letterbox formula?

[Alan]

Hi,
whats the formula to calculate arbitrary letterbox pixel dimension for any given resolution?

for instance 2.25 letterbox in NTSC

Thanks,
Alan

[cyril conforti]

but this doesnt take into account the pixel ratio. if i use the crop guides to find out this info it gives me a Y value of 286. but 720/2.25*.9 is equal to 288.

scott212 wrote:

I’ll take that challenge:

to get the new vertical size, use this equation:
hRes / crop = vRes[w/crop]

to find out how far to move the picture vertically to be sure that it’s aligned in the center, use the following:
(vRes – vRes[w/crop]) / 2 = vEdgeDist

ex. 2.25 letterbox in NTSC (720×480)
720 / 2.25 = 320 <– vertical image size
(480 – 320) / 2 = 160 <– distance from edge (letterbox size)

ex. 2.25 letterbox in 720 (1280×720)
1280 / 2.25 = 568.889
(720 – 568.889) / 2 = 75.5555

In shake, here’s what I do (macro waiting to happen!):
1. Resize the image (resize node)
2. Using the formula, set the desired dimensions based on your crop
3. Create solid black image (color node) at delivery res (1280×720)
4. Feed black image and resize node into an over node w/ black in back (duh)
5. Attach a Move2d node in between the resize and the over nodes.
6. Using the second half of the formula, input the vEdgeDist into the yPan parameter.

Sorry, no flame access, just a student.

Best,
Scott

[bnw]

Pixel aspect ratio for NTSC is actually 10/11 which is 0.909090909 etc, giving you 290 and a bit. The difference is maybe due to the fact that not all the 720 pixels of Rec. 601 video are active picture. Actually only the middle 710.85 pixels are. If you round that down to 710, since you can’t be sure that the .85 of a pixel will be reproduced, you get a 2.25 letterbox height of 286.868686 etc. Hmm. I can’t think of a good reason to round that down to 286 rather than up to 287 for a safe-zone guide though.

Aren’t analogue video standards horrible?

BTW what on earth are you doing with a 2.25 aspect? Surely 2.35? Aren’t analogue film standards horrible? 🙂

[Author]

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