Posted by Pete McBreen 15 Jan 2011 at 12:55
After losing count of the number of sites that use the browser to shrink massive images into the size of the thumbnails that the page needs (and waiting forever for these large images to load), it is time to say enough.
Shrinking images is easy, especially from the command line using ImageMagick. It is an open source tool, so there is no excuse not to use it, there are even precompiled binaries available for most platforms.
The most common use case people have for images is small thumbnails that are used as an image link to another page that has the full size image on it, or the page only has space for a small image and the original is too large to fit. Doing this with ImageMagick is easy, even if the original aspect ratio of the image was not right.
convert -thumbnail 200x100^ -gravity center -extent 200x100 original.jpg thumbnail.jpg
convert is the name of the ImageMagick tool,
-thumbnail 200x100^ creates an image that is at least 200 pixels wide and 100 pixels tall while preserving the aspect ratio of the original picture. This means that the resulting thumbnail can be larger than 200x100, but the image will not be distorted. The second part of the command
-gravity center -extent 200x100 specifies that the resulting image should only be 200x100 and that the image should be picked from the center of the image. The gravity option can also be any compass direction with NorthWest being the top right of the image.
Processing multiple images is trivially easy, just specify a set of images using a wildcard like
*.jpg and then the resulting images will be written out to a numbered set of files using
thumb-%d.jpg, giving filenames like
thumb-2.jpg and so on.
convert -thumbnail 200x100^ -gravity center -extent 200x100 *.jpg thumb-%d.jpg
So no more excuses for distorted or over size images in web pages.