YCbCr 4:1:0

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Note that these FourCCs and other "YUV" names only reflect that YCbCr is often falsely mixed up with YUV, which is not YCbCr but a different color space that is used in analog PAL-based stuff (analog TV, video tapes, ...) and not in digital media.

YCbCr 4:1:0 indicates a planar YCbCr colorspace where a 4x4 block of pixels in an image is represented by 16 Y samples, 1 for each pixel, but all 16 pixels share a Cb sample and a Cr sample.

If YCbCr 4:1:0 data is stored in YUV9 or IFO9 format, it is stored with all of its Y data first, then its Cb data, then its Cr data. YVU9 data swaps the Cb and Cr planes.

For trivia, the 9 in YUV9/YVU9 most likely refers to the number of bits needed to represent a single pixel. Since 18 bytes, or 144 bits represent 16 pixels, an average of 9 bits are required to represent a single pixel in this scheme.