Wierd Black lines in renders! PLEASE HELP!!

[nick_h]

I have done some modelling of a Roman style kitchen and have rendered a load of frames using mantra at full PAL resolution. When i then view the animation, there are loads of black lines/squares going towrds some objects in the kitchen. These objects are not transparent, they are just pots on shelves, modelled by revolving a curve around the Y axis. THe shader on these pots is just the modified VEX Gallery Granite.

Any help would be greatlty appreciated, as time is not on my side!

I've attached a still from the animation.

Cheers,

Nick

[keltuzar]

i think (THINK) it looks to me that you did not normalize the vectors when u created those shaders in vex and its giving problems during rendering...

Looks to me that normals coming from the small pot shader that is the issue...

(Ps I maybe wrong...)

[stevenong]

Hi Nick,

I would start troubleshooting by removing the shader from the geometry & render to see if the blacks lines are still there. If the lines are gone, it means something is wrong with the shader. If the lines are still there, then something is wrong with the geometry.

On this note, I noticed some of your geometry is too shiny. It seems like the specularity or diffuse of the VEX Glass shader is turned up too high on the two glass bottles. Also, one of the clay pots rings at the bottom right is too shiny/bright. I think you mirrored or duplicated one of them but didn't reverse the normals or maybe the shader wasn't applied? Also, is it supposed to contain something white or black? You can fix reversed normals by appending a Facet SOP to the end & turn on Orient Polygons (if you're using polygons) & Post-Compute Normals (for most geometry types).

Looking at the image again, I notice the black squares on the walls. Are you using a Displacement shader for the walls? If you are, you have to turn on Displace Bounds under the Object's Render tab. Toggle it on & you can set the value to be the same value as the shader's Height/Magnitude/Amplitude to help. You don't want to set the value more than it should because your render time will increase. It helps to separate the walls into another object, if it is not too difficult, so you can have better control.

I'm not sure if the black lines you have is caused by the "charcoal" geometry under the pot. It looks to be coming from underneath or not.

Another troubleshoot to do is to remove the shadow shader from the light to see if the lines are gone.

I hope the above helps! Good luck!

Cheers!

steven

[Jason]

Another long shot - do you have any geometry in your scene corresponding to those lines? Why I ask is did you know that mantra will still render open polygons and curves?

[Mario Marengo]

Another long shot...

A possible clue is that these artefacts are not antialiased. If a shader evaluates to "black", it gives you an antialiased black line, not those chunky "things" you see in that picture.

This seems to suggest that some portion of the shading failed to evaluate correctly. Which typically means that some instruction generated "Not-A-Number" (or NAN). This kind of stuff (*if* that's what it is) is hard to debug. But one place to look first is zero-length normals or vectors that are being used to displace, bump, or even calculate illumination -- but my first choice would be with the bump/displace side of things.

I would probably proceed as follows:

1. Remove all objects from the scene until you're left with only the ones causing the problem.

2. Check if they have one or more shaders in common.

3. Set all their shader parameters to default values. Hopefully at this point you have an artifact-free render.

4. Re-introduce your parameters one by one until you generate the problem again.

If the above fails to turn up the culprit, you might have to have a look at the shading code itself.

Good luck!

[Jason]

A possible clue is that these artefacts are not antialiased. If a shader evaluates to "black", it gives you an antialiased black line, not those chunky "things" you see in that picture.

This seems to suggest that some portion of the shading failed to evaluate correctly. Which typically means that some instruction generated "Not-A-Number" (or NAN). This kind of stuff (*if* that's what it is) is hard to debug. But one place to look first is zero-length normals or vectors that are being used to displace, bump, or even calculate illumination -- but my first choice would be with the bump/displace side of things.

17144[/snapback]

Ah! Good point there, Mario.

This reminds me, it's possible to run mantra with a verbosity option to check for NaNs. In your Render Command, use these flags:

mantra -V P

You can get a full list of supported mantra options from "mantra -h" - these are the verbosity-related options:

        -V val  Set verbose level (i.e. -V 2p)

                    0-9  Output varying degrees of rendering statistics

                    p    Turn on VEX profiling

                    P    Turn on VEX profiling and NAN detection

                    a    Turn on Alfred-style progress reporting

[nick_h]

WOW!! Thanks for all your advice guys/girls!! I wasn't expecting such a helpful response! I'm gonna go through it as you've suggested turning things off and trying again.

stevenong - thanks for the errors in my work , i noticed a few of them when i posted the pic, but not all!!

Thanks again all....a pleasant intro to this forum...looks like i'll be using it more as i start to try to become an animator!!