JPEG Images (*.jpg, *.jpeg) format is ideal for images that are to be used on the web. Since it uses average color values taken from groups of adjacent pixels to compress the image, it is not suitable for high-quality printing. However, on the web the diminution of image quality is more than made up for by the speed of download that compression allows. Typically, a download of several minutes is reduced to just a few seconds.
Note that the format works best with images that have some tonal subtlety, such as photographs of people, rather than ones with large areas of block or flat color (use GIF format instead for these).
Before saving as a JPEG file you can choose some options in this dialog:
The Quality slider and boxes at the top of the dialog allow you to determine the size and quality of the image. Basically, the smaller the file the lower the quality, the larger the file the higher the quality. The slider and the boxes are automatically adjusted together. Quality runs from 1-100 and can be set in the Quality box. File size can be set anywhere between Small file and Large file using the slider (or use the Size box to select from Low, Medium, High and Maximum).Typically a quality rating of between 40 and 50 will provide a satisfactory file size together with a decent image.
The Baseline options (in the Format Options section) cause the image to appear one line at a time in your web browser, starting from the top. Optimized slightly reduces the file size when compared to Standard (the default). Note that Baseline JPEGS are becoming rarer, since viewers get impatient with the amount of time it takes to see the whole image.
The Progressive option causes the image to appear in your web browser in a series of "passes". These allow you to see the image straightaway, albeit in faded form, before the image is fully refreshed. The number of passes is set in the Scan box: the higher the number the quicker the image begins to refresh, although the longer the refreshing takes to complete. Note that at present only a few browsers support Progressive JPEGS, though the situation should improve in the near future.
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