> 
>> Chrisoph added this bit of code in blob.cpp to avoid visible seams:
>>
>>    DBL depthTolerance = (ray.IsSubsurfaceRay()? 0 : DEPTH_TOLERANCE);
>>
>> I've been running with DEPTH_TOLERANCE at 1e-4 for a long time in my
>> personal version of POV-Ray which fixes all the blob accuracy issues
>> except for the subsurface one.
> 
> For subsurface rays it's safer to keep it at exactly 0.
> 
...
> 
> This problem is absent in rays earmarked as subsurface rays (presuming
> we have high enough precision) because such rays are never shot directly
> from the original intersection point, but a point translated slightly
> into the object. So subsurface rays already carry an implicit
> depthTolerance with them.
> 

Aside: 'exactly 0' unless we change blob.cpp code means >0.0.

I woke up this morning thinking more about your description of SSLT and 
though I picked up no major differences in result, results were not 
identical - but I didn't expect them to be. I'd not run the returned 
root depth threshold at both 0.0 and 1e-4 (MIN_ISECT_DEPTH) and compared 
results.

The top row in the attached image is without sturm the bottom with. The 
differences are very subtle, but can be seen after being multiplied by 50.

The left column is less my recent changes so min_dist and the return 
depth >0.0 with sturm automatically getting used often in the upper left 
image.

The middle column is the updated code so the middle top now using 
strictly solve_quartic() making the differences on the right are 
greater. The bottom, sturm always row best shows the effect of not 
returning roots <= 1e-4 from the ray origin.

My position is still the difference isn't large enough to be of concern, 
but if so, maybe we should more strongly consider going to >0.0 or some 
very small values for all shapes. Doing it only in blob.cpp doesn't make 
sense to me.

Aside: MIN_ISECT_DEPTH doesn't exist before 3.7. Is it's creation 
related to the SSLT implementation?

Bill P.

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