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Let's talk about chroma subsampling

1 Replies, 1502 Views

Here's a comparison I made quickly. Green text on red background. One is the original, the other is with color subsampled by resizing to 50%, then upscaling again.


In the worst case scenario (extreme color contrasts) this is what you lose with stuff like 4:2:0. The picture that originally looked sharp and defined and like true HD now looks more like a good DVD. Despite full luminance resolution.

Now that we have UHD Blu Rays, we can scale them down to 1080p and have full 4:4:4 colors. It doesn't make a difference in most scenes. But it can make a huge difference with some art elements or fine colorful details. 

Hence this thread shall stand as my personal advocacy for 4:4:4 encoding of projects wherever the source justifies it. Which includes, of course, 35mm scans.
(This post was last modified: 2018-06-05, 11:13 PM by TomArrow.)
That's not the worst case. Less bandwidth is given to red and blue compared to green, the formulas can be found on Wikipedia - for YUV in HD (using BT.709) the formula is:

Y: 0.2126R + 0.7152G + 0.0722B
U: -0.09991R - 0.33609G + 0.436B
V: 0.615R + 0.55861 - 0.05639B

Of course in high bitrate bluray encoding this looks very acceptable - at lower broadcast bitrates you can get extreme blocking around the bright colours-

[Image: 420-interlaced-still.png]

I see broadcasts that regularly look even worse than that - even in 1080i! And the worst part is that most of it could be fixed if the TV studios implemented simple wardrobe requirements and didn't use bright coloured lines on their charts etc.

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