I wanted to plug a bump map into the normal inputs of a Diffuse and a Glossy shader. But if that is done, for some unknown reason it renders only on part of the mesh.

A line across the image will appear. On the left side of the image below it looks fine, but on the right side it isn't. It looks as if it has rendered image without the Bump map plugged into the Normal input of the Glossy shader.

Rendered preview:

enter image description here

Bump node group:


I used spec map as factor of Mix shader:


enter image description here

I discovered that the problem is caused by Bump map plugged into Normal input of Glossy shader.


It seems that the artifact is caused by the last "Add" node in the Bump map group connection which is combining all free bumps together.

if the Blend type is changed to e.g. "multiply", the line across the image (the artifact) will disappear. However, the resulting image is much to dark. Which is obvious (wrong blend type).

So why does the blend type "Add" on the last node of Bump group cause this issue?

Bump maps are classic *.png images with greyish background color (#b3b3b3), content in a image has dark color (#0a0a0a)
I'm using Blender v2.69.

  • $\begingroup$ A .blend file would be very helful to get a straight answer, if convenient. $\endgroup$ Commented Jan 31, 2014 at 15:30
  • $\begingroup$ @Leon Cheung : Yeah, I would put it here if I would had faster internet connection than I have now (16kbit). So tough luck for me $\endgroup$ Commented Feb 1, 2014 at 22:24

1 Answer 1


In short, adding all those values together is making bump values that are larger than the reasonable range.

The add nodes don't do anything magical. They are doing exactly what it sounds like ;-) They're adding numbers together. In the case of bump, full normal distortion is 1 (tallest value), no normal distortion is 0 (shallowest value).

50% grey plus 50% grey is 100% white, which is full bump distortion. 75% grey plus 75% grey is.... a color that Blender doesn't know what to do with... which is a bump distortion that Blender doesn't know what to do with.

Those add nodes shouldn't have anything to do with color. You can't necessarily expect that the color you want is also exactly the bump value that you want. You should manipulate the color of your shader seperately from the bump of your shader, even if you have to input the same image twice.

I hope this helps!

  • $\begingroup$ I tried to change grey values but nothing has changed.Problem causes reflection of light (glossy shader). if I change Distribution type from Beckmann to CGX, reflection isn't there anymore as well as sharp line across image (In image all looks as in right side of image above). Beckmann distribution type wouldn't be as sharp as it is, right? So problem will be somewere else... But thanks anyway for reply! $\endgroup$ Commented Feb 1, 2014 at 22:40
  • $\begingroup$ @user2694295 Try multiplying the output of the final add vector node by a scalar .333 and plug the resulting vector into the normal. It's just a hunch, but if it works I will explain further. $\endgroup$
    – PGmath
    Commented Dec 16, 2014 at 20:59

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