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Possible Cycles render artifact
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Description

Windows 7 Pro 64-bit
Intel Core i7-4770K CPU 3.50GHz
ASUS nVidia GeForce GTX 760 2GB GDDR5 DirectCU Mini

Broken: Blender 2.70 19e627c

There appears to be a cycles render artifact in my animation. Please refer to the attached photo.

The arrow points to a thin, blue line runnning across the form of the object.
That line is not a reflection of any object in the scene. Also, it does not appear to be linked to the geometry of the object.
I have tried rotating the object, but the render artifact remains.

It is possible that this is somehow related to the material definitions (perhaps I have done something wrong??), but I have no way of knowing this.
As you can see, the 'artifact' does not appear in a render of the same frame in Blender 2.69.
The render from 2.69 appears to be completely different and I have no way of knowing if the underlying code has been changed between 2.69 and 2.70 to produce those visual changes.

If you would be so kind as to examine this matter. Thank you.

Details

Type
Bug

Event Timeline

Ignatz (ignatz) added a project: BF Blender.
Ignatz (ignatz) set Type to Bug.
Ignatz (ignatz) added a subscriber: Ignatz (ignatz).
Ignatz (ignatz) created this task.
Ignatz (ignatz) raised the priority of this task from to Needs Triage by Developer.

The world uses an environment texture (17167972-lg.jpg) that we do not have, so it's hard to reproduce. Could you please upload the image? Maybe also strop_001.tif.
I get a purple image due to the missing file and that might be (partially) the cause of the problem.

After removing the reference to the missing images, I get the same difference between the program versions regarding the brighter spots, like in your screenshot. So, I can confirm this. (on Win7/64)

Thomas Dinges (dingto) triaged this task as Normal priority.

Sorry! Forgot to include the texture file.
Here is an appropriately blurred-out copy of same. It gives more or less the same results.

In fact, the texture doesn't seem to play an important part in this problem.
I rendered the same file with a number of different background settings.
In each case that silly little line shows up.
I'm still confused.

I found the problem: The "layer weight" node does not have a "Normal" input in 2.69. It's been added in 2.70. Have a look at it in both versions. Can I assume you've reconnected the nodes in 2.7? Because, if I load your file in 2.69, there's no connection at all between the texture coordinate node and the layer weight node, hence the difference.

@Willi (willi) - Thanks, I had not noticed that difference.
The file was made in 2.70 and I only used 2.69 as an extra test.
However, this still doesn't seem to explain that extra line artifact in the render.
Am I missing something here? Is that line an expected part of this render material set-up?

If I reduce the material node setup to this...

and switch off all light sources but the world background (set to white), I get these images depending on the "Blend" value of the "Layer Weight" node:

blend=0.05
blend=0.35

So, I'd say this is normal and not a bug.

@Willi (willi) - I cannot disagree with what you have pointed out. However, my confusion centers on the following question:

Why does that line appear? If that line artifact is linked to the so-called normals of the texture of that particular object

then is stands to reason (?) that rotating the object should (//in theory//) also rotate that line artifact.... but this is not happening.

So, I am remain confused.

But, OK, assuming (and I stress the word, assuming) this is normal behaviour of the renderer and not some sort of bug...

then where would I go to find some documention to explain to me why this happens...
   so that I don't butt my head up against it in future.

PS. Sorry about the previous comment. I didn't intend for that yellow formatting to happen.

You're using the normal of the texture. If the blend value is small, the direction of the normal vector has a bigger influence, hence the small line. But let's wait what the real experts here will say about it.

Brecht Van Lommel (brecht) claimed this task.

In this case you should use the Normal output from the Geometry node.

The Normal socket from the Texture Coordinate node is for texture mapping, so that textures stick to the mesh as you rotate the object. For BSDFs / fresnels however that will not give a good result.