@jak.willis

Hi Jack.

As far as I'm aware there is no decompression of videos on playback.

I think you have the wrong idea of compression in this context.

The compression throws away bits of data that are combined into larger blocks of colour where there is less detail within each frame. It is not recoverable, which is why there are non compressed formats to work with if needed before exporting but these only have the information of the video it is captured from, so if the original file is not a raw video format it has already been compressed when recorded from the camera.

Hi there, thanks for your response to my question.

Perhaps I’m getting the word “decompression” confused with the word “decoded”?

When I Google this stuff a lot of posts from various people claim that when you play back a video it has to be decoded or decompressed back to its “original form”. That’s why I have been confused because I couldn’t understand what the point of compression was if it then has to be decompressed upon playback.

@jak.willis

Hi Jak.

Yes, the file has to be decoded for the format it has been saved to as most video file formats are not compatible with each other as they use different logarithms to encode the files. But decoding neither adds nor subtracts from the quality of the file. It is simply an instruction set to tell windows how to play the data. What pixel goes where at what time etc.

Ray.

@jak.willis

Hi Jak.

Yes, the file has to be decoded for the format it has been saved to as most video file formats are not compatible with each other as they use different logarithms to encode the files. But decoding neither adds nor subtracts from the quality of the file. It is simply an instruction set to tell windows how to play the data. What pixel goes where at what time etc.

Ray.

Hi again, Ray.

Okay, I think I see what you’re saying. So when I hear the word “decompress” it doesn’t literally mean that?

@jak.willis

Hi Jack.

They are probably using the wrong term.

You can compress a video file to send to someone using something like Winzip that does compress a file that has to be decompressed at the other end before playing but the compressed file itself cannot be played. They are two different subjects.

One exception may be newer file types as those recorded on newer mobile phones that use variable frame rates but again they are not compressed in the normal sense. The coded information speeds up and slows down frame rates according to the amount of movement in each frame. The more static the action, the slower the frame rate becomes, saving memory space. That has to be played back with a similar codec, possibly unique to the phone's manufacturer and not commonly available to Windows editing programs. Although some video playback programs can play them, most video editing packages cannot handle the files. Do not blame the programs. It's possible the phone manufacturers are not allowing the codecs to be used by third party developers yet. They are battling for sales.

Video is becoming increasingly complex in the newer formats being offered. A type of new file format war. Not in my opinion, the best outcome. One reason I never use phone video footage for serious projects. Even Go-pros are now doing it. It will only get worse before it gets better.

Ray.

@jak.willis

Hi Jack.

They are probably using the wrong term.

You can compress a video file to send to someone using something like Winzip that does compress a file that has to be decompressed at the other end before playing but the compressed file itself cannot be played. They are two different subjects.

One exception may be newer file types as those recorded on newer mobile phones that use variable frame rates but again they are not compressed in the normal sense. The coded information speeds up and slows down frame rates according to the amount of movement in each frame. The more static the action, the slower the frame rate becomes, saving memory space. That has to be played back with a similar codec, possibly unique to the phone's manufacturer and not commonly available to Windows editing programs. Although some video playback programs can play them, most video editing packages cannot handle the files. Do not blame the programs. It's possible the phone manufacturers are not allowing the codecs to be used by third party developers yet. They are battling for sales.

Video is becoming increasingly complex in the newer formats being offered. A type of new file format war. Not in my opinion, the best outcome. One reason I never use phone video footage for serious projects. Even Go-pros are now doing it. It will only get worse before it gets better.

Ray.

Ah, I see what you mean about them being two different subjects.

So when it comes to playing a video file on, for example, a Blu-ray Player, then the file has to be decoded - not decompressed?

What is the difference between decoding and decompression?

@jak.willis

Hi

The term pairs compression/encoding and decompression/decoding are used interchangeably when referring to creating the video file and playing it.

A video comprises of a sequence of frames (still images), if all the image data was stored in the video file it would be very huge in size.

Using MP4 h.264 encoding as an example:

Compression/encoding works on multiple levels by discarding data from frames.

