Common Tasks

## Converting an image to another format

Converting an image to another format is as simple as writing the image to a file. ImageMagick uses the output filename suffix (".jpg" for JPEG, ".gif" for GIF, for example) or prefix ("ps:" for PostScript, for example) to determine the format of the output image.

## Making thumbnails

RMagick gives you three different methods for resizing an image: resize, sample, and scale. All three are equally easy to use. Specify the number of columns and rows you want the thumbnail to have, like this:

       img = Image.new "bigimage.gif"
       thumb = img.scale(125, 125)
       thumb.write "thumb.gif"

Alternatively, just pass a single `Float` argument that represents the change in size. For example, to proportionally reduce the size of an image to 25% of its original size, do this:

       img = Image.new "bigimage.gif"
       thumb = img.scale(0.25)
       thumb.write "thumb.gif"

The resize method gives you more control by allowing you to specify a filter to use when scaling the image. Some filters produce a better-looking thumbnail at the expense of extra processing time. You can also use a blur argument, which specifies how much blurriness or sharpness the resize method should introduce.

Finally, the sample method, unlike the other two, does not do any color interpolation when resizing.

## Converting a color image to grayscale

Use the quantize method with the Magick::GRAYColorspace argument. If you want real "grayscale," quantize the image to 256 colors. If you want to convert a color image to black-and-white, use 2 colors. See demo.rb.

## Making a drop shadow

Here's one way to make a drop shadow behind text. Use the gaussian_blur method to blur a light gray copy of the text, position the "shadow" slightly to the right and down, then draw the foreground text slightly to the left and up. (Click the image to see the Ruby program that created it.)