|Getting Started with The GIMP: an informal tutorial: CSLUG and Rock Eagle Presentations, October 2002|
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The GIMP can open almost any kind of image format available today, and it can also save in almost any format. But unless you're absolutely sure you'll never want to edit the picture again, save in the GIMP's native format, XCF. XCF files save all the layers, channels, and paths you've created. It even stores undo information, and it's not a lossy format. As a result, XCF files tend to be very large.
Use the "File -> Save As ..." dialogue to save the image. The default setting for "Save Options" is "Determine File Type: By Extension" which means that you add the extension of your choice and it uses that as the file type. So enter "filename.xcf" in the text box and say "OK."
XCF files can't be viewed by web browsers, so do "File -> Save As ..." again, this time using "filename.jpg" as the file name. You'll be confronted with the following dialogue:
The "Export file" dialogue box.
As this states, the JPEG file format can't deal with transparency. It also can't deal with layers, so the GIMP pushes all the layers into one before saving as a JPEG. If your background has transparent areas (not the case here) the GIMP will fill it in with some colour - your mileage may vary. Since we have transparency but it's not in the Background, the result will be a JPEG that looks almost exactly like what we see in the GIMP. Click on "Export." But you're not done yet: most file formats have their own set of options, and JPEG is no exception:
The "Save as JPEG" dialogue box.
Here you can adjust the "Quality" or compression - and several other things. The higher the compression, the smaller the image - and the poorer the final image quality. Note that it says near the top of this window "Preview (in image window)". You can take a look at the Preview to see what damage JPEG compression is doing to your work. "75" is usually pretty good for web display, but this depends on a lot of things: if you have fine text in the image, you'll find that you have to use extremely high quality to prevent compression artifacts from being visible (in English: it doesn't look good, and your viewers will notice this). This is because JPEG was designed for photographic images (on which it works superbly) and doesn't work as well with clear lines. To give you some idea of the trade-off, take a look at the following three images:
The image saved at a quality of 75 (50714 bytes).
The image saved at a quality of 50 (33856 bytes).
The image saved at a quality of 25 (21311 bytes).
Notice in the final image that the sky is becoming visibly chunky: this is how JPEG saves space, by reducing the number of colours and painting large areas a single colour. If you look at the three images closely, you should also notice that artifacts (visual defects) appear first around the text: as I said earlier, JPEG doesn't handle text well. In fact, I originally intended to use PNG images (a lossless image format) for all of the images in this paper, but the size was prohibitive and I decided to accept the reduction in quality so that you would be able to download the paper more quickly.