At the top level video is composed of up to 3 different types of frame

Intra (I) frame this is the full image and the 1st frame is always an I frame

Predicted (P) frame - this only hold the differences from the previous frame

Bidirectional (B) frame - this only holds the differences between the current frame and both the preceding and following frame.

As you can see the P and B frames only hold data for the difference, the rest is thrown away - this saves storing unnecessary data and reduces the file size.

The sequence of I, B and P frames make a Group of Pictures (GOP) - the sequence of the 3 types of frame varies depending on the codec doing the compressing/encoding.

@CubeAce comment - that are combined into larger blocks of colour - is another level of compression which involves complex mathematical matrix calculations which is beyond this basic description of compression/encoding.

Decompression/decoding does the reverse of compression/encoding ie it reconstructs each individual complete frame from the IBP sequence.

The first frame being displayed is the I frame, the next frame B or P, containing only the differences, is added in to the previous I frame to create the next frame. This process of adding the differences repeats for the subsequent frames until the next I frame is reached when the process starts all over again.

Hope this clears up any confusion.

John EB

@jak.willis

Hi Jak.

Decoding is an instruction set to tell a program how to put information into something usable instead of a string of ones and zeros. Whether it's text, audio or images.

Compression and decompression is about making files smaller. There are limits as how much you can compress any data.

In computer terms, to reduce storage space of a file that would normally take up more space. Normally such files have to be decompressed and placed back onto a disk / memory card, or similar before they can be played back. They cannot normally be used in their compressed form.

Ray.

They cannot normally be used in their compressed form.

But why is that?

@jak.willis

It's a different form of compression. Not a video format but a format to reduce file sizes for storage.

@jak.willis

Hi

What is the difference between decoding and decompression?

In the context of compressed video files - it means the encoded data needs to be decoded in order to decompress it. Codec actually means Co(mpressor)dec(compressor). All consumer and most professional cameras use a codec via an encoder to create a compressed video file instead of the raw RGB data and frame information, in order to store the images within in a reasonably compact file size. To display this compressed information in a player or in an editing program a codec is used by a decoder to recreate the RGB information so that the monitor can display a picture.

Because data is lost to a more or lesser extent during compression by lossy codecs like MPEG2 & MPEG4, depending on the efficiency of the encoder and codec and the target bitrate chosen, the decoded picture will look fairly true to the original scene - or rather how it would have looked if it was recorded as uncompressed RGB.

Perhaps to get your head around it you could look at it this way - the compression is not what loses the quality as this is reversed by the decompressor (decoder), but more the codecs algorithms deciding what you can afford to lose without noticing it, in the act of compressing.

It is worth considering that during playback of the timeline MEP - must to display each frame - decode and uncompress the frames information - process any effects or transitions you have applied - and display at 25 frames every second or higher. Because this is computationally intensive, the processing of the effects and the decoding has been outsourced to the GPU's hardware decoder chips, shader cores, and memory rather than trying to accomplish all of this within the CPU cores and main memory. MEP 2021 and VPX 12 have achieved an appreciable increase to the preview frame rate possible with compressed codecs, by outsourcing the decoding to a suitable GPU.

Peter

@Scenestealer

Wow!

Concise and eloquent as always Peter.

I think what I was trying to get across is the quality is always baked in once encoded. Data at that point, if lost remains lost. Also a more efficient compression - decompression format will always be more resource intensive.

That was what I was finding on my old system. It was not coping with H265 very well. 4K was almost impossible to play back let alone work with.

Most cameras have had similar problems and use codecs to slow down data rates to memory cards.Heat dispersion can be another problem when dealing with camera data transfer rates. Some of my memory cards get quite hot in use.

Ray.

depending on the efficiency of the encoder and codec and the target bitrate chosen, the decoded picture will look fairly true to the original scene - or rather how it would have looked if it was recorded as uncompressed RGB.

Yes, that’s what confuses me. When you compress a video and lose quality then how can the decoded picture look similar to the original?

