I am trying to render an animation in Cycles render but make the background transparent. In the render tab I have gone to film and made sure transparent was checked, but when I render the image the background comes out black!? What am I doing wrong?
Are you rendering to a file format that supports transparency ?
Rendering to an image sequence can use .pngs
The majority of movie formats do not support transparency. Quicktime animation or Quicktime .png codecs should do. If you don’t have these options in the render output settings then render to an image sequence and convert to these formats with another application.
I am trying to render it out to .png files so I can compile it later, I have it set to RGBA, colour-depth 8, compression 15%
I am rendering out to .png file. I have it set at RGBA as well…
Switch to Blender Render (Blender Internal). Open Properties>Render>Shading and switch the Alpha dropdown box from Sky to Transparent. Switch back to Cycles Render, make sure Film>Transparent is enabled, and go for it. Why? Dunno, it just works for me when Cycles needs to render a transparent BG.
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so do you have to export as PNG please? it won’t work as AVI?
We don’t render the 3D scene directly to a video file, because:
- if the render crashes, with images you can continue where it left but with video file you’re screwed
- you can stop the image render and continue where it left later
- image files can be lossless, a png is lossless in the visual range of values (0-1). Using already encoded video to make adjustments or as part of another video results to re-encoding and loss of quality
- an image file can contain high range of values (such as .exr) so they allow more flexible adjustment ranges, or may contain data instead, which is useful in post-processing
- it’s easier to save an image with alpha transparency than finding a video format that supports it
- any good video viewer/editor can also handle image sequences so the handling in the application is the same as handling one file, and can be compiled to a video file afterwards.
AVI is a video container format, it’s the encoding format that says what the video looks like and what properties it supports. http://blender.stackexchange.com/questions/2050/any-video-formats-that-render-with-alpha-channel
ok thank you, so basically its much better to export to PNG if you want a transparent background?
Is using PNG the best bet for the best quality export as well please?
No. OpenEXR is the only lossless image format available. That being noted, any file format containing 16 bits per channel is considered to be totally acceptable for compositing. Png is more than adequate if you use the option for 16 bits per channel.
ok great, thanks so much for replying 
re the 16 bits, thats referring to the color bit depth per channel right?
Yes, it is 16 bpc.
16bit png is totally adequate if you can live with clipped highlights. If not, forget png, tif and all the other crap formats and go with exr. Unless you use log encoding, integer storage does not work well as intermediate format because you lose the ability to keep values above 1.0 which are necessary to do even the simplest things like dof and bloom correctly in comp.
thank you so much.
thank you for taking the time to reply 
also sorry guys i forgot to ask, whats better out of .exr and .exr multilayer please? is multilayer better quality please?
Multilayer isn’t any better quality. Both are 32 bpc. Standard exr can be half type (16 bpc) and offers three different compression algorithms as well. Multilayer offers only 32 bpc with no option to compress. It offers to include as many layers as you want and a wide array of channels/passes within each layer. The only channel options with standard exr is RGB, A & Z.
Ok cool, never used this format so will give it a test today. Thanks so much.
Perfect. Thanks!