I’m still on the 1.23b demo, so perhaps this has been addressed in V1.5…
Is it possible to output an image so that the background/canvas/clipping plane whatever you want to call it is a transparent alpha if saved as a PSD?
I’d like to be able to composite a ZBrush image into another image, do things like independent color correction of foreground or background elements, etc.
Is it possible?
Are you saying you want to export a greyscale map of the depth in your scene? If so, use the MRGBZGrabber (with Auto crop turned off) to grab your scene. This generates a texture (the colors in your scene) and an alpha (the depths in your scene.)
<font color="#949494" size=“1”> December 06, 2002 Message edited by: Muvlo </font>
Here is actually a better approach for this purpose, using the above sample image. After making the depth capture using the MRGBZGrabber (thanks, Muvlo!), you will see that it has various shades of grey:
This isn’t always helpful when you’re using the alpha as a mask for compositing.
You could use Photoshop to adjust the levels and create a pure black and white image from the depth grab. But there’s an easier way! Choose white for your color and the Flat Color material. Paint over your scene with ZADD turned off and MRGB on. This will paint everything white, but it will also remove all shadows (since the Flat Color material doesn’t receive lighting information). The result is the image below, which you can export and use as your mask.
I’ve pretty much thought of or tried all those (along with magic wanding the background and creating an alpha channel in PS), but there’s still a pretty strong aliased line around the objects.
Thanks for the help!
I like to use a blue/red/green screen for my back ground or foreground and then chroma key it(selectively deleting colors based upon variables). This works really good with ZB, since it does not export alphas. It takes a little more thought when you create your painting, but if you keep strickly in mind with what you are doing, you can make some really cool stuff! I hope this gives you a little direction. 
Just make sure you create the Blue/green/red screen as a different layer. This is best used for a background layer, so you can add animated skies behind the image, and composite anything over the ZB image, if you use blue screens etc. 
<font color="#949494" size=“1”> December 06, 2002 Message edited by: sirquadalot </font>
Second edit, try working with the blue screen thing but also work at twice the resolution of your final output.(this anti aliases the image really well!) Basically cut the image size in half but make sure the dpi doubles. ZB 1.5 makes this simple to do. ZBrush is limited to 72 dpi. When you work at twice the final output and then reduce it by half, while painting a certain contrasting background color, you should be able to composite it correctly, depending upon all the wonderful variables out ther. HeHeHee!
<font color="#949494" size=“1”> December 06, 2002 Message edited by: sirquadalot </font>
Also it’s possible to export the the file as a psd with mrgb on, or something like that, and Zbrush gives you an opportunity to save the materials as a separate alpha channel with the document. This alpha can be opened in photoshop and the levels command used to separate the various materials. This makes a mask that is pixel accurate. Resulting selections in photoshop can then be booleaned. This is good for doing accurate selections of touching objects that have different materials.