[warning, long post] On Fri, 02 Feb 2007 14:27:25 +0000, Andrew Church wrote:
>>According to the transcode manual, original picture P * Q @ r1 >>should be zoomed to intermediate M * N @ r1, then pad back to >>X * Y @ r2. I've attempted many time trying to understand it, >>but the more I looked into it, the more chaos I seemed to get. > > Does http://www.transcoding.org/cgi-bin/transcode?Aspect_Ratio seem > confusing to you, Actually, what confused me is that each person has his own way of doing the calculation. You have one there in the wiki, JoRN REDER has one in LINUX-MAGAZINE, Rick Harris has one in this mlist archive, but all seems to me are not agree with what the transcode manual says. So far I've got more than 4 ways to do the calculation but I haven't find two that can agree with each other. > particularly the section starting with "As another > example,..." which is intended to explain this calculation? If so, > how is it confusing (so I can fix it)? Ok, let me try, There are two kinds of aspect ratios in the wiki. ,----- | pixel aspect ratio or PAR (also known as "sample aspect ratio" or SAR), is | the ratio of the width to the height of a single pixel in the video image. `----- This seems contradict with the pixel aspect ratio definition used in the transcode manual: ______________________________________________________________ --export_par C[,N] set export pixel aspect ratio to C[,N] To encode anamorphic material, transcode can encode the target pixel aspect ratio into the file. This is NOT the actual aspect ratio of the image, but only the amount by which every single pixel is stretched when played with an aspect ratio aware application, like mplayer. ______________________________________________________________ The way PAR is defined in transcode manual is more widely accepted. Most video converting tools stick to such definition. E.g., mplayer, tovid, ffmpeg, etc. ,----- | One is display aspect ratio or DAR. this is the ratio most commonly referred | to by the term "aspect ratio", and is the ratio of the video frame's | physical (displayed) width to its height, regardless of the number of pixels | used to represent the video image. `----- Ok, here is another aspect ratio, DAR. What I can't make out is that neither of above 2 aspect ratios are the commonly used 4:3 or 16:9 video aspect ratio according to the wiki. seems to me there are 3 aspect ratios implied in wiki, the most commonly used is unnamed. The transcode manual however, explains how to do the resizing without those different aspect ratios. The example that begins with "Lets assume that you have an NTSC DVD" in transcode manual explains how to do the resizing clearly, at least to me it make sense, and I can follow -- just having a difficult time trying to apply the knowledge. ,----- | The PAR may be determined by the display hardware. for example, most LCD | televisions and monitors have a PAR of 1:1. (CRT monitors and televisions | are able to change their PAR on the fly, because the analog signals which | they use do not have the concept of "pixels".) `----- What I like to know is, since my monitor is CRT monitor and I watch DVD on TV, what is the PAR for CRT monitor and TV respectfully. Without this value, I can't follow the wiki to do the calculation. ,----- | DAR width | --- = ------ | PAR height `----- hmm... any reason behind this? I can't figure why there is such equation. Further, since most LCD televisions and monitors have a PAR of 1:1, so mostly DAR = DAR / 1:1 = width / height. but this will contradict with the DAR definition above. ,----- | As another example, suppose you have a 672x272 video file on your computer | that you want to transcode to an NTSC (720x480) DVD. `----- The "common-sense" aspect ratio, ie 4:3 or 16:9, for this 672x272 video file is not mentioned in the wiki. Would the calculation that follows still hold, if the video file is of 4:3 or 16:9, or change between them? Andrew, I know you were trying to explain the hard question from a different prospective, so as to allow people have more fully understand of the problem. And I know you were trying to condense the vast relevant knowledge into the wiki -- you once said "I started writing about this, but changed my mind..." about the pixel aspect ratio of 525-line video or 625-line video... all in all, I think it is a very convoluted topic, and appreciate your effort trying to strait it out. I have two suggestions. - would it be possible to base your wiki according to transcode manual? if the wiki is entirely different than the transcode manual, people would have a hard time trying to follow both. - for your "As another" example, I like the fact that you choose a none-standard size, 672x272. Would you finish it with another none-standard size, say 704x480, and change the "common-sense" aspect ratio, ie 4:3 or 16:9, between the source & destination? thanks -- Tong (remove underscore(s) to reply) http://xpt.sf.net/techdocs/ http://xpt.sf.net/tools/
