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Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance.[1]

It is used in many video encoding schemes – both analog and digital – and also in JPEG encoding.

Gamma-corrected signals like Y'CbCr have in issue where chroma errors "bleed" into luma. In those signals, a low chroma actually makes a color appear less bright than one with equivalent luma. As a result, when a saturated color blends with an unsaturated or complementary color, a loss of luminance occurs at the border. This can be seen in the example between magenta and green.[3] To arrive at a set of subsampled values that more closely resembles the original, it is necessary to undo the gamma correction, perform the calculation, and then step back into the gamma-corrected space. More efficient approximations are also possible, such as with a luma-weighted average or iteratively with lookup tables in WebP and sjpeg's "Sharp YUV" feature.[12]

Out-of-gamut colors

Another artifact that can occur with chroma subsampling is that out-of-gamut colors can occur upon chroma reconstruction. Suppose the image consisted of alternating 1-pixel red and black lines and the subsampling omitted the chroma for the black pixels. Chroma from the red pixels will be reconstructed onto the black pixels, causing the new pixels to have positive red and negative green and blue values. As displays cannot output negative light (negative light does not exist), these negative values will effectively be clipped, and the resulting luma value will be too high.[3] Similar artifacts arise in the less artificial example of gradation near a fairly sharp red/black boundary.

Other types of filtering during subsampling can also cause colors to go out of gamut.

Terminology

The term Y'UV refers

Gamma-corrected signals like Y'CbCr have in issue where chroma errors "bleed" into luma. In those signals, a low chroma actually makes a color appear less bright than one with equivalent luma. As a result, when a saturated color blends with an unsaturated or complementary color, a loss of luminance occurs at the border. This can be seen in the example between magenta and green.[3] To arrive at a set of subsampled values that more closely resembles the original, it is necessary to undo the gamma correction, perform the calculation, and then step back into the gamma-corrected space. More efficient approximations are also possible, such as with a luma-weighted average or iteratively with lookup tables in WebP and sjpeg's "Sharp YUV" feature.[12]

Out-of-gamut colors

Another artifact that can occur with chroma subsampling is that out-of-gamut colors can occur upon chroma reconstruction. Suppose the image consiste

Another artifact that can occur with chroma subsampling is that out-of-gamut colors can occur upon chroma reconstruction. Suppose the image consisted of alternating 1-pixel red and black lines and the subsampling omitted the chroma for the black pixels. Chroma from the red pixels will be reconstructed onto the black pixels, causing the new pixels to have positive red and negative green and blue values. As displays cannot output negative light (negative light does not exist), these negative values will effectively be clipped, and the resulting luma value will be too high.[3] Similar artifacts arise in the less artificial example of gradation near a fairly sharp red/black boundary.

Other types of filtering during subsampling can also cause colors to go out of gamut.

Terminology