scott <sco### [at] scottcom> wrote:
> 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!

Doh! It's happens too easily around here ;)

> 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.

Well I was thinking that if it worked with straight raymarchers, it would help
with pov's isosurface as well. Editing/compiling pov's source is something I
intend to do... someday. But if somebody wants to beat me to the punch and come
up with an isosurface shadow ray fix (and maybe convince clipka to add it to
UberPOV), it won't hurt my feelings one bit :)

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