Every photo on your screen is a carefully built illusion. The Joint Photographic Experts Group standard, released in 1992, starts by exploiting a weakness in human vision: our eyes are sharp on brightness and contrast but poor at fine detail in color. The algorithm converts an image from red, green, and blue into luma and chroma, keeps the brightness at full fidelity, and throws away half or more of the color data by making neighboring pixels share a value. Like a coloring book with crisp outlines, the picture still looks right even when the crayon strays outside the lines.

This episode walks through the rest of the trick. The image is cut into blocks of 8 by 8 pixels, and the discrete cosine transform, proposed by Nasir Ahmed in 1972, turns each block into a map of visual frequencies. Quantization then rounds most of the fine detail down to zero, and a zigzag reading order packs those zeros into shorthand. The story then turns to the patent claims that tried to tax the format a decade after its release, and to why technically better successors have never managed to replace it.

• The quality slider in photo software controls how aggressively quantization divides and rounds. The blocky squares in a bad JPEG are the visible borders of those 8 by 8 blocks.
• Every save reruns the lossy math, so repeated edits degrade an image like a photocopy of a photocopy. Cropping or rotating exactly along block boundaries avoids the loss.
• In 2002, Forgent Networks asserted a 1986 Compression Labs patent and collected about $105 million from roughly 30 companies before the patent office found it invalid on prior art in 2006.
• Later claimants included Global Patent Holdings, which sued companies and even the Green Bay Packers in 2007 over downloading JPEGs on websites.
• JPEG 2000 removed the blocky artifacts but failed in the market. Instead, projects like MozJPEG, Guetzli, and Jpegli squeeze 20 to 35 percent more efficiency from the original standard.