Strange bump mapping problem (MTOR)

[minortemujin]

Hi all,
I'm facing a very strange MTOR/PRman problem. I texture mapped an object, then assigned a simple bump map on it. A strange effect shows up on my render ( http://members.bellatlantic.net/~vze2c7rt/2.jpg ). I took the bump map away, everything's fine ( http://members.bellatlantic.net/~vze2c7rt/1.jpg ). I've changed the pixel samples, shading rate, etc.., nothing works! The shader is a standard Blinn with a standard Simple displacement. Anyone has any idea?

Thanks,
Stanley

[dan-mcfarland]

Stanly,
Off the top of my head, did you try monkeying with the displacement bounding?Is the displacement using the same manifold as the blinn? If you used a noise generator try adjusting the frequency, iit sort of looks like the manifold or frequency is off.
Good luck,
Dan

[minortemujin]

Both of the Blinn and the displacment are useing the same manifold from Maya, so it shouldn't be a problem. I'm not using any true displacement so I didn't touch the displacement bounding. I'm using a printed texture for bump mapping so the noise generator thing simple isn't the case. It's very strange, isn't it?

[gdc]

All right this is just a guess off of the top of my head. I'm thinking that perhaps your texture is being accessed outside of the s&t range. When this happens the texture call will return black for the color. One way to test this is when you convert your texture to a .tx try this:
txmake -mode clamp -resize up- texture.tif texture.tx
-mode clamp will force the texture call to return the last pixle value when the texture call calls outside of the s&t range. There are other options to -mode so you may want to read up on them in the doc's if this fixes the problem.
The resize option is just something I've gotten into the habit of doing. If you use txinfo on an image without the resize option you should see something like this:

s mode: CLAMP
t mode: CLAMP
Channel datatype: TX_BYTE
Pixel aspect ratio correction factor: 1.481481

note the Pixel aspect ratio correction factor: 1.481481 line. You usually want this to be 1
With the resize option you should see something like this:

Pixel aspect ratio correction factor: 1.000000

Much Better!
Like I said it's just a guess.
--g

[minortemujin]

In fact, I did use Clamp for all my textures. I tried reside up- and all other modes, nothing is working... I found out that the Kb value can affect the weird effect. It seems like bigger the Kb is, bigger the black area. Anyone has any idea?

Thanks,
Stanley

[gdc]

Well, like I said it was just a guess. I was looking at the pictures you posted and I noticed something about the "black" areas. They seem to happen in areas where the model would have either close isoparms to get a shaper transistion or some sharper point weights to pull the surface tighter. Because of this I'm thinking that perhaps the problem is related to the model in some way. Is the modle thin like a grid or does it have a backside like a torus? ( Hope that makes sense) One thing I would try is to create a torus because the shape is very close to one and assign the shaders you are using to the torus and render that. Does the same thing happen?
Making Kb larger should make this problem bigger because you've already confired the fact that it's the bump mapping causeing it, so adding more bump will amplifly it. If you use a negitive Kb does it go away?
Sorry for so many questions.
--g