General description (actual behavior can vary for different options and settings)
scale%
Height and width both scaled by specified percentage.
scale-x%xscale-y%
Height and width individually scaled by specified percentages. (Only one % symbol needed.)
width
Width given, height automagically selected to preserve aspect ratio.
xheight
Height given, width automagically selected to preserve aspect ratio.
widthxheight
Maximum values of height and width given, aspect ratio preserved.
widthxheight^
Minimum values of width and height given, aspect ratio preserved.
widthxheight!
Width and height emphatically given, original aspect ratio ignored.
widthxheight>
Shrinks images with dimension(s) larger than the corresponding width and/or height dimension(s).
widthxheight<
Enlarges images with dimension(s) smaller than the corresponding width and/or height dimension(s).
area@
Resize image to have specified area in pixels. Aspect ratio is preserved.
{size}{offset}
Specifying the offset (default is +0+0). Below, {size} refers to any of the forms above.
{size}{+-}x{+-}y
Horizontal and vertical offsets x and y, specified in pixels. Signs are required for both. Offsets are affected by ‑gravity setting. Offsets are not affected by % or other size operators.
Image Geometry
Enlarges images with dimension(s) smaller than the corresponding width and/or height dimension(s).
Image Geometry
Compose Image
ImageMagick Command line options