PAM (portable arbitrary map)

[omniplex]

PAM is a close relative of PNM (portable any map) supported by FFmpeg (example). Like the binary PNM formats (P4..P6, the ASCII versions are P1..P3) it consists of a small text header followed by a binary bitmap. Arguably parsing a PAM header (P7 format) is simpler than parsing a PNM header.

In theory PAM can replace the PNM formats, and it "officially" supports 16 bits (MAXVAL 65535) instead of only 8 bits (MAXVAL 255) for gray maps and pix maps. Of course folks use 16 bits also for P2/P5 (PGM) and P3/P6 (PPM), so that's not very interesting. The rather odd P1/P4 format (PBM) with one bit per pixel (FFmpeg pixel_fmt monob, 1 = black) ends up as a MAXVAL=1 gray map in PAM (pixel_fmt monow, 1 = white, one byte per pixel).

The interesting PAM feature is RGBA (DEPTH=4) with an alpha channel, and DEPTH=2 for a gray map with transparency. I use FFmpeg to convert PNG to PAM, decode this in a script, "do stuff" (e.g., extract tiles from a grid in a Freeciv tileset), and eventually I convert the output PAM(s) with FFmpeg back to PNG(s). If XnView could display PAM I could skip some PNG -> PAM -> PNG -> PAM -> PNG steps.

[xnview]

do you have some samples?

[omniplex]

do you have some samples?

I saved your forum avatar as XnView.png, and converted it to PAM with...

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ffmpeg -hide_banner -i xnview.png -c:v pam -pix_fmt rgba xnview.pam

The forum won't let me upload the XnView.pam, I put it into a ZIP and attached that as XnView.ZIP. The input PNG is 100x100 RGBA (3009 colours.) The output PAM header is...

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P7
WIDTH 100
HEIGHT 100
DEPTH 4
MAXVAL 255
TUPLTYPE RGB_ALPHA
ENDHDR

The line ends are bare LFs, the header consists of 69 bytes. The PAM size is 40069=100x100x4+69. The MD5 hash of the raw bitmap determined by ffmpeg -i xnview.p?? -f md5 is 690035cc9aef9c3fae4626d65d7fc09f

[omniplex]

do you have some samples?

The infamous SignedShortLosslessBug.jpg published by the Grassroots DICOM project, MediaWiki and XnView cannot display it, but FFmpeg works:

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ffmpeg -i SignedShortLosslessBug.jpg SignedShortLosslessBug.pam

The PAM header is:

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P7
WIDTH 512
HEIGHT 512
DEPTH 1
MAXVAL 65535
TUPLTYPE GRAYSCALE
ENDHDR

The MD5 for the raw bitmap (JPG input and PAM output) is 2c81619015e9356b7349e3e8aa26e7bc. On Wikimedia commons you can see a corresponding gray16be PNG, but the MD5 is different.