<p>A single layer DVD has about 4.5 GB of capacity, a dual layer DVD has about 8.5 GB of capacity. Some of this space is taken up by menus and special features, but that still leaves plenty of room for video with minimal compression.</p>

<p>Take a look at the "blocky" videos - they have probably been re-compressed down to 700MB (so they can fit on a CD-R blank) or 1GB (because people love round numbers). Shrinking the video down requires more aggressive compression settings, which results in lower quality video. Sometimes this lower quality exhibits itself in terms of large blocky dark regions.</p>

<p>The other issue is that the compression algorithms used on DVD video are lossy. When you transcode to a lower bitrate codec, the software uncompresses and then recompresses the video. Since the compression is lossy, the uncompress step recovers imperfect video, and that imperfect video is then compressed. This slightly compounds the quality problems.</p>

<p>If you want to make a perfect copy, just rip out the original video. Don't transcode it to a new format.</p>

CC BY-SA 3.0
2012-08-04T14:33:38.873Z
The problem with this, is that overly compressed video tends to have artifacts throughout it. These files only have artifacts in the dark regions, and even then light areas in the dark region are fine.
Canageek
CC BY-SA 3.0
2012-08-04T15:48:02.873Z
Overcompression isn't a binary thing. If you pull off the original file and never transcode it, the video quality won't change. You are either transcoding the video to a codec with similar quality settings (causing mild degradation) or turning up the compression.
ObscureRobot
CC BY-SA 3.0
2012-08-05T14:32:52.123Z
So why does it hit the black areas first?
Canageek
CC BY-SA 3.0
2012-08-05T18:26:45.293Z
The effect happens everywhere, but the human vision system detects differences in black shading before it detects differences in colors. It is also possible that your display's Gamma curve is set such that it exaggerates small differences in black level.
ObscureRobot
CC BY-SA 3.0
2012-08-02T02:53:01.353Z
2012-08-02T02:53:01.353Z
ObscureRobot
<p>When a DVD is ripped to a file on a computer the dark scenes typically get compressed to the point of illegibility; huge squares of black are all set to the same value, removing all the detail. This is often seen in video files posted to the internet.  </p>

<p>I get why it happens; The difference between two shades of black is quite easily to perceive, but to the compression algorithm it is a tiny difference, so the two shades are set to one value to save space. </p>

<p>However, this effect is obviously avoided on the DVD itself. How do they do this, and why don't the compression methods used when ripping the DVD to a file also do this? I mean, the DVD format and codecs are old, far older the the ones used online today. So why does this problem appear when ripping the file?</p>

CC BY-SA 3.0
2012-08-04T15:49:37.630Z
Strictly speaking, ripping is the process of pulling data off the disc. If the video ends up in another codec, you have been transcoding the video as well. Transcoding is the likely source of the blockiness.
ObscureRobot
<compression>
How do DVDs avoid the blockyness that plagues dark scenes in most video files?
Canageek
# How do DVDs avoid the blockyness that plagues dark scenes in most video files?

(Asked by: username_0 on 2012-07-31)

When a DVD is ripped to a file on a computer the dark scenes typically get compressed to the point of illegibility; huge squares of black are all set to the same value, removing all the detail. This is often seen in video files posted to the internet.

I get why it happens; The difference between two shades of black is quite easily to perceive, but to the compression algorithm it is a tiny difference, so the two shades are set to one value to save space.

However, this effect is obviously avoided on the DVD itself. How do they do this, and why don't the compression methods used when ripping the DVD to a file also do this? I mean, the DVD format and codecs are old, far older the the ones used online today. So why does this problem appear when ripping the file?

**Tags:** compression

> **Comments:**
> **username_1**: Strictly speaking, ripping is the process of pulling data off the disc. If the video ends up in another codec, you have been transcoding the video as well. Transcoding is the likely source of the blockiness.  

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## 1 answers

### Answer by username_1 on 2012-08-02. Score: 3

A single layer DVD has about 4.5 GB of capacity, a dual layer DVD has about 8.5 GB of capacity. Some of this space is taken up by menus and special features, but that still leaves plenty of room for video with minimal compression.

Take a look at the "blocky" videos - they have probably been re-compressed down to 700MB (so they can fit on a CD-R blank) or 1GB (because people love round numbers). Shrinking the video down requires more aggressive compression settings, which results in lower quality video. Sometimes this lower quality exhibits itself in terms of large blocky dark regions.

The other issue is that the compression algorithms used on DVD video are lossy. When you transcode to a lower bitrate codec, the software uncompresses and then recompresses the video. Since the compression is lossy, the uncompress step recovers imperfect video, and that imperfect video is then compressed. This slightly compounds the quality problems.

If you want to make a perfect copy, just rip out the original video. Don't transcode it to a new format.

> **Comments:**
> **username_0**: The problem with this, is that overly compressed video tends to have artifacts throughout it. These files only have artifacts in the dark regions, and even then light areas in the dark region are fine.  
> **username_1**: Overcompression isn't a binary thing. If you pull off the original file and never transcode it, the video quality won't change. You are either transcoding the video to a codec with similar quality settings (causing mild degradation) or turning up the compression.  
> **username_0**: So why does it hit the black areas first?  
> **username_1**: The effect happens everywhere, but the human vision system detects differences in black shading before it detects differences in colors. It is also possible that your display's Gamma curve is set such that it exaggerates small differences in black level.  

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