How does JPEG compression shrink your photos?

JPEG shrinks photos by throwing away the details your eyes are worst at noticing. A typical JPEG is about ten times smaller than the raw image, and the loss is usually hard to spot.

It works in a few steps. First it separates brightness from colour. Our eyes are much more sensitive to fine detail in brightness than in colour, so JPEG stores the colour information at a lower resolution, often a half or a third, and keeps the brightness sharp.

Next it cuts the picture into blocks of 8 by 8 pixels and rewrites each block as a mix of simple patterns, from smooth gradients to fine stripes, using a piece of maths called the discrete cosine transform. Then comes the step that actually saves space: the finest, fastest-changing patterns are stored with less precision, or dropped entirely at low quality settings, because we barely see them. That step can't be undone. Finally the remaining numbers are packed tightly with a lossless code that gives short codes to common values.

The key idea goes back to 1972, when Nasir Ahmed, an engineer at Kansas State University, proposed the discrete cosine transform for compressing images. He worked out a practical version with two of his PhD students and published it in 1974. A committee called the Joint Photographic Experts Group built on it and released the JPEG standard in 1992. By 2015, several billion JPEG images were being made every day.