Im getting some black pixels rendering on a surface with a simple bump map on it.I tried maxing out the super samples and shading quality of the surface however this did not help at all. I slapped the same shader onto a grid and get black holes as well.

Nothing special, polygon surface with just a texture map and a bump map that is normalized. Both are using uv 1 shading layer. I tried using subd and non subd. Turned off shadows… etc.



ps. Probably something dumb obvious with the shader. What I have noticed is that these holes only exist in little dips/valleys of the surface this shader is applied to.

It might be something as simple as turning up the amount of bump too far. I find that you can only push normals around so far before you get nasty artifacts. If you need a heavy bump you might be better off using a displacement instead.

The bump map is only set to 0.1 depth and wrap is set to decal.

Wren
The bump map is only set to 0.1 depth and wrap is set to decal.

Hey Wren,

This could also be your illumination model. Are you using Oren-Nayar by any chance?
Not just diffuse, it could also be bad reflection, or bad specular, or….
Why don't you remove the illumination model altogether (set it to constant), or do something safe like Lambert only, and then see if the black dots are still there – at least eliminate the possibility.

Cheers!

Cant see it with constant unless I adapt the white points. Then very light grey lines are there. With lambert the problem is evident as it is with blinn or phong obvious.

ps. does this problem relate to?

“Monday, September 27, 2004
Houdini 7.0.203: A serious bug was found and fixed which may have caused pixel artifacts in mantra.”

Wren,

The lines you're seeing are the result of a very subtle problem in mantra we discovered during The Day After Tomorrow. Almost all of the time you will never notice these subtle vertical lines unless you go and do what we did: composited an underexposed helicopter against a high-dynamic-range sky. Because the lines appear in the alpha channel too, the sky becomes visible in lines cutting through the helicopter. The fix was provided by SESI and you can avoid it by rendering floating point images by specifying the depth in upper case - i.e.


mantra -b FLOAT


Here is the snippet from mantra help output you can get with “mantra -h”:

-b val Specify bit-depth of output image (color image only)
0, any - Use the ‘natural’ bit depth
8, byte, char - Generate 8 bits per color channel
16, short - Generate 16 bits per color channel
half - Half precision floating point
HALF - Rounded half precision floating point
32, float - Floating point data per color channel
FLOAT - Rounded floating point data (uppercase)
The natural depth is dependent on the target image format
When rounded floating point data is generated, minimal
rounding is performed on the floating point data before
it's written out. Values will be slightly less precise,
but values like 0.9999998 will be changed to exactly 1.0.



This will solve Wrens “grey lines” discovery but this is not related to black pixels. Maybe thats the support journal entry? Try it and tell us!

8)

No luck with that jason…. but it was worth a shot.

This scene is really simple. Head of a character that is polygonal with a vex surface shader containing a bumpmap. One light with no shadows.


ps. If I lower the depth of the displacement to 0 the problem goes away so it is definitely related to that.

Wow!…. I had no idea about the “mysterious grey line” problem; so thanks for the tip Jason!

Warren,
My suspicion is that it is the shader and not Mantra, but I'd really want to know if it was the other way around! So; could you post the shader, and/or the hipfile – maybe a simplified version that shows the problem?

Mario Marengo
Wow!…. I had no idea about the “mysterious grey line” problem; so thanks for the tip Jason!

Whats an Alpha discrepancy of 0.00002 between friends?
:wink:


PS. In our render scripts I always force “FLOAT” instead of “float” now. There are just too many things for an artist to remember and I decided I never wanted to see those lines again. Ever. (Neither do our compositors who were much perplexed, and first took the problem to our Nuke developers.)