Aspect ratio confusion with MeGUI

[nofsky]

Hi, first post here :)

I have started to convert some of my DVDs to x264 MKVs so I can view them on my new HTPC without touching the discs. I have been using MeGUI to do this and I think I'm doing things correctly to keep a good quality and proper aspect ratio. All this about aspect ratio is a bit confusing though so I was hoping some of the pro's on this board could tell me if I am doing something wrong.

I'll use the movie Cube as an example. On the back of the dvd it says this movie is in 1.85:1 aspect ratio. I am aiming for 1/3 DVD sizes for all my movies (1.45 GB).

Ok, so I make a d2v from the VOB in MeGUI...
In the AviSynth script creator in MeGUI it says at Input DAR "ITU 16:9 NTSC (1.822784)".

I press autocrop which cuts 4 on left & right, 8 on top and 10 on bottom.

Then I check resize and suggest resolution. It suggests 720x384 which is 1.875:1 aspect ratio.

I resize with spline36 and minimal noise filter and analyze for deinterlacing.

Then I press to save the script, AviSynth script creator closes, and a new preview window pops up. In this window it now says 1.875 DAR so I change it to 1.85 as it says on the back of the DVD.

Is this a proper way of making sure the aspect ratio is correct and the video is good quality?

[Inspector.Gadget]

No. If you're going to encode anamorphically after cropping, you need to keep the original PAR (pixel aspect ratio) value, which is tied to the original DAR (display aspect ratio) that MeGUI detects. Once you crop from the video*, you don't have the original DAR but you may retain the original PAR by using MeGUI's anamorphic encoding function. As a result, the output dimensions will not be the same as the original, but the output pixel dimensions will be the same, resulting in a properly stretched picture.

*Except proportionally to the original DAR, which isn't the case here.

[nofsky]

Ok, I've tried to use the clever anamorphic encoding option with resize to mod16.

MeGUI suggests resolution 720x464.

When I save the script the new preview window says 1.873... do I change this to 1.85 (like the back of the dvd says)?


It seems to me that I'm wasting a lot of space by encoding to anamorphically though....

[Inspector.Gadget]

When I save the script the new preview window says 1.873... do I change this to 1.85 (like the back of the dvd says)?

No. MeGUI has automatically re-calculated a new, correct DAR. Leave it as is.


It seems to me that I'm wasting a lot of space by encoding to anamorphically though....

No. You're encoding with pixels that are stretched on playback, resulting in encoding fewer pixels than you would if encoding everything with square pixels. You may realize further, slight gains in efficiency by overcropping to mod16 instead. This should also (perhaps imperceptibly) increase video quality.

[nofsky]

It seems to me that I'm wasting a lot of space by encoding to anamorphically though....

What I meant is wouldn't encoding anamorphically reduce the overall video quality since when anamorphic the video size is 720x464 and when not it's 720x384 and I'm using the same bitrate for both (to fit into 1/3 DVD)?

Maybe I should accept the slight aspect ratio error when encoding as I originally did or maybe I should allow bigger file sizes...

[Inspector.Gadget]

What I meant is wouldn't encoding anamorphically reduce the overall video quality since when anamorphic the video size is 720x464 and when not it's 720x384 and I'm using the same bitrate for both (to fit into 1/3 DVD)?

No. Each pixel is still encoded as if it were square, but stretched on playback. There is no quality cost. I suggest you research anamorphic video further to understand what actually happens.

[nofsky]

No. Each pixel is still encoded as if it were square, but stretched on playback. There is no quality cost. I suggest you research anamorphic video further to understand what actually happens.

But I am encoding more pixels when I'm using the clever anamorphic option in MeGUI... 720x464 pixels versus 720x384 when not anamorphic. That is more pixels. Playing the finished anamorphic video in VLC displays a stretched 868x464 picture. When I'm watching them fullscreen it makes no difference except the anamorphic has slightly more accurate aspect ratio but also less video quality because of more pixels encoded into the same file size as the smaller resolution non-anamorphic video.

[Inspector.Gadget]

If you're resizing to a smaller size when not using anamorphic than when using anamorphic then of course you are using fewer pixels. But that's a totally worthless comparison, and if you want the smaller size there's no reason not to preserve the anamorphism at the smaller size. For instance:

Start with an (always anamorphic) 4:3 NTSC DVD. It is cheaper, in terms of number of pixels, to encode at 720x480 anamorphic than to resize and encode at 720x528. Obviously, it is cheaper in pixels terms to resize and encode at 640x480 at a 1:1 pixel aspect ratio - but then you're not even talking about the same video dimensions. Your hypothetical doesn't concern anamorphism but merely resizing, and that is totally up to your preference.

[nofsky]

Ok, thanks for your help.

I think I'll just resize and not concern myself with perfect aspect ratio.