ImageMagick gets seam carving.
Just as Sampling an image resizes by directly removing or duplicating whole columns and rows from an image, the special IM operator "-liquid-rescale" also removes or duplicates columns and rows of pixels from an image to reduce/enlarge an image. The difference is that it tries to do so in a more intelegent manner.
First the instead of removing a simple line of pixels, it removes a 'seam' of pixels. That is the column (or row) that could zig-zag through the image, at angles up to 45 degrees.
Secondly it trys to remove seams that have the 'least importance' in terms of the images contents. How it selects this is in terms of the images energy, or more simply, the amount of color changes a particular 'seam' involves. The 'seam' with the least amount of changes will be removed first, followed by higher 'energy' seams, until the image is the size desired.
