PDA

View Full Version : tiff and alpha


pingo
03-10-2008, 01:10 PM
hey all.
If I render a picture out from my 3d app with an alpha and save it as a tiff, photoshop (CS3 Mac) cuts the alpha away when opening it. This is not good, cause I mostly have shadows and I want to use the alpha for further compositing use.

This doesn't happen with TGA files, but I want the 16 bit tiff option.

Is there someway to make Photoshop ignore the alpha? - like Aftereffects (and old Photoshops)

cheers!

pingo

mykhel
03-11-2008, 10:06 AM
Hmm, that's strange. Are you saying the alpha is missing in PS? Do you actually see the alpaha channel in the channels palette when you open it in PS? Do you have 5 channels when you open it--rgb, r,g,b and the alpha channel? Or just 4?


I am using MAX and when you save out a tif you must tell it to save the alpha with it, otherwise, it just saves out 4 channels. Maybe your app works the same.

pingo
03-11-2008, 11:33 AM
No, maybe I wasn't being clear.
example: a rendered sphere casting shadow on a grid. I force my 3d app to only render an alpha on the sphere. It's essentially a simle object buffer. Now I can tweak the sphere in post without touching the grid.
Except if I render as a tif, it seems Photoshop tries to help and cuts out the sphere, and puts it on a transparent background. So now my grid is gone.
(to answer you, I end up with 3 channels, rgb, no alpha, but transparent background)
This doesn't happen if I render out to TGA.
I'm not doing anything wrong cause if I open the image up in Aftereffects, or whatever other image app I can get my hands on, the alpha and the grid is there (of course!!)
I really don't like the way Photoshop tries to be clever doing stuff like that. There must be some setting I'm missing. Especially since I can't find anything about this on the web.

Go ahead and try this. It'll all make sense: Render a simple scene with an alpha. Photoshop will delete the alpha and make you a nice (and very anoying) transparent layer.

BTW I'm not interested in why photoshop does this. I want it to not do it:-)

cheers!

pingo

mykhel
03-11-2008, 03:29 PM
I'm sorry. I'm having trouble following you. I would like to though. It might help me avoid headaches in the future. What software are you using? Can you post an image of the alpha render?

I rendered out an alpha and the only surprise I had was that it rendered out the whole scene in alpha and not just the selected elements. I thought you could just render out specific objects, but I guess I used to do that by chosing a different render option.

Anyway, I am curious as to what's going on. If you have the time keep me posted. It is always good for me to learn something.

Absinthe-Graphics
03-11-2008, 05:16 PM
Yep I know this problem too. If you save a tiff with alpha, PS will cut out the alpha when opening. I work around that with saving and loading the alpha as separate file. But you are right: there must be some solution within photoshop.

Absinthe-Graphics
03-11-2008, 05:46 PM
I found something about this in another forum. There the solution was to turning on/off the video alpha(requires hardware support) checkbox at the transparency & gamut preferences
This seems to have solved the problem for these guys using PS 7, but I am using CS3 and there is no such video alpha checkbox at the preferences?! So for me this is not working.:banghead:

gavalian
03-14-2008, 09:05 AM
But you can simply make new laver above that and fill color ! I think TIFF alpha also keeps shadows etc...

CGTalk Moderation
03-14-2008, 09:05 AM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.