Rendering AVCHD - basic question

LittleGreyCat

Standard Member
Reading loads of threads about AVCHD editing there is a concensus that one realy long and heavyweight stage is the rendering of the finished video.

I have just started wondering why rendering is so complicated.

Is this specifically because of the AVCHD format?

AIUI there is a full frame, then a series of frames just describing the differences from the full frame, then another full frame. And so on.

Now if you are just assembling a simple edit by chopping out unwanted bits then nearly all the video prior to final rendering will be AVCHD in the above format.

The only areas which won't fit will be the points where the video is cut and joined, where the sequence of frame/differences/frame will be interrupted.

So why cant the joins be fettled then the rest of the clip just copied?

I presume the timeline has to be re-written so you can position anywhere in the finished movie by time, but most of the data seems to be unchanged.

Is it that all the bits are so closely intertwined that you have to unpick and recode everything instead of just updating discreet parts?

Tried Google but didn't find any obvious description.

Cheers

LGC
 

PhilipL

Member
Hi

Reading loads of threads about AVCHD editing there is a concensus that one realy long and heavyweight stage is the rendering of the finished video.

I have just started wondering why rendering is so complicated.

Is this specifically because of the AVCHD format?

AIUI there is a full frame, then a series of frames just describing the differences from the full frame, then another full frame. And so on.

Now if you are just assembling a simple edit by chopping out unwanted bits then nearly all the video prior to final rendering will be AVCHD in the above format.

The only areas which won't fit will be the points where the video is cut and joined, where the sequence of frame/differences/frame will be interrupted.

So why cant the joins be fettled then the rest of the clip just copied?

I presume the timeline has to be re-written so you can position anywhere in the finished movie by time, but most of the data seems to be unchanged.

Is it that all the bits are so closely intertwined that you have to unpick and recode everything instead of just updating discreet parts?

Tried Google but didn't find any obvious description.

Cheers

LGC
Yes AVCHD is compressed over time, so you get one frame than around 20 or more that only describe the differences. To build these 20 frames can me looking back and looking forward in time.

Smartrendering is often the term used to describe only rendering the bits that have changed. You can get software that does this with AVCHD files, for example Corel VideoStudio.

The problem is AVCHD is pretty complicated, and different manufacturers might record in different ways, so just chopping it up and splicing in a rendered bit doesn't always work that well.

The other issue is because you are splicing in bits that have been rendered with other bits that haven't, this can give rise to a visible change to the picture colour or quality as you enter and leave the spliced in bit.

It also seems to be the case that splicing in or out bits of an AVCHD stream gives rise to a broken stream as it just isn't designed for this sort of editing. This means depending on what you use to play it back, it might be broken. I've never had a glitch free smart render from VideoStudio when I've tried it.

In addition if you are applying colour correction etc to the clip, it can't be smart rendered anyway.

So the most consistent render is by decompressing all the AVCHD as it goes, applying the effects and cuts, and then recompressing again. Providing you use good software with good quality settings set you'd be hard to tell the difference in output, with the benefit it hasn't been hacked together.

Regards

Phil
 

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