Portable Pix Map, an image format of the PNM family.  It is a part of
[PBM Plus] - a large numbers of image converters and manipulators available
on the internet.

Every pixel is represented by three bytes for red, green, and blue channel. No compression, no transparency, as far as [RS] is aware.

[Tk] images can be read and written in PPM format using [photo] [[as of which version?]].  See also [PGM].

[RS] 2006-06-02: Here is a reader that parses a "P3" format PPM string (like read from a file) and creates a [photo] from it:

 proc ppm-photo ppm {
   regsub -all {#[^\n]*\n} $ppm " " ppm ;# strip out comments
   foreach {type w h max} $ppm break
   foreach {r g b} [lrange $ppm 4 end] {
      set r [expr {int(255.*$r/$max)}]
      set g [expr {int(255.*$g/$max)}]
      set b [expr {int(255.*$b/$max)}]
      lappend row [format #%02X%02X%02X $r $g $b]
      if {[llength $row] == $w} {
         lappend rows $row
         set row {}
      }
   }
   set im [image create photo]
   $im put $rows
   set im
 }
#-- Testing:
 set data {P3
 4 4
 15
 0 0 0 0 0 0 0 0 0 15 0 15
 0 0 0 0 15 7 0 0 0 0 0 0
 0 0 0 0 0 0 0 15 7 0 0 0
 15 0 15 0 0 0 0 0 0 0 0 0
 }
 ppm-photo $data
#-- produces the following image (zoomed by a factor of 9):
[http://mini.net/files/ppm.jpg]
A larger ray-tracing image to test with is at http://www.cpsc.ucalgary.ca/~cherlin/sample.ppm - I was happy to see that IrfanView and my little [proc] agreed on how to render it :^)

[aricb] From P3 PPM to photo image in 17 lines!  I'm impressed.
----
[Category Graphics]