4.4. Compressing Images
| 4. Common Tasks |

### 4.4. Compressing Images

图 3.21. Example Image for JPEG Saving

If you have images that take up a large space on disk, you can reduce that space even without changing the image dimensions. The best image compression is achieved by using the JPG format, but even if the image is already in this format, you can usually still make it take up less space, as the JPG format has an adaptive compression scheme that allows saving in varying levels of compression. The trade-off is that the less space an image takes, the more detail from the original image you lose. You should also be aware that repeated saving in the JPG format causes more and more image degradation.

To save you image as a JPG file, therefore, use File → Save As to open the “Save As” dialog.

图 3.22. “Save As” Dialog

The dialog opens with the file name already typed in the Name box. If the image is not a JPG image, delete the existing extension and type JPG instead, and GIMP will determine the file type from the file extension. Use the file extension list, circled in the figure above, to see the types supported by GIMP. The supported extensions change depending on your installed libraries. If GIMP complains, or if “JPEG” is grayed out in the Extensions menu, cancel out of everything and step through the 第 4.7 节 “Change the Mode”. Once you have done this, click Save. This opens the “Save as" ” JPEG dialog that contains the quality control.

The “Save as JPEG” dialog uses default values that reduce size while retaining good visual quality; this is the safest and quickest thing to do.

图 3.23. “Save as JPEG” dialog with poor quality.

Reduce the image Quality to make the image even smaller. Reduced quality degrades the image, so be certain to check “Show preview in image window” to visually gauge the degradation. As shown in the figure above, a Quality setting of 10 produces a very poor quality image that uses very little disk space. The figure below shows a more reasonable image. A quality of 75 produces a reasonable image using much less disk space, which will, in turn, load much faster on a web page. Although the image is somewhat degraded, it is acceptable for the intended purpose.

图 3.24. Dialog for Image Saving as JPEG

Finally, here is a comparison of the same picture with varying degrees of compression:

图 3.25. Example for High JPEG Compression

Quality: 10; Size: 3.4 KiloBytes

Quality: 40; Size: 9.3 KiloBytes

图 3.26. Example for Moderate JPEG Compression

Quality: 70; Size: 15.2 KiloBytes

Quality: 100; Size: 72.6 KiloBytes

4.3. Change the Size of an Image for print |  | 4.5. Crop An Image

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