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JPG Converter

JPEG. JPEG is a lossy raster format built for photographs, with no transparency and quality that degrades on every save.

JPG, also spelled JPEG, is a lossy raster image format defined by the Joint Photographic Experts Group. It transforms blocks of pixels with a discrete cosine transform and quantises the result, discarding detail the eye is least sensitive to. It has no alpha channel, so transparency is flattened onto a background.

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What a JPG file is

JPEG compression works on blocks of pixels. Each block is converted from spatial values into frequency coefficients by a discrete cosine transform, those coefficients are divided by a quantisation table and rounded, and the rounding is where the data goes. A lower quality setting uses a coarser table and throws away more. The result is small and, for photographs, visually close to the original.

Two consequences follow from that design. It handles smooth gradients and natural detail well and handles hard edges badly, which is why text and screenshots in a JPG grow halos around the letterforms. And because quantisation runs again on every save, repeatedly opening and re-saving a JPG degrades it each time, a loss no later conversion can undo.

.jpg and .jpeg name exactly the same format. The three-letter spelling is a holdover from filesystems that allowed only three-character extensions, and the media type for both is image/jpeg.

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