when scaling between formats is it width or height that I should be calculating from?

[Nicholas Natteau]

I have a PP CS6 project that will demand mixing 4K, 1080P, 720P and 480i footage in a 720p timeline.

But I don't know if it's the width or the height that counts when scaling between formats.
Of course if your scaling among different size formats that have the same aspect ratio, it's one thing.
But the scaling % changes when you're dealing with formats that have different aspect ratios.

For example 4K (4096 x 2160) is a 1.9 aspect ratio whereas 1080p (1920 x 1080) is 1.78 aspect.
To calculate the correct percentage when scaling down 4K to 1080P should I be looking at the width or the height?
1920 is 47% of 4096, but 1080 is 50% of 2160. So is it 47% or 50% that I should be scaling down to?

In other words is the scaling ratio of 1080p 47% the size of 4K or 50%?
Conversely, is 4K 213% bigger or 200% bigger than 1080p? What is that counts in the scaling process, width or height?

 

[nosys70]

it depends where you want to loose.
from 4k to 1080, you will loose few pixel to get rid of the eventual black bars so it is probably not so important. Especially if next you will downres to 720p.
from 1080 to 720 there should be no problem.
You can even decide not to loose anything by changing aspect ratio of picture.
the only real problem is from 480i (provided it is 4:3 because it coul also be 16:9 anamorphic at best or 16:9 letterbox at worst) to 720p because here you will get huge pillarboxed picture, with wide black bar on the side.
and zooming in the picture will make you loose a big part of it and make it worse.

 

[Razz16mm]

720p is the vertical resolution. Scale once to your project timeline vertical resolution so image height is constant 720 for all formats, Crop width to 1280HD for wider formats after scaling. Let the 4:3 remain native ratio with pillar box.