It is worth highlighting that Rec. 709 and sRGB share the same primaries. However, their transfer functions and luma coefficients differ
By drawing a triangle into the CIE XZY color space, you just defined something called primaries, an essential property of your color space. The primaries define the exact boundaries of your space. In this case, we picked R=[x:0.64, y:0.33], G=[x:0.30, y:0.60], and B=[x:0.15, y:0.06].
You might remember from Part I of this blog post that our human vision isn’t linear
Unfortunately, the rules for camera sensors are a bit different. A camera might indeed see our nice ocean picture very linearly
For example, yuv422 says, “take 4 pixels, in the first line and combine 2 of them into 1, in the second line combine 2 of them into 1”. yuv420 says, “combine 4 pixels, take 2 pixels from the first line, take 0 pixels from the second line and combine them into 0”. yuv420 essentially combines the chroma information for 4 different pixels (2 in each line) into only 2 pixel values. The number of lines that chroma subsampling affects is always 2 in these cases. It might be a bit confusing, but this 2 is not contained in the acronym
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