PyPNM - PPM and PGM image files reading and writing

Overview and justification

PPM and PGM (particular cases of PNM format group) are simplest file formats for RGB and L images, correspondingly. This simplicity lead to some adverse consequences:

• lack of strict official specification. Instead, you may find words like "usual" in format description. Surely, there is someone who implement this part of image format in unprohibited, yet a totally unusual way.

• unwillingness of many software developers to provide any good support to for simple and open format. It took years for almighty Adobe Photoshop developers to include PNM module in distribution rather than count on third-party developers, and surely (see above) they used this chance to implement a separator scheme nobody else uses. What as to PNM support in Python, say, Pillow... sorry, I promised not to mention Pillow anywhere ladies and children are allowed to read it.

As a result, novice Python user (like me) may find it difficult to get reliable input/output modules for PPM and PGM image formats; therefore current PyPNM module was developed, combining input/output functions for 8-bits and 16-bits per channel binary and ascii PGM and PPM files, i.e. P2, P5, P3 and P6 PNM file types. Both greyscale and RGB with 16-bit per channel color depth (0...65535 range) are supported directly, without limitations and without dancing with tambourine and proclaiming it to be a novel method.

Format compatibility

Current PyPNM module read and write capabilities are briefly summarized below.

Image format	File format	Read	Write
16 bits per channel RGB	P6 Binary PPM	✅	✅
16 bits per channel RGB	P3 ASCII PPM	✅	✅
8 bits per channel RGB	P6 Binary PPM	✅	✅
8 bits per channel RGB	P3 ASCII PPM	✅	✅
16 bits per channel L	P5 Binary PGM	✅	✅
16 bits per channel L	P2 ASCII PGM	✅	✅
8 bits per channel L	P5 Binary PGM	✅	✅
8 bits per channel L	P2 ASCII PGM	✅	✅
1 bit ink on/off	P4 Binary PBM	✅	❌
1 bit ink on/off	P1 ASCII PBM	✅	❌

Target image representation

Main goal of module under discussion is not just bytes reading and writing but representing image as some logically organized structure for further image editing.

Is seems logical to represent an RGB image as nested 3D structure - (X, Y)-sized matrix of three-component RGB vectors. Since in Python list seem to be about the only variant for mutable structures like that, it is suitable to represent image as list(list(list(int))) structure. Therefore, it would be convenient to have module read/write image data to/from such a structure.

Note that for L images memory structure is still list(list(list(int))), with innermost list having only one component, thus enabling further image editing with the same nested Y, X, Z loop regardless of color mode.

Note that for the same reason when reading 1 bit PBM files into image this module promotes data to 8 bit L, inverting values and multiplying by 255, so that source 1 (ink on) is changed to 0 (black), and source 0 (ink off) is changed to 255 (white).

Installation

In case of installing using pip:

pip install PyPNM

then in your program import section:

from pypnm import pnmlpnm

then use functions as described in section "pnmlpnm.py functions" below.

In case you downloaded file pnmlpnm.py from Github or somewhere else as plain .py file and not a package, simply put this file into your program folder, then use import pnmlpnm.

pnmlpnm.py

Module file pnmlpnm.py contains 100% pure Python implementation of everything one may need to read/write a variety of PGM and PPM files. I/O functions are written as functions/procedures, as simple as possible, and listed below:

• pnm2list - reading binary or ascii RGB PPM or L PGM file and returning image data as nested list of int.
• list2bin - getting image data as nested list of int and creating binary PPM (P6) or PGM (P5) data structure in memory. Suitable for generating data to display with Tkinter.
• list2pnm - writing data created with list2bin to file.
• list2pnmascii - alternative function to write ASCII PPM (P3) or PGM (P2) files.
• create_image - creating empty nested 3D list for image representation. Not used within this particular module but often needed by programs this module is supposed to be used with.

Detailed functions arguments description is provided below as well as in docstrings.

pnm2list

X, Y, Z, maxcolors, image3D = pnmlpnm.pnm2list(in_filename)

read data from PPM/PGM file, where:

• X, Y, Z - image sizes (int);
• maxcolors - number of colors per channel for current image (int);
• image3D - image pixel data as list(list(list(int)));
• in_filename - PPM/PGM file name (str).

list2bin

image_bytes = pnmlpnm.list2bin(image3D, maxcolors)

Convert nested image data list to PGM P5 or PPM P6 (binary) data structure in memory, where:

• image3D - Y*X*Z list (image) of lists (rows) of lists (pixels) of ints (channels);
• maxcolors - number of colors per channel for current image (int).
• image_bytes - PNM-structured binary data.

list2pnm

pnmlpnm.list2pnm(out_filename, image3D, maxcolors) where:

• image3D - Y*X*Z list (image) of lists (rows) of lists (pixels) of ints (channels);
• maxcolors - number of colors per channel for current image (int).
• out_filename - PNM file name.

list2pnmascii

Similar to list2pnm above but creates ascii pnm file instead of binary.

pnmlpnm.list2pnmascii(out_filename, image3D, maxcolors) where:

• image3D - Y*X*Z list (image) of lists (rows) of lists (pixels) of ints (channels);
• maxcolors - number of colors per channel for current image (int).
• out_filename - PNM file name.

create_image

Create empty 3D nested list of X*Y*Z sizes.

viewer.py

Program viewer.py is a small illustrative utility: using pnmlpnm package, it reads different flavours of PGM and PPM files, and allows saving them as different types of PGM/PNM, i.e. it can read ascii PPM and write it as binary PPM or vs. Also this program shows images using pnmlpnm and Tkinter. No, there is no mistake: it does not feed PPM files to Tkinter directly. Instead, it uses nested 3D list data loaded using pnmlpnm to generate in-memory bytes object of PPM structure using preview_data = pnmlpnm.list2bin(image3D, maxcolors), and then feeds this in-memory bytes object to Tkinter as preview = PhotoImage(data=preview_data) (note using data=, not file=). This way it shows, for example, ascii PPM which Tkinter itself cannot handle.

As a result, you may use pnmlpnm and Tkinter to visualize any data that can be represented as greyscale or RGB without huge external packages and writing files on disk; all you need is Tkinter, included into standard CPython distributions, and highly compatible pure Python pnmlpnm.py taking only 16 kbytes.

References

1. Netpbm file formats description.

2. PyPNM at PyPI - installing PyPN with pip. Does not contain viewer example etc., only core converter.

3. PyPNM at Github containing example viewer application, illustrating using list2bin to produce data for Tkinter PhotoImage(data=...) to display, and of open/save various portable map formats.

4. PixelArtScaling - usage example, pure Python image rescaling applications using Scale2x and Scale3x, PNG I/O is based on PyPNG, and PPM/PGM I/O - on PyPNM, thus making all applications cross-platform.

5. POVRay Thread: Linen and Stitch - usage example, contains image filtering application «Averager», implementing non-standard adaptive image averaging. Filter before/after preview based on statically linked PyPNM list2bin code and Tkinter PhotoImage(data=...) class.