What's with rewinding digital video???

[cosmichobo]

G'day,

This applies to Quicktime, AppleTV, pretty much every digital player I've come across...

Play - fine.
Fast Forward - easy.

But when you hit reverse, it will stall, "play" really choppy, crash...

I don't get it.

How hard can it be to read the digital data in reverse?

Can someone explain the issue to me?


cheers


cosmic

 

[blevins321]

It's because the player has already buffered what is coming next. It has to reread and process the data from disk again when you rewind.

 

[Mal]

In addition, some types of digital video use what's known as key frames in the compression process. Basically, every nth frame will contain all the information to display that frame, but then the next n-1 frames will contain only the changes from that previous frame. When going backwards, it has to then go back and figure out what that frame should look like by reading the information from several frames before and then all the changes up to that point, and that can take longer to calculate than it does to actually display an image normally, so you get choppiness and stuttering as it tries to keep up as best it can.

jW

 

[cosmichobo]

Mel's answer washes better with me than a caching issue... I understand the basics of key-frames certainly... Hadn't thought about the issue of working through them backwards...

Surely though, some clever programmer could reverse the process... Instead of "building" upon the original keyframe, "subtract" from the final one(s)...

I guess there's a reason I didn't complete my IT degree....

 

[Mal]

Surely though, some clever programmer could reverse the process... Instead of "building" upon the original keyframe, "subtract" from the final one(s)...

The problem is that all the video files would have to be re-encoded to support reverse keyframes, and it's not an issue most people are worried about. Can't see it ever happening.

jW