PNG compression — Input checklist

Worked example

Before compressing a PNG, check for transparency, the kind of picture and the pixel count, because they decide how much can be saved.

Take a designer with a 1024 × 1024 PNG logo on a transparent background that must go into an email footer. Open it over a coloured backdrop first, not a white one, so you can see the soft anti-aliased edge around the letters. Remember what that edge looks like, because you will compare it against the compressed file.

The content type predicts the result. A flat logo, an icon or a UI screenshot with large blocks of colour is what PNG stores efficiently, and a re-encode may trim a little. A photograph saved as PNG is very heavy, but squeezing it again within PNG cannot change that much, since the format keeps every pixel exactly.

A frequent mistake is expecting a quality slider to shrink the file. PNG here is re-encoded losslessly and the Quality control does not change a PNG, so moving it will not alter the result. If the file is a 2880 × 1800 screenshot of 3 MB, the realistic levers are fewer pixels or another format, not a number.

One quick check before you start: record the dimensions and the byte size, and note whether there is an alpha channel. If the source is above 64 million pixels the tool will ask you to reduce it first, and a PNG that has already been through an optimiser may simply come back unchanged.

PNG logo with alpha

PNG; transparency retained when supported

Check transparency against both a light and a dark backdrop before and after compressing, since a white page hides halo problems.