http://bugs.winehq.org/show_bug.cgi?id=12717
--- Comment #17 from knan-wine@anduin.net 2008-09-22 12:55:53 --- (In reply to comment #14)
Created an attachment (id=14720)
--> (http://bugs.winehq.org/attachment.cgi?id=14720) [details]
a "broken" sample file
May be a 24-bit image stored as 32-bit. Since the alpha channel is all 0 (which in tga is "fully transparent"), it may be interpreted as fully transparent all over.
But a random googled specification (http://local.wasp.uwa.edu.au/~pbourke/dataformats/tga/) says: "The 4 byte entry contains 1 byte each of blue, green, red, and attribute. For faster speed (because of the hardware of the Targa board itself), Targa 24 images are sometimes stored as Targa 32 images."
So an alternative interpretation is "crap alpha channel data, ignore it" - a valid interpretation according to the tga standards. File looks quite valid, just a bit strange.
tgatoppm just throws away the alpha channel, clearing up any confusion.