<p>Those are <a href="https://en.wikipedia.org/wiki/Moir%C3%A9_pattern" rel="nofollow noreferrer">moire patterns</a>. They are  an aliasing artifact that usually occurs when sampling on a regular grid. Did you jitter the positions of your samples? If you just sampled an evenly spaced 10x10 grid within each pixel, that could explain it. Also, numerical errors or inaccuracy could it.</p>

CC BY-SA 4.0
2019-04-11T06:12:12.253Z
To be honest it looks more like moire and insuficent shadow bias
joojaa
CC BY-SA 4.0
2019-04-11T16:02:19.893Z
Interesting! I've not seen that happen before. Very cool.
user1118321
CC BY-SA 4.0
2019-04-15T15:02:32.933Z
I also think it may be a shadow bias issue, as I've seen this happening on some of my renders. Just bias the ray starting position an epsilon away from the surface when tracing the reflected rays.
vgs
CC BY-SA 4.0
2019-04-11T04:42:12.117Z
2019-04-12T02:27:54.017Z
2019-04-12T02:27:54.017Z
user1118321
<p>I followed the book <a href="http://www.realtimerendering.com/raytracing/Ray%20Tracing%20in%20a%20Weekend.pdf" rel="nofollow noreferrer">Ray Tracing in One Weekend</a> and implemented a simple ray tracing procedure. There are 100 <strong>random</strong> samples every pixel for antialiasing, but the render result looks weird: There is a unnatural circle at the center of the screen, and a strange line lies on the center of the ground, Why?</p>

<p><img src="https://i.sstatic.net/uMb3s.png" alt="render result"></p>

CC BY-SA 4.0
2019-04-11T06:38:24.300Z
Seems like either numerical error or you have a mistake in your impl.
lightxbulb
CC BY-SA 4.0
2019-04-11T09:00:04.093Z
@lightxbulb You are right! The numerical error caused self intersection. I have fixed it now. https://i.imgur.com/mGwTjGA.png
YuChang
CC BY-SA 4.0
2019-04-12T13:25:40.983Z
@YuChang you should explain what the problem was and how you fixed it in an answer
Sebastián Mestre
<raytracing>
Why the ray tracing result looks weird?
YuChang
# Why the ray tracing result looks weird?

(Asked by: username_0 on 2019-04-11)

I followed the book [Ray Tracing in One Weekend](http://www.realtimerendering.com/raytracing/Ray%20Tracing%20in%20a%20Weekend.pdf) and implemented a simple ray tracing procedure. There are 100 **random** samples every pixel for antialiasing, but the render result looks weird: There is a unnatural circle at the center of the screen, and a strange line lies on the center of the ground, Why?

![render result](https://i.sstatic.net/uMb3s.png)

**Tags:** raytracing

> **Comments:**
> **username_1**: Seems like either numerical error or you have a mistake in your impl.  
> **username_0**: @username_1 You are right! The numerical error caused self intersection. I have fixed it now. https://i.imgur.com/mGwTjGA.png  
> **username_2**: @username_0 you should explain what the problem was and how you fixed it in an answer  

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## 1 answers

### Answer by username_3 on 2019-04-11. Score: 1

Those are [moire patterns](https://en.wikipedia.org/wiki/Moir%C3%A9_pattern). They are an aliasing artifact that usually occurs when sampling on a regular grid. Did you jitter the positions of your samples? If you just sampled an evenly spaced 10x10 grid within each pixel, that could explain it. Also, numerical errors or inaccuracy could it.

> **Comments:**
> **username_4**: To be honest it looks more like moire and insuficent shadow bias  
> **username_3**: Interesting! I've not seen that happen before. Very cool.  
> **username_5**: I also think it may be a shadow bias issue, as I've seen this happening on some of my renders. Just bias the ray starting position an epsilon away from the surface when tracing the reflected rays.  

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