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This image size is just 21.8 KB without lack of quality. Why is that? How is it done?

Image of man on plain background

  • Lack of quality from what? Without the original image you can't really compare quality with anything. – Scott Jun 9 '14 at 7:41
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Here a lot of artifacts = quality loss compression. Since it have plain gray background the final size of JPG is quite small and artifacts are not so obvious.

enter image description here

  • Well put, you could use some more explanation tough. The point of jped is what you describe. That even tough at close inspection theres indeed quite a loss of quality, but human eye seems not to mind. Sometimes the result is more pleasing than other times. (you can also use a mask to control quality) – joojaa Jun 9 '14 at 8:03
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This sort of photo is great for a JPEG compression algorithm. Excerpts from the JPEG FAQ:

Generally speaking, JPEG is superior to GIF for storing full-color or gray-scale images of "realistic" scenes; that means scanned photographs, continuous-tone artwork, and similar material. Any smooth variation in color, such as occurs in highlighted or shaded areas, will be represented more faithfully and in less space by JPEG than by GIF.

JPEG has a hard time with very sharp edges: a row of pure-black pixels adjacent to a row of pure-white pixels, for example.

75% of this image is one color. The photo of the man in this photo is actually pretty uniform as well - gray shirt, black pants. The only place where the JPEG image really needs to add complexity to not look terrible are the lines that separate the man from the background, the tie from the shirt, etc.

Give the guy a colorful plaid shirt and watch the file size grow quite a bit! Same story if there was a background with a large depth of field.

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