Anamorphic SD true aspect ratio

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Level 6

12,595 points

Anamorphic SD true aspect ratio

While experimenting with effects applied to different aspect ratios I came across an interesting "anomaly".

It turns out that 720 x 480 anamorphic widescreen video has an actual aspect ratio of 1.81818...: 1, even when applied to a 16:9 (1.7777... : 1 a.k.a. 16:9) project. The horizontal edges are obscured by the project size "crop" by ~9.5 pixels on each side (but not removed/cropped).

The resulting width, normally expected to be 853 (.333... but there's not such thing as a fractional pixel) is actually 872 (which is measurable if you know how).

Typically, in SD "broadcast" there are allowances for "overscan". However, I do not understand this particular case, and so far, it seems to be ONLY this case. (Any explanations?) And, there is NO overscan vertically!

To make the video fit precisely into 16:9, this requires a horizontal scaling of 97.8% (give or take a few 100ths... again... fractional pixels). [I arrived at 97.8 first by trial and error, then to get to the math involved, if you divide "should be width" of 853 by the actual width of 872, you get 0.97821101 (times 100 to get to percent), but you don't have to be that precise.]

As an example, I created a "border" effect (applied to the media) which has equal distances from the edges horizontally and vertically. In 480 anamorphic, the result is this:

The top border width is much wider than the left border width.

After applying the 97.8% Scale>X correction

the borders are equal widths again - and *notice* that the image does not sustain the project size "crop" inherent in all other clips with the "Fit" Spatial Conform setting. The media is *there* to be able to fill the space!

I have searched everywhere I know to look and have not found a reasonable explanation for this. Is this an industry wide practice? (stretching/scaling the horizontal aspect for an overscan) or is this just Final Cut?

[Not that it should make any difference anymore...haha...]

If there are no explanations for this behavior, then hopefully, there is some meaningful information here for you if you find yourself ever having to deal with anamorphic widescreen 720x480 media.

Posted on Oct 4, 2023 10:56 AM

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Answer 1

fox_m Author

Level 6

12,595 points

Oct 4, 2023 4:46 PM in response to Tom Wolsky

Hi Meg, Tom!

I just had discovered a very unusual condition where animorphic widescreen NTSC goes beyond the left/right borders of an HD project bounds by approximately 1.1 % on each side (2.2% total) and does NOT get cropped in the process (with Fit Spatial Conform). This doesn't happen with any other type of media, not even the standard NTSC formats. The aspect ratio of the anamorphic widescreen is 1.8167:1 when with Fit Spatial Conform it should be 1.777778:1 — but only the 1.777778:1 portion is displayed.

I really don't know how to explain this, apparently. I guess it has to be seen to believed*. However, I can't seem to find any publicly available sample SD media to be used as an example. I was curious to find out if anyone knew if an overscan was used automatically.

*If you have an old SD anamorphic widescreen clip available - place a color solid underneath the sd widescreen anamorphic clip - Fit Spatial Conform - Anamorphic Override set to widescreen. Reduct the Transform > Scale > X parameter until the color solid background is revealed. It shows just how much of the footage extends beyond what would normally be a cropped boundary.

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Answer 2

Oct 4, 2023 10:55 PM in response to fox_m

I have used this page for my calculations when converting rectangular pixel DV to square pixels:

http://web.archive.org/web/20140218044518/http://lipas.uwasa.fi/~f76998/video/conversion/

...

So for example (different numbers for PAL):

NTSC 4:3 .dv to 640x480 square pixels: Source must be resampled to 648x480 and cropped 4+4 pixels from both sides to 640x480 (or cropped 4+5 pixels from both sides to 711x480 and scaled to 640x480).

NTSC 16:9 .dv to 854x480 square pixels: Source must be resampled to 865x480 and cropped 5+6 pixels from both sides to 854x480 (or cropped 4+5 pixels from both sides to 711x480 and scaled to 854x480).

NTSC 16:9 .dv to 1920x1080 square pixels: Source must be resampled to 1945x1080 and cropped 12+13 pixels from both sides to 1920x1080 (or cropped 4+5 pixels from both sides to 711x480 and scaled to 1920x1080).

NTSC 16:9 .dv to 1280x720 square pixels: Source must be resampled to 1296x720 and cropped 8+8 pixels from both sides to 1280x720 (or cropped 4+5 pixels from both sides to 711x480 and scaled to 1280x720).

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Answer 3

fox_m Author

Level 6

12,595 points

Oct 4, 2023 11:49 PM in response to Matti Haveri

Thanks Matti!

It's going to take me awhile to digest all this information... I downloaded it and will study more (it's going to take me several times through it to get it all.)

I haven't found exactly what FCP is doing in this article... but it's probably something along the same basis. NTSC 16:9 in FCP is 872 x 480 (at least measured from the "inside out" so to speak.) What's actually displayed is 853 by 480 but what actually gets me is that there is no reason to create the overscan for analog display destinations — everything is digital now. Plus, the extra 19 pixels are not cropped which is what is the most unusual part of this.

The closest to what's going on in FCP is NTSC 16:9 to 1920x1080 (format .dv doesn't really matter here, I don't believe) where the Source is resampled to 1964 x 1080 - basically that same 19 pixels past the bounds, and on FCP, usually cropped 10 left 9 right — if my calculations are correct (!).

BTW, in a 720x480 NTSC anamorphic widescreen project, the pixels are "fit" horizontally with no overscan. My measuring tool still shows 872x480 as the dimensions.

As for my original project, I guess I'll have to live with the weirdness and instruct users that if they're upscaling 480 widescreen, to make the Scale X adjustment. [I don't do Snapshots.]

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Answer 4

Oct 5, 2023 1:06 AM in response to fox_m

My related calculations are on the spreadsheet below. Correct me if there is an error somewhere.

I have done an experiment by taking PAL .dv footage of a circle and converting it with Photoshop or ffmpeg's scale & crop filters to square pixels and the output was a perfect circle with those calculations. AFAIR FCP and QuickTime Player by default are not exactly the same but very close.

https://www.dropbox.com/scl/fi/b6hb1b0t4tvcgjge90c4j/resolution_aspect-ratio.xlsx?rlkey=r403jty1fx9ug25t223i55lk4&dl=0

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Answer 5

fox_m Author

Level 6

12,595 points

Oct 5, 2023 1:11 PM in response to Matti Haveri

From the standpoint of a precision effect that places specific graphics at specific pixel locations based on horizontal resolution, Final Cut's handling of 480 Anamorphic Widescreen, although technically no different than any other resolution, actually expands beyond the edges of the viewport (without cropping!). The precision effect appears broken. However, after a **manual** adjustment to the horizontal scale, the effect will appear at it's designated orientation.

I can't seem to wrap my head around this because there are no analog scan lines being displayed. The actual physical width of the video output is a known quantity. Adjustments for "Picture" do not require turning knobs to account for the width of a CRT screen like the old days. And it just puts an asterisk on my claim that the effect will work in any aspect ratio. It does, but you have to make an adjustment... :P

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