On 08/07/2015 21:00, Samuel Benge wrote:
> Orchid Win7 v1 <voi### [at] devnull> wrote:
>> On 08/07/2015 06:14 PM, Samuel Benge wrote:
>>> scott<sco### [at] scottcom>  wrote:
>>>>> The attached shows artifacts present at various accuracies for the function
>>>>> sqrt(x*x+y*y+z*z)-1.>
>>>>
>>>> Assuming you've got no pattern displacement in any of those images, it
>>>> looks to me like it's something to do with the shadow ray being started
>>>> too close to the surface (and hence the tracer immediately thinks the
>>>> shadow ray has been blocked by the isosurface itself).
>>>
>>> I should have guessed that, since I ran into shadow ray problems last summer
>>> with the raymarching stuff. The shadow ray's origin had to be spaced from the
>>> initial hit point (by backing it up toward the camera or outward from the
>>> surface), or else artifacts would appear.
>>
>> Question: Does the problem go away if you make the spheres bigger? Like,
>> say, 10 units across instead of just 1?
>
> I just ran some tests. Scaled the isosurface and zoomed the camera; adjusted the
> function radius and scaled the object; multiplied/divided the function to change
> the gradient's falloff rate... all at values between .01 and 100. No change. You
> end up just needing to adjust the accuracy and max_gradient, but the end result
> is always the same.
>
>> As I recall, POV-Ray tends to dislike rendering either very tiny or very
>> large objects. Floating-point precision issues and all.
>
> I was hoping that with 64 bit would come a more precise epsilon value, but it
> didn't turn out that way :( Was it a misunderstanding on my part, or to keep
> POV-Ray rendering the same on different architectures, I wonder?

This is completely off-topic!

If you start the shadow ray a little further out, you'll just get the 
same artifacts but in a slightly different place. With a point light 
casting a hard shadow-line on a curved surface like that it's always 
going to amplify any numerical precision issues, as all the pixels 
around the shadow-line will have very similar shadow ray direction 
(which are very close to tangent to the surface).

A realistic solution is to use a very small area light, to average out 
the area around the shadow-line.

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