How is 720x480 4x3 format?

[AMCooper63]

I was trying to calculate cropping a 720x480 (4x3) recording that is really a 16x9 show. I know that 4x3 should be 640x480. But MPG stadard is 720x480. For a 4x3 ratio it should be 720x540. I'm trying to figure out if I should crop the video to 720x405, or if I should crop to 720x360 then stretch it to 720x480 and call it 16x9 non square pixel.


720/16*9=405
640/16*9=360

[nielm]

Originally Posted by AMCooper63

I was trying to calculate cropping a 720x480 (4x3) recording that is really a 16x9 show. I know that 4x3 should be 640x480. But MPG stadard is 720x480. For a 4x3 ratio it should be 720x540. I'm trying to figure out if I should crop the video to 720x405, or if I should crop to 720x360 then stretch it to 720x480 and call it 16x9 non square pixel.

its your choice, and depends on the software you have, and the file format target.... If you are recompressing to AVI, most AVI players do not support aspect ratio flags properly, so in most cases you have to re-scale to 'square' pixels

I am guessing you are starting with a 720x480 letterboxed 16x9 show (top/bottom black bars)...so you have 720x360 cropped, which you need to rescale to either 640x360 or to 720x405 (as you have already calculated) to get a 16x9 square pixel video.

Tools like AutoGK do this automatically. GordianKnot provides you with a nice interface to do this manually.

If your target is MPEG2, then you just crop to 720x360 and tell the mpeg2 editor that the output is 16x9, which will set the appropriate 16x9 flag in the output stream (which Sage should respect if in 'source' aspect ratio).

One final option is to keep 720x360 and use sages aspect ratio settings to manually (or semi-automatically with my plugin) to correctly resize the video on output.

[SHS]

640x480 is PC 4x3 which is not that as TVset 4x3

[AMCooper63]

I'm not sure I got all of what I wanted. I think I understand that I need to crop my 720x480 MPG2 video to 720x360. Then because that is not a valid aspect ratio for MPG2, I have to stretch the video to 720x480 and mark it as 16x9 so that decoders will use non square pixel rendering and stretch the video to 853x480.

Does anyone know why 720x480 is used for 4x3 video? 720x480 is actually 3x2 format. How is it decoded so it's not distorted? Is it actually decoded to 640x480?

[nielm]

Originally Posted by AMCooper63

I'm not sure I got all of what I wanted. I think I understand that I need to crop my 720x480 MPG2 video to 720x360. Then because that is not a valid aspect ratio for MPG2, I have to stretch the video to 720x480 and mark it as 16x9 so that decoders will use non square pixel rendering and stretch the video to 853x480.

... but before any stretching will need you to deinterlace (or IVTC) before so that you dont get any loss of data. Scaling interlaced, or telecine'd movies without deinterlacing will result in a mess!!

720x480 is just a 'profile' for DVD MPEG2 video, the actual mpeg2 compression format does not care what the size is, as long as it is in multiples of 8...

If you can crop to 720x360 and mark as 16x9, you may be OK in Sage. (DVD players may not like it though)

Originally Posted by AMCooper63

Does anyone know why 720x480 is used for 4x3 video? 720x480 is actually 3x2 format. How is it decoded so it's not distorted? Is it actually decoded to 640x480?

it is marked as 4x3 in the mpeg2 stream and re-scaled on output by decoders that know what they are doing.

x=720 was probably chosen for some complicated compromise between compression ratio and perception of pixels and encoding noise...
y=480 is because this is the number of lines in SDTV.