LuxCoreRender Forums

While setting up some scene, I came across a strange behaviour with a mirror and Sky-background. Looking somewhat perpendicular at a mirror gives me the following type of artefact:
Blend file attached, consisting of a clean setup of only a mirror and camera, the rest is left pretty much default.
Here is what I can say so far from quick playing around with it:

- happens with all bidir/path + /sobol/MLT/Random combinations
- The artifact is half the size using BiDir than using path
- It can be different between preview and actual render
- Mirror and Glossy material behave the same
- The pattern heavily depens on the camera-mirror distance
- The pattern heavily depends on clipping start, but not clipping stop
- camera FL changes nothing other than zooming in on it
- Using volumes or not makes no difference
- The pattern changes with orientation of the camera/mirror pair. It then appears to be partly cut off (by what??). You can test this by selecting camera+mirror and rotating them around the origin.

What I haven't tested a lot yet is various light sources, i.e. if this only an issue with the background sky.

Your stuff is exactly at the world origin. This causes floating point precision problems.
Solution:
- move everything a bit
- or change the min. epsilon (multiply it by 10)

ok thanks, that makes sense and moving stuff around changes things.
It seems to me that it is more of a relative issue though. When I shift the mirror to x=100, I can still get the pattern, although the camera has to be much further away (>10000, compared to 150 previously). In this case, your min.-epsilon method only helps if I multiply it by 1000, whatever all the consequences of adjusting that parameter are...

I think due to the way IEEE floats are specified, you will always have "bad spots" in the coordinate grid.

CodeHD wrote: ↑Wed Jan 02, 2019 7:24 pm It seems to me that it is more of a relative issue though.

Floating point number precision is not linear or uniform, for instance, it is a lot higher in some range than others. The distance between 2 consecutive values also keep increasing farther you are from the 0.0 and they exhibits many other strange/counter intuitive behaviors when compared to true real numbers.
Unfortunately, it is unavoidable when you approximate the infinite set (i.e. Real numbers) with just 2^32 possible different values.

Dade wrote: ↑Wed Jan 02, 2019 8:47 pm Floating point number precision is not linear or uniform, for instance, it is a lot higher in some range than others. The distance between 2 consecutive values also keep increasing farther you are from the 0.0 and they exhibits many other strange/counter intuitive behaviors when compared to true real numbers.
Unfortunately, it is unavoidable when you approximate the infinite set (i.e. Real numbers) with just 2^32 possible different values.

Yeah, Im aware of that, I worded it a bit too mich like a guess I only really wanted to point out the min-epsilon behaviour I observed in case anyone else comes across and reads this.