Need understanding of modules, anamorphic, pixel aspect ratio

[JeremyBrown]

Lately I have been taking unimportant 1080p videos and re-encoding them down to 720p's x264s at around 2500 Kbps in Handbrake.

When doing the resize, I notice that the videos tend to start with a PAR of 1x1. Thinking that this may be important, I will crop a couple of pixels from top and bottom to get it back to a PAR of 1x1. These are usually letterboxed videos. I also notice that I can also choose between mod 2, 4, 8 or 16 and this affects the width and height. Handbrake says to just use 2 but I think I've heard that having mod 8 or even mod 16 is important is important for encoding efficiency or something. I also notice that the dimension of the display size and source may be different even when not resizing and that VLC also shows 2 sizes for the videos (resolution and display resolution).


Should I worry too much about all this stuff or just leave it at modules 2 and not worry about getting a PAR at 1x1? Is there a simple guide that explains this stuff? I've read some stuff on the web but some just talk about modules and some just talk about pixel aspect ratio so it's all a bit confusing for a newbie.

[DB83]

Pixel Aspect Ratio (PAR) has nothing to do with the MOD of the video.

MOD is simply the common divider of the width/height of the entire video which could have square pixels ie a PAR of 1:1 or non-square pixels ie anamorphic.

So consider a dvd which will use non-square pixels to display letter-box/widescreen against Blu-Ray which would use square pixels to achieve the same.

MOD becomes important since not all codecs support every MOD.

[hello_hello]

Anamorphic encoding is mostly used for anamorphic sources such as DVDs where the resolution and display aspect ratio (or display size for Handbrake) are different (they don't have a 1:1 PAR), the idea being not to resize or change the shape of the pixels much, but when you are resizing it allows the PAR to be altered if need be to compensate for any aspect error. When encoding non-anamorphic sources with a 1:1 PAR it no doubt still works the same way.

As an example, 1280x720 is exactly 16:9 (1.777778). If you were to resize to a width of 720 while keeping to mod16, the nearest resizing would be 720x400 which is an aspect ratio of 1.8:1, so there's a slight aspect error. You could fix it by using anamorphic encoding to change the shape of the pixels a little so 720x400 will display as 16:9, or you could crop a little picture from the source to make it 1.8:1 and then you could resize to 720x400 without any aspect error. Cropping 2 pixels from the width and a total of 10 from the height of a 1280x720 source would give you 1.8:1 and that's effectively what you're doing when you crop to keep the PAR 1:1.

Mod16 isn't required for x264 encoding and the mod makes almost no difference to encoding efficiency. If you want a 1:1 pixel aspect ratio, the lower the mod, the more accurate the resizing can be. I don't use Handbrake and haven't seen the latest version but it sounds like you're using "anamorphic loose". If you were to switch to "anamorphic none" the output should always have a 1:1 PAR, although I don't know if Handbrake displays any aspect error when resizing that way. Normally you'd select a resize width and the height would be adjusted to match.

It really doesn't matter too much how you do it. Some hardware players assume the PAR is always 1:1 for MKVs or MP4s etc, in which case the display aspect ratio won't be quite correct if the PAR isn't 1:1, but on the other hand anamorphic encoding allows the PAR to change so resizing won't cause any aspect error, as long as the player supports displaying anamorphic MKVs or MP4s correctly.

If a source has a 1:1 PAR, I keep it that way when resizing and crop a bit of additional picture as required to keep any aspect error close to zero, but how you do it is up to you.