Alpha background?

I’m just getting my feet wet with Blender with VERY little exposure to computer graphics (and I think I’m in love. :D) My friend wants to use a variation on a logo animation I made with Blender. He’s a Mac user in his profession, so I learned how make the file ‘output’ as a Quicktime file he can use readily.

He asked if I could “put the background on an Alpha channel,” because he wants to put the logo animation on another background. Is there a way to do something like that, or to have only certain objects render, or . . .?

Use the RGBA option on the Output tab and also enable the “Premul” optionon the Render tab. Optimally you don’t want to use the quicktime option because it only supports premultiplied alpha channels which always result in a “Halo” effect unless your buddy is a sophisticated end user and knows how to remove these halos. A better option would be to send your bro an OpenExr sequence with the “Key” option enabled on the Render tab and the “Half” option and “Zip” compression enabled on the output tab. Just about any compositing software should be able to read that format and the file sizes shouldn’t be verry large. Using “Pixar” compression with the half option enabled rather than “Zip” will further reduce the file size (16bits per channel V.S. 12bits per channel).

Wow! Thanks, RamboBaby. That seems to have worked beautifully. In fact, I don’t see any halo in the quicktime version. I’ll mess with it a bit and see what more I can learn (or ask.) :wink:


Can you give a summary info on what is open exr file format, i am totally
new to it ?

You won’t see it in QT, you’ll see it when you composite the QT over other images/video, especially if you used a colored “Sky” background instead of the all black “Premul” option, the latter having only a slight uniform gray halo.


Multipass OpenExr files are Blender’s native rendering format. All images imported into Blender are converted on the fly to RGBA float, even if the input image doesn’t contain an alpha channel - such as .jpg format. This makes Blender unique in the world of compositing software except for Adobe AfterEffects which also natively assigns an alpha to all input images (but does not convert them to 32bits per channel float like Blender).

OpenExr format is capable of containing an infinite number of arbitrarially assigned channels such as Normals and Vector (available from Blender) or camera tracking data (not yet available from Blender). This amazing degree of flexibility has made it THE industry standard for digital imaging. 32bits per channel however seems to be a bit of overkill since it encompasses just about every shade of color that human eyes can process and a whole lot more. The precision level is amazing too, to the point of only failing in an exceptionally minor academic sense though your eyes will never be able to differentiate such minute flaws.

While 32bpc is overkill for most work, standard Openexr format in Blender (not to be confused with MultilayerOpenExr files) include an option for Half type or 16bits per channel files. These allow extreme levels adjustments without introducing banding artifacts, are more than adequate for compositing, and save an incredible ammount of disk space as opposed to their MASSIVE 32bpc cousins.

More info:

Okay, thanks SO much for the info/education. :smiley:

You won’t see it in QT, you’ll see it when you composite the QT over other images/video, especially if you used a colored “Sky” background instead of the all black “Premul” option,
I did use the “Premul” option as you suggested. I don’t know if it matters, but I’m running Linux (Mepis 8 Beta2) and used KMPlayer to view the rendered video, since proper Quicktime is apparently unavailable for Linux.

I don’t know what software my friend will use for compositing, but when I was talking to him about OpenEXR, I think he said he has After Effects (if that’s the right name.) I’ll send him a couple different renders using your suggestions. He may have to learn something or other to use the EXR file(s).

Does it make sense to Zip the directory containing the .exr sequence in order to email it? It’s only around 100 frames.

Again, I can’t thank you enough. You’ve been very helpful! :yes: