Resizing and AR question

[horrormaster34]

I'm just having trouble understanding why it's always recommended to round to a multiple of 16 when resizing. It seems to have more of a negative affect on the original aspect-ratio of the video.

If you have a full-screen 1280x720 video and want to resize it down to a width of 640, it would seem you would use a height of 360 so that the aspect-ratio remains the same of the source (1.7778). But, 360 isn't a multiple of 16, so instead you have to use 352. That would screw with the original aspect-ratio and yield 1.8181.

What's the point in using a multiple of 16 when it just screws with the original aspect-ratio?

[Keiyakusha]

You can resize to 640x360 and then crop it to 640x352 so aspect ratio will be fine.

[horrormaster34]

The video takes up the full frame so cropping the height 8 pixels will remove parts of the video, not black space.

[Corpsy]

I'm sure he realizes that. I think he just assumed that cropping the height by 2.23% wasn't likely to remove any important details.

Not all aspect ratios are going to be multiples of 16 in both height and width, and there's no law that says you have to abide by that guideline. I think it just has to do with the efficiency of most compressors and being able to use 16x16 macroblocks in all parts of the image. Basically, I believe that compressing a video that's 640x360 is probably the same as compressing one that's 640x368. I doubt it's very significant.

[CWR03]

Resizing to a number evenly divisible by 16 is only a recommendation geared toward compatibility with certain hardware, such as standalone players that support MPEG-4 files. If your main use for your output will be PC, it makes no difference because it will generally display at exactly the encoded AR.