I understand that my title requires further specification, so allow me to elaborate.

I recently finished a model in Blender and rendered 1000 frames to PNG with alpha. The total size was roughly 2 GB for all frames.

However, attempting to store them as a MOV file (QT-rle video codec, no audio) results in a whopping 5 GB file. That's two and a half times the original frames' storage size, for the exact same frames.

I imagine that this is because MOV is an extremely inclusive container with an extremely inclusive chosen codec, and PNG may use a compression scheme that it does not... but beyond an alpha channel, I'm not sure I need any of these things. It would be just as easy to just use the lossless PNGs, wouldn't it?

Normally I would blow this off as the price of video editing, and just delete the file when I know I'm done with it...the problem is, I can't even play it back, full speed, on my computer without it frequently pausing to buffer more of the file into memory. This makes it very difficult to work with, and this is not a slow computer. It's perhaps a year old.

Why does QT-rle in MOV take so much space? How can I adjust its features to take up less? For now, I'm sticking with Matroska/H.264 and PNGs, but the format is likely preferred for a reason and I would like to understand what I'm looking at here.

Thank you kindly.

EDIT: After further research, apparently "rle" stands for "run length encoding". This makes sense, as I've also seen it called "Quicktime Animation", and this style of encoding would do well for drawings with large swatches of consistent color across rows, but terribly for high-detail or HDR images. This might be what I'm looking for... I just chose an inappropriate format, which is bending over backwards to handle my not-animated-looking video. Further study will tell.

1 Answer 1


OK, I've come to understand the issue with QT-rle, and it isn't that it isn't compressed; it's that it's the worst kind of compression for the type of video I'm making.

QT-rle uses "run length encoding", which relies on long strips of pixels having the same color. In videos in which major hue, saturation, and lightness changes are regular, it packs the file with metadata which inflates its size to unusable levels. If I were working with an animation that had vast patches of solid colors, it would likely work better.

So, that's why I'm dealing with crippling file sizes. Alternatives include storing frames as JPEG2000 or PNG, as I will be now, or other video codecs which happen to include an alpha channel; though there are admittedly few. The other possibility is to composite the alpha to a mask video and use that in post.

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