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Please take a look at these images











Is there something known that causes this? Max2009 with forestpack3
Model from Revit.
As you can see, only the simple physical sky is correct.
The fourth image, with hdri background seems ok but the fifth image, again with hdri, is messed up.

You seem to be experiencing the problem described in this FAQ thread. It's not a bug, it's how it's supposed to work for compositing to function properly.

It seems that the problem described there, is the black background.... My problem seems to be a little different.... OK, I will compose the image to see...

It's about material setup and textures. How did you use the textures and how do they look like?

i have same problem:
- trees that have sky behind RGB render channel look "biten".
- alpha channel looks ok
- transmittance texture in tree material is ok as content of image (purely white/black, no grayscales antialias and so on...) and as format (.png)
- as you can see this happens also if tree is reflected by AGS surfaces (as in my saple below) or physical glass surfaces.

to work around this i use STAMP tool in photoshop in RGB render layer to fill empty space, and then merge alpha mask.

::: click to download max scene (max file+textures):::
http://www.lucafrizzarin.com/maxwell/testtrees.zip

and...here the result of a fast rendered scene.

OK, all you need to do is turning off image filtering for the transmittance image. You can do that from the image picker of MXED or by setting bitmap filtering to "none" under Bitmap Preferences in 3dsmax.

This doesn't seem to be my problem..... Look at the two examples.... The only thing I did, is changing the background from phys. sky to HDRI

JTB wrote:This doesn't seem to be my problem..... Look at the two examples.... The only thing I did, is changing the background from phys. sky to HDRI

Yes, I replied Luca's post. So, tell us about your material or link a failing scene and let me help.

tom wrote:

JTB wrote:This doesn't seem to be my problem..... Look at the two examples.... The only thing I did, is changing the background from phys. sky to HDRI

Yes, I replied Luca's post. So, tell us about your material or link a failing scene and let me help.

Here is the scene.... I found out that when screen mapped is off, the problem disappears... But then, I can't have a correct background.
Also, Sky dome works OK, so it must be something wrong with HDRIs

DivShare File - itootest.zip

tom wrote:OK, all you need to do is turning off image filtering for the transmittance image. You can do that from the image picker of MXED or by setting bitmap filtering to "none" under Bitmap Preferences in 3dsmax.

W O N D E R F U L !

thanks Tom

(i really didn't think that that bitmap option was used by Maxwell i never disabled it before today, thanks)

Luca_Studioaltieri wrote:W O N D E R F U L !
thanks Tom
(i really didn't think that that bitmap option was used by Maxwell i never disabled it before today, thanks)

As a fix, filtering will be auto-disabled for clipmaps in the next version.

JTB wrote:Here is the scene.... I found out that when screen mapped is off, the problem disappears... But then, I can't have a correct background.
Also, Sky dome works OK, so it must be something wrong with HDRIs

You're right and as I test the scene I confirm the bug with screen mapping, this will be fixed in the next version. (-edit: Later posting this I found a mistake in your setup so now I have to say it was not a bug of Maxwell, sorry.) Meanwhile I will make a suggestion to save your work. Please follow these steps:

1) Open your opacity map in PS
2) Layer / Adjustments / Threshold and Hit OK
3) Create a new material for the tree made of one BSDF
4) Put color map into Ref-0
5) Put opacity map into Trans (check invert if it needs)
6) Set Nd=1, Roughness=100

Thanks a lot for your time.... Btw, what are the pros/cons for this way of making materials with clipmaps and the one I used? The material you suggested is easier to make but I remember that the method I used was always suggested as correct... Can you find a reason to use/not use one of the two methods? (Just curious)

JTB wrote:Thanks a lot for your time.... Btw, what are the pros/cons for this way of making materials with clipmaps and the one I used? The material you suggested is easier to make but I remember that the method I used was always suggested as correct... Can you find a reason to use/not use one of the two methods? (Just curious)

Np Well, there's no correct method, let me explain how it works.

1) CLIPMAPPING
- Use 1 BSDF
- Set Nd=1
- Use black/white texture in transmittance (no grey pixels)
PROS: Faster, easier to setup
CONS: Needs pure black and white maps, otherwise the edges will fail.

2) WEIGHTMAPPING
- Use 2 BSDF
- Set your texture and material in one of them
- Set the other BSDF as "Ghost", which means setting Nd=1 and Trans=255
- Don't use other textures inside ghost.
- Set positive mask to first layer as weight and negative to ghost layer as weight
PROS: Can do true transparency, supports shades of grey in mask
CONS: Somehow harder to setup and renders a bit slower

So, now let me correct my last post above. In your material setup you've done weightmapping at the same time with clipmapping so your second layer was not pure ghost which was unveiling unwanted reflections. So, just unload transmittance texture in your opacity layer and the problem will be gone.

Thank you! I will correct this!