Reversed video is 10 times bigger than the original

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I used the reverse option in properties and selected the lowest quality possible. My original file is 100 MB in size, but the reversed video is more than 1 GB. Why does this happen? Is there a way to fix this?

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This is by design.

The 100 MB file is small because it has a high GOP (Group Of Pictures). This means it writes Frame 1, then for Frame 2 it only writes what changed from Frame 1. For Frame 3, it only writes what changed from Frame 2. And on and on the daisy chain goes for about 150 frames. Then it starts over with a complete frame, and daisy chains about 150 more frames off of it before writing the next complete frame. This is sometimes called Long-GOP or IPB encoding. An “I-Frame” is a complete frame, whereas P/B frames contain only the minor changes.

Writing the minor changes between frames takes up less space than writing the entire frame each time. This is why the file is small. But it also makes it slow to edit. If Shotcut needs to look up Frame 143 for display, then it must load Frame 1 to get a complete base, then apply all the changes that happened in Frames 2 to 143 in the order they happened to construct Frame 143. This is a very slow way to look up a frame for editing, and it makes Shotcut appear slow and unresponsive. It’s not Shotcut’s fault. It’s the way the file is encoded to save space.

“Convert to Edit Friendly” and “Reverse” undo that compression by writing a complete frame in every frame slot. So looking up Frame 143 is nothing more than looking up Frame 143 directly. There is no reconstruction that requires going back to Frame 1. This is sometimes called “All-Intra” or “All-I” encoding.

Since a complete frame is written every time, the file size will appear to explode. However, it will actually play and process faster even though the file size is larger.

Shotcut does not offer a way to change this behavior.

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3

@Austin very clear and informative explanation. This answers a couple of questions that I was too lazy to ask :slight_smile:

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