Is Exporting Alpha Channel in h.264 Possible?

[Matt Long]

Awesome Dave! Thank you. That’s exactly what I needed. A definitive answer. I would have preferred it the other way, but I’m glad to know now.

I’m trying to create a video for iPad that retains alpha information. According to Apple they support:

– H.264 video up to 720p, 30 frames per second,
– MPEG-4 video, up to 2.5 Mbps, 640 by 480 pixels, 30 frames per second
– Motion JPEG (M-JPEG) up to 35 Mbps, 1280 by 720 pixels

We’ve ruled out h.264, but would either of these other two formats support alpha, to your knowledge? Clearly I’m new to this so if I’m asking the wrong questions, please bear with me and let me know what I’m missing.

Thanks again.

[Todd Kopriva]

> but would either of these other two formats support alpha, to your knowledge?

No.

> Clearly I’m new to this so if I’m asking the wrong questions, please bear with me and let me know what I’m missing.

Here’s an excerpt from After Effects Help that gives you what you need to know to find out for yourself if you can export an alpha channel in a particular format:

“Many file formats can include an alpha channel, including Adobe Photoshop, ElectricImage, FLV, TGA, TIFF, EPS, PDF, and Adobe Illustrator. AVI and QuickTime (saved at a bit depth of Millions Of Colors+), also can contain alpha channels, depending upon the codec (encoder) used to generate the images stored in these containers. For Adobe Illustrator EPS and PDF files, After Effects automatically converts empty areas to an alpha channel.

When specifying the color depth for an output image, the plus sign (as in Millions Of Colors+) denotes an alpha channel. Similarly, choosing to output to 32 bits per pixel implies an output depth of 8 bits per channel for each of four channels: RGBA.”

BTW, if you’re new to After Effects, I recommend starting here.

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Todd Kopriva, Adobe Systems Incorporated
Technical Support for professional video software
After Effects Help & Support
Premiere Pro Help & Support
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[Mo Dejong]

Matt, short answer is yes, it is possible to create a h.264 with an alpha channel for delivery on iOS.

This blog post shows how it can be done. A few things though. h.264 is a delivery codec, it should not be used as an intermediate in the video production process. Export from AE using the Quicktime Animation codec at 32 BPP (Millions+) to retain the alpha channel. Dave is correct that h.264 does not support an alpha channel by default, one needs to actually create 2 videos, one holds the RGB and the other holds the alpha pixels as a grayscale video. All the code to split the videos, convert to h.264 via ffmpeg+x264 and then recombine the videos on the iPad are already included with the AVAnimator tools. I think your will be quite impressed with the results as the generated file sizes are quite small and the .m4v files generated can be included directly to your xcode project.