Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
To select a custom region as you describe you need a clip mask. The area you want to apply effects to should be opaque and the remaining area transparent. Use the -clip option to apply the clip mask followed by your effect.
