ImageMagick - a GUI, command line tools, and an API make this one handy package.
ImageMagick (IM) is a suite of command line tools such as Convert, Composite, Animate, etc. You can use these tools to do basic tasks such as changing the format of an image, rotating an image, cropping and resizing images, etc. IM also has a GUI called Display.
Using ImageMagick, you can make mosaics of images , 2D/3D images. You can also create movie animations.
Because IM is a command line suite, its biggest advantage is that it can be used with a number of programming languages. You can make excellent scripts that use the IM suite. There are many programming APIs available for IM such as RubyMagick, chmagick, PHPWand, etc.
- Inverting colors on a PDF:
convert -negate input.pdf output.pdf
The ImageMagick suite of command line tools support a wide array of file formats. There is currently support for about 90 different formats. Some of them are,