Because compression is about losing data, ideally without losing quality!

In a series of images recorded at 25fps there is often little change in the picture contents from one frame to the next, so the compression is removing the blocks of pixels that do not change and only keeping information about the blocks that do change.

Because compression is about losing data, ideally without losing quality!

In a series of images recorded at 25fps there is often little change in the picture contents from one frame to the next, so the compression is removing the blocks of pixels that do not change and only keeping information about the blocks that do change.

Okay. So regardless of how decompression works, you don’t get back the original source quality how it was before you compressed it?

@jak.willis

Hi Jack.

It should be the same but re-rendering the file when editing will bin some of the original data. But there is a lot going on in the editor to try to make that minimal if the rendering settings allow. IE, not trying to set the data rates available too low when re-rendering or using a codec that bins too much information.

The second one is subjective but one reason I would never contemplate using WMA.

It's the reason Editors never alter the source files, so you have something to go back to with as much information baked into the file as it was when recorded.

Ray.

@jak.willis

Hi Jack.

It should be the same but re-rendering the file when editing will bin some of the original data. But there is a lot going on in the editor to try to make that minimal if the rendering settings allow. IE, not trying to set the data rates available too low when re-rendering or using a codec that bins too much information.

The second one is subjective but one reason I would never contemplate using WMA.

It's the reason Editors never alter the source files, so you have something to go back to with as much information baked into the file as it was when recorded.

Ray.

Hi Ray,

Okay, so otherwise when you playback the video it will be the quality that you rendered it in? Depending on what bitrate you’ve used, etc?

@jak.willis

Short answer is Yes Jak. The trick to keeping file sizes from getting too large or too small and not losing quality that you could notice easily in normal playback is to set them at roughly the same settings as the original settings of the source files, assuming you are using the same form of codec. It gets a bit trickier if trying to use a newer codec that is more efficient. This is why MEP provides base level settings that their programmers have felt gives the best balance of performance verses quality. They are good in most instances or a reasonable point to start tweaking from if you don't agree and feel you can do better.

I personally don't know if there is a 'correct way' to tweak the parameters or not but I look for variation in sharpness where detail can alter and colour graduations in large portions of colour such as a clear blue sky where banding can occur or in shadow areas where blocking is more common. Then for me it becomes a bit of a 'Dark art' similar to processing a stills raw image. A thousand ways to get it wrong and maybe a few where it works better.

So an ideal project for me does not contain files of differing frame rates or codec types or other types of variation but all taken a roughly the same time from the same source material. That rarely happens so I have to accept certain sections of the video when finished may look less sharp or more noisy or not quite the same colour balance depending on how well I process and blend those clips together.

Ray.

To display this compressed information in a player or in an editing program a codec is used by a decoder to recreate the RGB information so that the monitor can display a picture.

And if it didn’t do this then what would be displayed on the screen instead? A string of 1’s and 0’s?

@jak.willis

To display this compressed information in a player or in an editing program a codec is used by a decoder to recreate the RGB information so that the monitor can display a picture.

And if it didn’t do this then what would be displayed on the screen instead? A string of 1’s and 0’s?

Nothing would display as it would be a corrupted file.

@jak.willis

To display this compressed information in a player or in an editing program a codec is used by a decoder to recreate the RGB information so that the monitor can display a picture.

And if it didn’t do this then what would be displayed on the screen instead? A string of 1’s and 0’s?

 

Nothing would display as it would be a corrupted file.

 

 

Ok. And if it wasn’t decoded would it display a string of 1’s and 0’s?

Forgive me if I seem to be going on about this, however I am still learning.

Thanks

@jak.willis

Again no, it would be machine code.

Try going to an unwanted jpeg and open it with notepad to see what I mean. I say unwanted or a copy because it may corrupt the file once notepad is closed.

I'm not suggesting looking at a movie file. That may cause problems. The file will be huge.

Never any offence taken over questions genuinely asked. Just if you don't open a comment with a greeting (Hi Jak) some of us will reply in kind in case it is a cultural thing.

Ray.