WHAT IS LONG-GOP VIDEO?
This is a very specialized encdoding process in the capture device, used by some cameras, most drones, nearly all phones, and many screen capture applications like OBS. In phones, drones, and cameras, there are typically specialized computer chips that do the encoding of the video data to write to the file.
To save space, a two-part process is used.
1) Basic Compression
The first part is "simple" compression of a frame. Typically using block data reductions, such as say a group of 4, six or eight or nine ... whatever number is chosen ... is compared. If they are all pretty close in data values, say within a point of a "central" number for that group, I'll create an example of "close to" 24, 15, 42 in R, G, and B values. The device then encodes all of the pixels as that central value.
Most of the time, you won't notice the change as is't pretty small-ish, and in small sections. BUT ... if it is in a sky or wall, without other texture, you may well notice as a visible pattern especially when expanding contrast, increasing saturation, or 'lifting' an area of the image to a brighter view.
At that point, you can start seeing 'banding' ... visual lines of brighter/darker bits across the sky or a wall, or 'macro-blocking', where you see small, odd shaped blocks of identical values.
Why do they use this type of compression?
Because in the next step, the creation of the 'long-GOP' structure ... the above process reduces the amount of data needing to be stored. And it's a truly massive data reduction.
2) The long-GOP format structure
"GOP" is "group of pictures". The chip in the device selects groups of images that seem to have the same basic data in the same places. And it creates a few actual frames to record to file, called iframes. These will be from 9 to more than a hundred "frames of video" apart. And are only compressed as in step 1 above.
It's what happens in-between those iframes that both gives this format it's incredible data compression rates, and ... causes such problems in NLE use ... non linear editor programs, like PrPro and Resolve and Avid.
Data-sets versus actual image frames
You can only compress an image so far, before you start losing noticeable detail. BUT ... if you simply throw away all 'repeated' data in a video frame, you can save massive space. And I do mean to literally throw away data. To save massive space.
Take a group of pictures ... maybe twenty frames ... the camera and scene are pretty stationary, there's not much movement, and that movement that does happen is predictable. Great! This is an ideal candidate.
The chip analyzes the first image frame against the following group, and looks for the pixels that pretty much stay the same. In all the following images, those are simply thrown away. Yes ... they no longer exist in the written file! As now, all that's needed for that "frame" ... is a small data-set!
But it does record three types of data-sets, that list what does change ... in three ways.
1) pixels that have changed since the last iframe;
2) pixels that will change before the next iframe, and
3) both.
So you have a file that includes an actual, highly compressed image frame only every 9-100 'frames' of the clip. But in-between are only charts of data of the three types listed above ... no actual images. So your long-GOP video file only has a few images in it. Very few, in fact. It's mostly simple data-sets.
And yes, that is the entire reason for the file size differences between a lightly compressed ProRes and your typical H.264/5 "long-GOP" file of the same scene. A long-GOP codec has very few frames stored to file. But in ProRes, or any other "intraframe" codec, every frame is a complete, if compressed, image. All frames of the video are complete in and of themselves.
And the chips that are created to do this long-GOP encoding process, in cameras or devices, are fast at performing those computations, and then writing the small amount of resultant data to the card or disc.
Playing back long-GOP video files
The 'normal' playback of such video files does take some computing horsepower, though for "basic" playback of one file, not an incredible amount. What's the process?
The computer starts with decompressing the first iframe ... then looks for the p and b frames that are the data-sets of information between that iframe and the next iframe. It checks to see if it has data-sets referencing the next iframe in that group, and if so, decompresses that next iframe also.
Both those iframes are stored to RAM or cache, and now ... it starts going through the data-sets of changed pixels, and recreating the frames by comparing the iframes to the data-sets. And storing each frame as it is recreated to RAM/cache.
Now ... it has some frames of video, and can start playback. But that computation process has to continue all the way through playing back that one file.
What about an NLE versus a "simple" video player?
It is often noted that video players like VLC don't struggle with playing back a long-GOP file. So why does Premiere, Resolve, or Avid, far more capable programs, struggle at times with long-GOP media?
It's very simple. And in two parts. First, video players are themselves very simple applications, with very low demands on computer resources.
But second ... they are only playing one video file. An NLE, playing back a sequence, is grabbing bits and pieces from a whole bunch of files of perhaps different formats/codecs, framesizes, framerates, adding effects, and playing that, which is now a very heavy computational load, on your screen. So in basic operational use, an NLE requires a ton more computer power.
A real look at long-GOP playback in an NLE
Ok ... start with the bit above about the computer having to re-create the vast majority of 'frames' you see on the screen by comparison of random but 'real' frames and intervening data-sets. Remember, those in-between images are being fabricated, and stored somewhere for access, from data-sets ... on the fly, and in real time!!!! ... now, let's further complicate things.
Let's grab a few, selected frames, from this clip, throw some sizing and scaling and color/tonal changes on them, then grab a few frames from this other clip, but only do some color changes, and go on down the sequence this way.
But remember ... the computer has to use RAM, cache, or other resources to store the frames it has to continually fabricate (decode), from data sets and comparative analysis, besides doing all the work of the NLE, the grabbing bits, smooshing them together, and adding effects.
Yea, that's a load.
And some devices now use partial iframes! ... in other words, they keep part of a video image, and do the rest of that partial iframe via data-sets linked to other iframes!
Which means for some drones and cameras, the computer has to find up to over 100 frames, comb through for iframes, partial iframes, and data-sets, and fabricate that entire group of images before the first frame can be played back.
It's almost insane what we expect computers to do these days, and that they do it at all is amazing.
But ... if ... your computer has the right hardware bits to handle the decoding of long-GOP files as a 'hardware process', rather than by "manual calculations" doing the fabrication work, it can be ... not too bad.
Until you start throwing too many other effects on top of the decoding work.
6
Replies
AdChoices
