How Image Compression Works

Image compression feels like magic—how can a file shrink to a fraction of its size while looking almost identical? Here is a simplified explanation of the technology.

The Problem with Raw Images

A digital image is made of pixels. A 1000x1000 pixel image has 1,000,000 pixels. If each pixel uses 3 bytes (for Red, Green, and Blue), the uncompressed image is 3 Megabytes. This is too large for efficient web browsing.

Lossless Compression

Lossless compression works by finding patterns. If there is a solid blue sky with 1000 identical blue pixels in a row, the computer doesn't need to save "blue, blue, blue..." 1000 times. It simply saves "1000 blue pixels". This reduces file size without losing a single drop of quality. PNG uses this method.

Lossy Compression

Lossy compression (like JPG) takes it a step further. It takes advantage of human vision limitations. The human eye is very sensitive to changes in brightness, but less sensitive to subtle changes in color.

Lossy algorithms group similar colors together. If two neighboring pixels are very slightly different shades of red, the algorithm might make them the exact same shade. By discarding these tiny details that we can barely see anyway, the file size drops dramatically.

You can test this yourself using our Image Compressor. Adjust the quality slider and notice how the file size drops much faster than the perceived quality.