Longer explanations of image formats, compression settings and the practical decisions behind them.
Quality 80 is not 80% of anything. It is an index into a quantisation table. Measured on one photograph: dropping from 100 to 90 removes 77% of the file and 4.2 dB of fidelity; dropping from 40 to 30 removes 17% and costs almost as much.
8 min read
Resize before you compress. Measured on one photograph: half the width at quality 80 produced a 16.4 KB file, while full width at quality 40 produced 20.6 KB and looked far worse. The order of operations matters more than the settings.
8 min read
WebP produced a 27.0 KB file against MozJPEG's 41.1 KB on the same photograph at slightly higher measured fidelity — 34% smaller. It also produced a larger file than JPEG on random noise. The advantage is real and content-dependent.
9 min read
PNG stored the same photograph in 1274.1 KB where JPEG needed 67.5 KB — nineteen times larger. On a flat graphic PNG won. On a smooth gradient PNG lost to JPEG by a factor of two, which surprises most people.
9 min read
A photograph from a phone can carry the coordinates of the place it was taken, the camera serial number and the exact timestamp. Removing that data is straightforward. Removing it carelessly rotates your photos sideways.
9 min read
Attachments are base64-encoded in transit, which inflates them by about a third. A 20 MB limit is really a 15 MB limit. Resize to 1600 pixels on the long edge and encode at quality 80, and a photograph lands near 150 KB.
9 min read