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Why Is Contrast Lost In Compression?


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#1 Macks Fiiod

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Posted 07 December 2016 - 02:54 PM

Been scratching my head on this for a while now.

 

Whenever I download a (poorly) compressed video file and view it, it's always a bit grey by default. All the hard whites and hard blacks more resemble light greys and dark greys. What is the reason for this? Surely the most basic of computer programs can render absolute blacks and whites, right?

 

Although I understand why some saturation would be lost.

 

Thanks for all input.


Edited by Macks Fiiod, 07 December 2016 - 03:07 PM.

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#2 Phil Rhodes

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Posted 07 December 2016 - 03:07 PM

Short answer: it isn't.

 

The problem comes from mishandling of video levels by compression software, not specifically from the compression itself.

 

This is a problem because flags to indicate the encoded video levels are either absent or unreliable in many (most) video files, and so a  lot of software just makes the least-destructive decision, which leads to a buildup of milkiness.

 

It's a very common issue, but it's particularly a problem in software like the open-source tool ffmpeg, or more to the point the libraries that make it up, which are very widely used. The people who write this software are expert software engineers, but like many open source people they are very reluctant to take outside guidance. It seems that the group that works on that particular project don't have a very good understanding of video levels (or gamuts) and the software handled them poorly for years; I believe it's since become a little better, but it still largely relies on users manually setting things up.

 

Worse yet, regardless of the underlying libraries, a lot of user-facing software doesn't even include settings for this.

 

It is a problem.

 

P


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#3 Tyler Purcell

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Posted 08 December 2016 - 01:39 PM

As Phil points out, it's really a software/encoding glitch. I encode stuff every day and it looks flawless.

I've found the base level mpeg 4 engine to really suck. This is what many windows based software packages use as their "compression" type for making smaller files. It's only when you get into the .h264 engine, that things really become substantially better.

On the mac side, .h264 is standard, it's the base level compression. Since it uses the quicktime engine to encode, Apple can control the quality with automatic software updates, rather then customers downloading new versions of software constantly. In 2012, Apple released an all-new version of Compressor, which is the widest used program for transcoding on mac's. It has an all-new .h264 engine that works really well, though it's dog-ass slow. At 15,000Mbps (which is what I compress most of my media at for upload) double pass, advanced profile, 1080p, 23.98, if you put the Pro Res master and the .h264 next to each other, there is very little difference between the two. The .h264 is slightly softer and there are some motion related artifacts (adds to the softness), which only a trained eye could see. Also, the colors aren't as vibrant, they're a bit muted on the compressed version. But that's comparing the two files on a $30,000 color grading monitor. On your home computer screen, they will look identical. If mac's didn't automatically display file sizes constantly, I wouldn't know the difference between the two on my computer.
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