[Home]History of Calculating Frame Size And Aspect Ratio


Revision 6 . . February 4, 2007 4:52 am by Achurch [Oops, -j 0,8 was totally incorrect]
Revision 5 . . February 4, 2007 12:33 am by Achurch [Clarify that the PAR changes in Ex.2]
Revision 4 . . February 3, 2007 5:43 am by Achurch [More examples (part 3, finished)]
Revision 3 . . February 3, 2007 5:29 am by Achurch [More examples (part 2)]
Revision 2 . . February 3, 2007 4:50 am by Achurch [More examples (part 1, still editing)]
Revision 1 . . February 3, 2007 3:44 am by Achurch [Moved from aspect ratio page, not edited yet]
  

Difference (from prior major revision) (no other diffs)

Changed: 49,57c49
The size 1024x576 is unchanged from Example 3, since the height of the original video is unchanged, and will work just as well. Note, however, that the second size, 720x405, has an odd value (405) for the height. Trying to use this size will cause an error, because the XviD video data format does not allow an odd width or height (as is the case for most current video codecs). In order to transcode the video using a width of 720, the height would need to be adjusted to an even number, either 404 or 406.

One way to adjust the height is simply to change the size given to the -Z option. While this accomplishes the desired change with no extra processing overhead, it also makes a slight change to the DAR, so that the resulting video will look a little bit stretched or squished vertically (though the change will be imperceptible to most people).

To avoid this change in the aspect ratio, you can instead use the -j option to change the size of the input video before the zoom is performed. For example, by clipping 8 pixels from the left and right edges of the video:

-j 0,8,0,8

you can reduce the input video's frame size to 704x576, giving you exactly the same situation as Example 3 (where a direct reduction to 704x396 could be made). Since this only requires zooming the frame vertically, it is faster and introduces less distortion than zooming the original 720x576 image to 704x396.
The size 1024x576 is unchanged from Example 3, since the height of the original video is unchanged, and will work just as well. Note, however, that the second size, 720x405, has an odd value (405) for the height. Trying to use this size will cause an error, because the XviD video data format does not allow an odd width or height (as is the case for most current video codecs). In order to transcode the video using a width of 720, the height would need to be adjusted to an even number, either 404 or 406. This will, of course, result in a slight change to the DAR, making it 16:8.98 or 16:9.02 respectively; if this is a problem, you'll have to choose a different horizontal size, such as 704x396 as in the previous example. (This will be slower, since transcode will have to zoom both horizontally and vertically.)

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