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Definition
JPEG (Joint Photographic Experts Group) is the most widely used image format for photographs and complex images on the web. It uses lossy compression to achieve small file sizes, trading some image quality for dramatically reduced storage requirements.
JPEG has been the dominant photographic image format since its introduction in 1992. The format works by dividing images into 8x8 pixel blocks and applying discrete cosine transform (DCT) compression, which discards visual information that humans are less likely to notice. A quality setting (typically 1-100) controls how aggressively data is discarded.
At quality 80-85, JPEG produces visually excellent results at a fraction of the original file size — often 10-20x smaller than uncompressed data. This efficiency made JPEG the standard for digital cameras, web images, and photo sharing. The format supports 24-bit color (16.7 million colors) but does not support transparency or animation.
JPEG's main limitations are its lack of transparency support and the introduction of compression artifacts — visible as blurring, banding, or "blockiness" around sharp edges and text. Each time a JPEG is re-saved, additional quality is lost (generation loss). For images with text, sharp lines, or transparency needs, PNG or WebP are better choices. The file extensions .jpg and .jpeg are interchangeable.