What Is Chroma Subsampling (4:2:0), and Why Does My Text Look Smudged?

You save a screenshot or a logo as a JPEG and the edges of the coloured text go soft and blotchy — red letters grow a faint halo, a blue button bleeds into the white around it. The black text right next to it is perfectly sharp. That pattern, colour smeared while black and white stay crisp, almost always has the same cause: chroma subsampling.

Brightness and colour are stored separately

Most image and video formats do not store pixels as red, green and blue. They convert them to one brightness channel (luma, written Y) and two colour channels (chroma, written Cb and Cr). The reason is human vision: we see fine detail in brightness far better than in colour. A format can store the colour at lower resolution and, in a photograph, nobody notices.

What 4:2:0 means

The three numbers describe how often colour is sampled, over a block two rows tall and four pixels wide:

• 4:4:4 — colour at full resolution. Every pixel keeps its own colour.
• 4:2:2 — colour at half the horizontal resolution. Common in professional video.
• 4:2:0 — colour at half the resolution in both directions, so one colour value for every four pixels. This is the default for JPEG from most software, and for almost all video: H.264, H.265 and AV1 in normal use, and everything streaming sites deliver.

In a photograph, 4:2:0 is the right choice. Colours in a natural scene change gradually, the brightness channel carries all the edges, and you save a lot of space for nothing visible.

Where it shows

On content with hard colour edges — text, logos, user interface, charts, pixel art. A one-pixel red line on white has to share its colour value with its neighbours, so the red spreads into the white and the white dilutes the red. The brightness channel keeps the shape sharp, which is why it looks smudged rather than blurry.

It is especially visible on:

• red and blue text, where colour carries most of the edge;
• thin coloured lines in charts and diagrams;
• screenshots of interfaces with coloured buttons and badges;
• video of screen recordings and slides.

How to tell whether your file uses it

For a JPEG, the subsampling is written in the file header. The image quality check reads it, and when the picture is the kind where it matters — artwork, a screenshot, anything with flat colour and hard edges — the report says: “Colour is stored at a quarter the detail of brightness (4:2:0).” On an ordinary photograph it says nothing, because there 4:2:0 is the normal default and mentioning it would be noise.

How to avoid it

• Save graphics as PNG. PNG is lossless and never subsamples. For logos, screenshots and diagrams it is usually smaller than a good JPEG as well.
• If it must be JPEG, choose 4:4:4. Many editors hide this under an “advanced” or “chroma subsampling” option; some tie it to the quality slider and only switch to 4:4:4 at the top settings.
• For screen recordings, use a codec setting that keeps 4:4:4 if your editor offers one, or accept that coloured text will soften in delivery formats that only support 4:2:0.

Checking whether a re-save changed it

If you are not sure whether an export changed the colour detail, compare it with the original in the image comparison. The heatmap lights up along coloured edges, and when colour is the main difference the result says “Shapes and brightness match closely — the difference is in the colours.” For more on what that score is measuring, see how to find colour changes between two images.