All options can be abbreviated to their shortest unique prefix. You may use two hyphens instead of one to designate an option. You may use either white space or an equals sign between an option name and its value.
To rotate at other angles, use pnmrotate. It is much slower, though.
The input image is pnmfile, or Standard Input if pnmfile is not specified.
You must supply exactly one of the following options:
Before Netpbm 10.7 (August 2002), you could specify any number of these options, including zero. pnmflip would apply all the indicated transformations, in the order given. If you specified none, pnmflip would copy the input unchanged to the output. (Reason for the change: this kind of interpretation of options is inconsistent with the rest of Netpbm and most of the Unix world, and unlikely to have been exploited.)
The following options help pnmflip use memory efficiently. Some flipping operations on very large images can cause pnmflip to have a very large working set, which means if you don't have enough real memory, the program can page thrash, which means it takes a ridiculous amount time to run. If your entire image fits in real memory, you don't have a problem. If you're just flipping top for bottom or left for right, you don't have a problem. Otherwise, pay attention. If you're interested in the details of the thrashing problem and how pnmflip approaches it, you're invited to read a complete explanation in the source code.
When you specify -memsize and are doing a row for column type of transformation, pnmflip does the transformation in multiple passes, each one with a working set size less than the specified value.
If your estimate is even slightly too large, it's the same as infinity. If you estimate too small, pnmflip will use more passes than it needs to, and thus will slow down proportional to the underestimate.
If you do not specify -memsize, pnmflip assumes infinite real memory and does the entire transformation in one pass.
This option did not exist before Netpbm 10.7 (August 2002).
This option did not exist before Netpbm 10.7 (August 2002).