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What is JPG (or JPEG)? Before answering that question, we must answer "what is digital photo". Digital photo is represented by an array of fine dots, each with its own color. This representation is called bitmap (bmp). BMP is the most intuitive and simple method of storing pictures. BMP has a deadly disadvantage, that is, taking too much space. JPG is an algorithm that solves this problem. JPG is the so called "lossy compression", a JPG compressed picture is slightly different than the original. However, the difference is neglectable for human eyes. The gain is smaller file size. JPG compression has the quality option from 0 to 100. 100 is the best quality and largest file size. See the following examples.

quality 100 / 46,113 Bytes

quality 90 / 21,124 Bytes

quality 60 / 9,286 Bytes

quality 30 / 5,215 Bytes

quality 10 / 1,889 Bytes
Also, there is a sampling option, with YUV411, YUV422, and none to select. None is the closest to the original, with the largest file size. YUV411 is darker than original, and also blurrier, and takes the least space.

There are 3 modes: progressive, standard, and standard optimized. When downloading speed is slow or the image is big, the progressive JPG will first show a layer of the blurry version of the image, and than progressively improve. Standard or standard optimized don't have this process of "progress". When the image is shown partially, the part is the clear version. When viewing downloaded pictures, it takes longer to load the progressive JPG's, especially the larger ones. The difference between standard and standard optimized is that standard optimized takes less space, but is less compatible. Theoretically, some system cannot read standard optimized, but in practice, most computer can.

The "Save for web" function of Adobe Photoshop also has the quality option ranging from 0 to 100. But its meaning is different from the one I mention above.

Save for web

I choose standard optimized, sampling none, quality 90 (70 in save for web).

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