Mod16 question and DAR question...

[alcOre]

Ok...

I have recently purchased an iPod Touch (love it by the way) and I've been teaching myself a few things about converting videos to different formats that the Touch supports.

I've learned a lot but I still have a few questions.

First, my questions are general questions about video and not necessarily specific to my iPod Touch, so I decided this would be the best forum.

Background : My source of video has been backups of my own dvds and captured high def video from my cable DVR (Motorola DHC3416) captured using CAPDVHS. I've been using the following tools : MeGUI, mpeg2repair, DGIndex and VideoReDo.

Question 1 : First question is about resizing as it relates to Mod16. What I want to know is this : What am I loosing by not resizing to a number divisible by 16? At a given bitrate, will video look better if its resized to a Mod16 value? I know some codecs don't support non-Mod16, but x264 seems to. I know some players dont decode/display non-Mod16 video correctly, but the iPod Touch seems to. So what am I loosing if I choose to resize to a non-Mon16 value? Assuming I am loosing anything resizing to a non-Mod16 value, does it get worse the further away from divisible by 16 it gets? What I mean is, if I encode to a number divisible by 4, am I loosing more than by choosing a number divisible by 8? And divisible by 2 worse than divisible by 4? I'm very picky about how my video looks on my iPod, so I really don't want to loose any quality if I don't have to, but sometimes I want to resize to resolutions that are non-Mod16 to try to preserve the original aspect ration. I know I can use a DAR to preserve that aspect ratio, which leads to my next question...

Question 2 : I made 2 encodes of the same short clip of a movie. The first one, I re-sized to a resolution that was fairly close to the original aspect ratio, and DID NOT use a DAR (aka Clever Anamorphic Encoding in MeGUI.) I encoded it again, but this time I DID use a DAR to make the aspect ratio exactly match the original source's aspect ratio. My theory was that since the pixels have to be stretched during playback when using a DAR to get the correct aspect ratio, that the video would look slightly more blurry than one encoded with square pixels (ie my first test encode. ) I captured two stills of the video on the same frame, and to me it did appear that the video using a DAR to obtain the exact correct aspect ratio did look blurry compared to the video that had a slightly-off aspect ratio but encoded with square pixels, since no stretching of the pixels was required (other than the resizing, which was done to both samples.) PS...I did encode both clips at the same bitrate and my resolutions on both equaled roughly the same amount of total pixels encoded. My question is...is that conclusion/theory correct? Does using a DAR on video make it look blurred compared to square pixels? Like I said, I'm picky about quality, but I also want the correct aspect ratio. Hence my first question. If using a DAR does cause the video to look less sharp/slightly blurry, I would like to encode at non-mod16 so I can get to within 1 pixel of exact original aspect ratio using square pixels.


Thanks

[jagabo]

1) For a given bitrate you get slightly worse quality if you don't use Mod16. You get slightly larger files if you use quality based encoding. As soon as you go over Mod16 the bitrate requirement goes up almost to the next Mod16 size. I ran a quick test. A short 720x480 4:3 video was resized to each of the following resolutions and compressed in CQP mode (constant quality):

320x224: 9556 KB (Mod16)
320x226: 9908 KB
320x228: 9974 KB
320x230: 10058 KB
320x232: 10053 KB
320x234: 10078 KB
320x236: 10088 KB
320x238: 10079 KB
320x240: 10037 KB (Mod16)

So, for example, a change in frame size from 320x224 to 320x226 is only about a 1 percent change in the number of pixels but it required about 4 percent more bitrate. Almost as much as going up to the next Mod16 frame size, 320x240, which is about 7 percent more pixels.

As a practical matter, if this had been done in bitrate mode, ie, all encoded at the same bitrate, you would not be able to see any quality difference at normal playback speed. If you viewed enlarged still frames you would be able to pick out slight differences but you would be hard pressed to say which was better.

2) If you get 1:1 pixel mapping without the DAR flag then it will be clearer than with the DAR flag. Otherwise they're both being resized to fit the screen and it's not obvious which will look better.

[alcOre]

2) If you get 1:1 pixel mapping without the DAR flag then it will be clearer than with the DAR flag. Otherwise they're both being resized to fit the screen and it's not obvious which will look better.

I'm assuming what 1:1 pixel mapping means, if I were to just encode the video without a DAR and watch it with no resizing on my laptop for example, it would be clearer than a video with a DAR. But since I'm watching these videos on my iPod screen, and they're getting resized anyways, I might as well stick to using Mod16 resolutions and using a DAR to keep aspect ratio?

[jagabo]

Sorry, by 1:1 pixel mapping I wasn't refering to the pixel aspect ratio (square pixels) but rather that each pixel in the source video is displayed as one pixel on the LCD. Any other mapping means some kind of digital scaling (blurring and moire artifacts) will be taking place.

[alcOre]

Yes...that's what I thought. As in, if I encode using square pixels, at some resolution (lets say 608x288 for sake of arguement) and watch that on my laptop, it will be nice and crisp, since my laptop can easily fit that many pixels on the screen and the pixel mapping will be 1:1 (assuming I leave the player's window to display the video at actual size.) But since I'm watching/encoding these videos for my iPod and it has to resize the video down to fit on my ipod screen, I should just use a DAR and not worry about square pixels since the iPod has to resize the pixels anyways to fit on my ipod's screen/resolution. Correct? Thanks for your help...

[jagabo]

No, I mean if your converted videos are exactly the right size for the iPod screen they will be rendered more clearly than if they have to be resized during playback to fit the screen.

I'm not real familiar with the iPod Touch but let's say it has a 480x360 square pixel display, displays videos full screen, and with the correct aspect ratio (I'm not sure that any of these is true). In this case a 480x360 square pixel video will be mapped 1:1 onto the screen. A 480x272 square pixel video will also be 1:1 and the extra screen space will be filled with black letterbox bars. A 360x360 video will get 1:1 mapping too (with pillarbox bars). But a 432x324 or 512x384 video will be resized to 480x360 and won't render as perfectly.