Mar

Black faces error from camera clipping

I tripped over a nasty messy stuff on this new project I’m working on. I’m setting up an aerial view of an urban proposal and have some simple geometry with lots of instancing.

Here is a simplified wire of the scene:

First test render looked like shown above and took me over a precious hour to figure out what’s wrong. First I checked the geometry, and there seemed to be no overlapping faces. Then I erased some of it and modeled it again. Then I checked the sun and the AO. Next on the suspect line were the mirrors and arrays, thought maybe some unexpected merging had occurred. As in Murphy’s laws, the right reason came to me last, and it was the camera clipping setting. The camera is positioned at a distance of about 700m (= 700 blender units) from the scene and first I had changed the end clipping at 1000 to get it all in. To get rid of the ugly stuff I changed the near clipping from the default 0.1 to 1, then it cleared up!

I don’t know if this is a bug or maybe is there an underlying explanation that I don’t understand*. I myself felt really bugged by wasting a lot of time, but there was a bright side to it too, a new addition to the bug part of my “how to” blog section 🙂

*Check explanation in comments section, by reader Sparraza! So now that’s clear, I will move the near clipping plane closer to my scene.

Categories : Blender internal render engine, bugs, Rendering

Daniele De Luca

Hi! I’m glad that you solved this issue in your scene! The camera clipping is the biggest reason for flickering effects, but the explanation is quite simple:

Blender calculates the rendering scene from the start to the end of the clipping, so it interpolates Z position of 3d objects subdividing the camera Z-depth of the scene with a number, a floating point number.
Those numbers represent the position of the vertices of the geometry and they are always approximated to the near finite number, so with those approximation some vertices are seen like coincident… and here it is the flickering!
(eg: a vertex at 2/3 of the z-depth is seen like at 0.66667 and maybe is over a vertex placed precisely at 0.66667)

The camera clipping parameter must be regulated to contain the scene progressively increasing the Start and reducing the End in order to help Blender to improve the approximation

I’ve also made an article to avoid those type of errors, like backface culling or flickering, it’s in italian but you could translate with Google translate or interpreting it from the images 😀

http://discoverylinux.wordpress.com/2011/10/08/guida-eliminare-il-flickering-in-blender-2-5/

Byebye!
Sparazza
Oana

Thanks for explaining this, Sparazza! I felt it was not quite a bug but now it makes sense. I also imagine this happens in other software not just Blender so probably I met with it in max but forgot about it (I did scenes as this before, only not recently, not in Blender). I’ll edit the post a bit in light of this.
I also liked your article, it’s very clear!
Daniele De Luca

Too much gratitude! thanks also for the visibility!!

Byee
Sparazza
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Marco

Dear Blender Mama,

Thank you!!! I’m new to blender and was stumbling on this one too, I was looking everywhere (else), climbing the walls until I found your article. Thank you, thank you, thank you for sharing.

Regards,
Marco

Black faces error from camera clipping

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