Pamflip User Manual

# pamflip

Updated: 02 June 2025
Table Of Contents

## NAME

pamflip - flip or rotate a PAM or PNM image

## SYNOPSIS

pamflip { -leftright | -lr | -topbottom | -tb | -transpose | -xy | -reflect | -rotate90 | -r90 | -cw | -rotate270 | -r270 | -ccw | -rotate180 | -r180 | -null | -xform=xform1,xform2... } [-inverse] [-memsize=mebibytes] [-pagesize=bytes] [pamfile]

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.

## DESCRIPTION

This program is part of Netpbm.

pamflip flips a PAM or PNM image top for bottom or left for right, or transposes it horizontal for vertical, or rotates it 1, 2, or 3 quarter turns.

To rotate at other angles, use pnmrotate. It is much slower, though.

The input image is pamfile, or Standard Input if pamfile is not specified.

To flip/rotate a JFIF (JPEG) image losslessly, use jpegtran. jpegtran is part of the Independent Jpeg Group's compression library package, not part of Netpbm. The normal Netpbm way to flip a JFIF file would be to convert it to PNM, use pamflip, and convert back to JFIF. But since JPEG compression is lossy, the resulting image would have less quality than the original. jpegtran, on the other hand, can do this particular transformation directly on the compressed data without loss.

You can flip an image in chunks, by using pamdice and pamundice with pamflip. This might be useful for improved parallelism or resource usage. The index image feature of pamdice and pamundice is especially useful in this task, because you can use pamflip on the index image. There is an example of flipping an image in chunks, using an index image, in the pamundice manual.

## OPTIONS

In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common Options), pamflip recognizes the following command line options:

### Specifying the Transformation

You must supply exactly one of the following options:

pamflip's predecessor (before Netpbm 10.7 - August 2002) pnmflip did not have the -xform option and instead allowed you to specify any number of the other options, including zero. It applied all the indicated transformations, in the order given, just like with pamflip's -xform option. (Reason for the change: this kind of interpretation of options is inconsistent with the rest of Netpbm and most of the Unix world, and thus hard to understand and to implement).

-leftright
-lr
  Flip left for right.
-topbottom
-tb
  Flip top for bottom.
-transpose
-xy
  Transpose rows for columns. I.e. make the pixel at row X column Y be at row Y, column X. (Note that these are not Cartesian coordinate X and Y; the Cartesian Y coordinate increases going up, whereas row numbers increase going down).
-reflect
  Reflect the image in (rotate it about) a diagonal line that slopes up and to the right. This is complementary to -transpose, which effectively reflects the image in a line that slopes down and to the right. This option was new in Netpbm 11.11 (June 2025). Before that, you can use -xform to achieve this flip.
-rotate90
-r90
-ccw
  Rotate counterclockwise 90 degrees.
-rotate180
-r180
  Rotate 180 degrees.
-rotate270
-r270
-cw
  Rotate counterclockwise 270 degrees (clockwise 90 degrees)
-null
  No change. (The purpose of this option is the convenience of programs that invoke pamflip after computing the kind of transformation desired, including none at all).

  This option was new in Netpbm 10.13 (December 2002).

-xform=xform1,xform2...
  Apply all the transforms listed, in order. The valid values for the transforms are as follows and have the same meanings as the identically named options above.
  - leftright
  - topbottom
  - transpose

  This option was new in Netpbm 10.13 (December 2002).

-inverse=
  This causes the transform to be the inverse of whatever is specified by the other options. This means if you run pamflip on an image, then run it again on the output using the same options plus -inverse, you get back the original image.

  This option was new in Netpbm 11.11 (June 2025).

### Memory Management

The following options help pamflip use memory efficiently. Some flipping operations on very large images can cause pamflip 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 pamflip approaches it, you're invited to read a complete explanation in comments in the source code.

-memsize=mebibytes
  mebibytes is the size in mebibytes (aka megabytes) of memory available for pamflip. It is the lesser of the amount of real or virtual memory available. pamflip does nothing special to allocate real memory or control its allocation -- it gets whatever it gets just by referencing virtual memory normally. The real memory figure in question is the maximum amount that pamflip can be expected to end up with by doing that. This is just about impossible for you to know, of course, but you can estimate. The total real memory in your system should be a major factor in your estimate.

  If pamflip cannot fit the entire image in the amount of memory you specify, it does the transformation in chunks, using temporary files for intermediate results.

  In strict horizontal transformations (left for right) or the null transformation, pamflip never keeps more than one row in memory, so the memory size is irrelevant and pamflip doesn't use temporary files.

  In strict vertical transformations (top for bottom), even when you allow pamflip enough memory to keep the entire image in virtual memory, it accesses it in a single pass, which does not cause any thrashing; the chunks in temporary files thus don't help.

  The real memory is important when you do a column for row type of transformation (e.g. -rotate90). In that case, even if pamflip can fit the entire image in virtual memory at once, if it does not also fit in real memory, the program will thrash like crazy because of the order in which pamflip accesses the pixels, and that means it will take a ridiculously long time to run. A proper -memsize drastically reduces the paging.

  If you specify -memsize too large, pamflip may attempt to get more virtual memory than the system allows it and fail. If it can get the virtual memory, but -memsize is larger than the amount of real memory the system allows it and the transformation is row for column, it will page thrash and run very slowly. A value even slightly too high is the same as infinity.

  If you specify -memsize too small, the program will run slightly more slowly because of extra overhead in manipulating temporary files. Also, if your environment isn't set up to make temporary files possible, pamflip will fail.

  Doing the entire transformation "in memory" doesn't speed things up as much as you might think, because even with the temporary files, the data is just as likely to be in memory. Virtual memory gets paged to disk and disk files get cached in memory. In fact, the pixels fit much more compactly into memory when stored in a temporary file than when stored "in memory" because pamflip uses a more efficient format. So you're likely to have less disk I/O when you allow pamflip less memory.

  If you do not specify -memsize, pamflip assumes infinity.

  This option did not exist before Netpbm 10.7 (August 2002).

  Before Netpbm 10.42 (March 2008), this option applied only to real memory. pamflip would always keep the entire image in virtual memory and if it could not get enough virtual memory, it failed. pamflip accessed the pixels in an order designed to keep real memory use within the specified amount.

-pagesize=bytes
  bytes is the size in bytes of a paging unit -- the amount of memory that gets paged in or out as an indivisible unit -- in your system. The default is 4KiB.

  This option has no effect.

  Before Netpbm 10.42 (March 2008), pamflip used it to control its use of real memory.

  This option did not exist before Netpbm 10.7 (August 2002).

### Miscellaneous

-verbose
  This option causes pamflip to issue messages to Standard Error about its progress.

## SEE ALSO

pnmrotate, pamdice, pamundice, pnm, pam, jpegtran manual

## HISTORY

pamflip replaced pnmflip in Netpbm 10.13 (December 2002). pamflip is backward compatible, but also works on PAM images.

## AUTHOR

Copyright (C) 1989 by Jef Poskanzer.

## Table Of Contents

- SYNOPSIS
- DESCRIPTION
- OPTIONS
  - Specifying the Transformation
  - Memory Management
  - Miscellaneous
- SEE ALSO
- HISTORY
- AUTHOR