They allow multiple ways of representing the same subsampling but you should probably use the values from table 2. In conclusion, sampling factors are very flexible as they can express anything from typical chroma subsampling to different subsampling of C B and C R channels and luma subsampling. Here's a table of common chroma subsamplings with corresponding sampling factors: Next, let's investigate how we can express chroma subsampling with sampling factors.įor chroma subsampling the luma channel must have the largest sampling factors and the sampling factors of both chroma channels must be equal. This means that sampling factors such as 4×3, 2×1, 2×3 are invalid because \(4 \cdot 3 + 2 \cdot 1 + 2 \cdot 3 = 20\) is greater than \(10\). the third channel has 2⁄2 horizontal and 1⁄3 vertical resolutionĪdditionally, the following inequality must hold:.the second channel has 1⁄2 horizontal and 2⁄3 vertical resolution.the first channel has 2⁄2 horizontal and 3⁄3 vertical resolution.Then, the relative resolutions are the following: More formally, we first calculate the largest horizontal and vertical sampling factors: Here's a table of common chroma subsamplings in both of notations alongside a visual representation: Let's denote the relative horizontal and vertical chroma resolutions with H and V.
CHROMA MEANING FULL
This is either 0 or equal to a.Ĭhroma subsampling can also be expressed as fractions of the luma resolution.įor example 4:2:2 means that the chroma resolution is halved horizontally while vertically it has full resolution. b is the number of chroma samples in the second row.a is the number of chroma samples in the first row.J specifies a block of J × 2 pixels for luma.Let's start with the J:a:b notation which Wikipedia defines as: This article attempts to shed some light on the different notations and how they are related. There's a great deal of confusion around chroma subsampling notation.Ĭhroma subsampling is generally expressed using J:a:b notation but the meaning of the notation varies between different sources and software.įurthermore, JPEG uses a different format internally which is sometimes exposed in user interfaces. Chroma subsampling and JPEG sampling factors
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