If you have the ImageMagick package installed on a Linux system or Mac OS X, you can use the convert tool that comes with it to resize an image from a shell prompt. ImageMagick is also available for Microsoft Windows systems, but I haven't tried it on a Microsoft Windows system.
To resize an
image, one uses the -sample option for the convert tool as in
the example below, which resizes an image, input.jpg, to 80 pixels wide and
40 pixels high, storing the result in output.jpg.
$ convert -sample 80x40 input.jpg output.jpg
You can also specify percentages of the original file size rather than a size in pixels.
convert -sample 25%x25% input.jpg output.jpg
Note: when you use the -sample option to specify the size
of the output image in pixels, you may not get the exact sizes you specified
for the width and height of the image. This is because ImageMagick considers
the aspect ratio of the two sides of the image when it is resizing, keeping
the aspect ratio of the new image the same as that of the previous image.
So one of the values may not be the same as the number you put on the
command line.
You can use the identify utility that comes with ImageMagick to obtain the width and height of an image (the width value is displayed first followed by the height).
$ identify example.png example.png PNG 100x89 100x89+0+0 DirectClass 11kb
In the example above, the image is 100 pixels wide by 89 pixels high.
You can determine if ImageMagick and these associated tools are on the system by using the which command.
$ which identify /usr/bin/identify $ which convert /usr/bin/convert
References:
-
Graphics from the command line
Flip, size, rotate, and more with ImageMagick
By: Michael Still, Senior software engineer, Tower Software
Date: July 16, 2003
Linux - Technical resources for Linux programmers and system administrators
