How can I find out about width and height of an image in a file?

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I've written a class called ImageInfo that I've contributed to the Public Domain. Visit the ImageInfo page to find out more and download it. It understands quite a few image file formats and can also retrieve color depth, comments, physical resolution and number of images in a file (not all of this information may be available in all files).

You could of course load the image and call getWidth and getHeight on your Image object. However,

that approach is slower than ImageInfo,
depending on the image dimensions it may require quite a bit of memory,
the format of the file may not be supported (ImageInfo knows more file formats than Toolkit) and
your platform may have the problem with the missing X server.

That can all be avoided by using ImageInfo.