If colored text or fine edges look less distinct on a display, there can be several causes. One concept to understand is chroma subsampling, which changes the sampling dimensions of color components in some video formats.
1. Luma and Chroma Components
Video signals may represent an image with one luma component and two chroma components. Chroma-subsampling formats describe the chroma sample dimensions relative to luma; they do not by themselves identify the format sent to a particular display:
- Y (Luma): The luma component, representing image brightness.
- Cb: A blue-difference chroma component.
- Cr: A red-difference chroma component.
ITU-T H.273 specifies the relative chroma dimensions for these formats. The visible effect of subsampling depends on the source image, filtering, scaling, and display path; it is not necessarily noticeable in every scene. See ITU-T H.273 for the format definitions.
2. Understanding the Ratios: 4:4:4, 4:2:2, and 4:2:0
Chroma subsampling formats are expressed as a three-part ratio:
- 4:4:4: The chroma planes have the same horizontal and vertical sample dimensions as luma. This describes sampling, not whether the entire signal is compressed.
- 4:2:2: Each chroma plane has half the horizontal sample dimension of luma and the same vertical dimension. Fine colored edges may appear different after reconstruction and display processing.
- 4:2:0: Each chroma plane has half the horizontal and vertical sample dimensions of luma. Sample alignment and reconstruction depend on the signal format and processing.
3. How Chroma Sampling Can Affect Fine Text
High-contrast colored edges and fine text can expose differences in chroma detail more clearly than some natural-image regions. The result also depends on the source, scaling, and display processing.
With 4:2:2 or 4:2:0, reconstructed chroma can have lower spatial detail than luma. This may change the appearance of saturated text edges, but does not guarantee visible blur in every signal or display setup.
The interactive Chroma Subsampling Test renders browser patterns for visual comparison. It does not inspect your graphics configuration or identify the display signal's chroma format.