5. Controlling image size
At this point, my picture of Niobe is looking pretty good:
My wife opines that I perhaps overdid the Unsharp Mask a tad, and I think so too, but that serves my educational purpose because you can clearly see that it made the image sharper. It's more obvious in the full-size image.
It's not mind-blowingly great photography, but I think you'll agree that it's better than what we started with, and this level of improvement was easy to achieve. We could do more by playing with layers and selections and more complicated sequences of filters-- we could put Niobe on Jupiter and turn her fur green and make lasers come out of her eyes, or, with more effort, we could make the picture better yet completely natural-looking-- but at this point you know enough to start playing with your digicam photos and making them prettier. I recommend the tutorials on the GIMP site if you want to learn more advanced manipulations.
No, instead I'm going to cover the much more important topic of how not to annoy your friends and family!
Modern digicams can produce images so enormous that, if e-mailed raw, they can be hard to download over slow dial-up connections. If your audience is just looking at your pictures on screen, it's unlikely that you really need to send all that data. GIMP (or the equivalent) makes it easy to make the picture files a fraction of the size that they were before. There are two ways you can do this: by scaling them down, and by adjusting the JPEG quality level.
You change the size of your picture with the Scale Image dialog under the Image menu:
The important controls here are the "Pixel Dimensions" ones. You can specify the width and height of the image in any of several units (pixels are most useful), or you can scale the image by a selected ratio.
That chain icon connecting the X and Y ratios means that the image will always be scaled so that it is the same shape as before. If you want to make strange Silly Putty effects, you can click on that chain to make it go away, and then control the stretching of width and height independently. Normally you'll want the chain turned on. Then all you have to do is type in the new pixel width, and the appropriate height will calculate itself automatically when you click in the field.
I tend to scale images to 640 x 480 or, for the ones I really like, 800 x 600 for e-mail use. (I'm actually feeling a little guilty for using slightly bigger images in this tutorial, but it helps to get across what I need to illustrate.) If the recipient wants to use the image as desktop wallpaper or print it out, you might want to go higher, and if the recipient has broadband Internet it doesn't matter so much. But for mass mailings to family members, some of whom are on dial-up with flaky phone lines, I keep 'em small just to be safe, especially if I'm going to be sending several at once.
The other thing you can do is control the quality level when you save the image. For sending photographs over the Internet, the best format to use is JPEG. If you save the image as a JPEG in the Save As dialog (either because you explicitly picked JPEG, or because you gave it a .jpg extension and selected By Extension) you get a dialog with a Quality slider:
This slider controls the JPEG image compression process, and it represents a trade-off between image quality and small file size. The value on the slider is likely to start out pretty high, maybe 85 percent or so, because most consumer-level digicams save their images as JPEGs with a high quality level. But actually you can usually turn it down quite a bit without hurting the image much. If you check the Preview box, the dialog will both calculate the projected size of the image, and show you what it will look like when saved and reloaded. If you drag the slider way down below 10 percent, you can see the image break up into colored blocks and it looks like garbage. But 50% is often more than enough for photographs intended to be viewed onscreen; I usually settle for something around there when e-mailing big pictures.
If you take the time to do these things, you'll be head and shoulders above 95% of the people who e-mail photographs, and people are bound to appreciate it!
Here's a final tip. You'll often want to save off working versions of a file while you are working on them-- either to make absolutely sure you have a backup before you do something dangerous, or so you can stop working and come back to them later. When it's time to send your photographs to other people, JPEG is the best format to use. But repeatedly loading a JPEG, modifying it and saving it again can cause image artifacts to accumulate, because of information lost when compressing the image. Under most conditions, if the JPEG quality level is turned way up, the effect is not large. But you can avoid it entirely.
For saving your working versions, you'll want to tell the Save As dialog to use a format that preserves all the information in your picture. GIMP's native format is called .xcf; if the Save As dialog has file-type selection by extension turned on, you can select this just by giving the file a .xcf extension. The .tiff or .png formats will also work fine in most cases (though if you're doing complicated things beyond the scope of this document, they may not preserve all your level and selection information); they are also more likely than .xcf to be readable by things other than GIMP. The files will be much larger than JPEGs, which is why you don't want to use these formats to send photographs to people. But you can save, modify and re-save them as many times as you like without fear of the quality deteriorating.