Strange Keyboard (take 2)

From: der Mouse <mouse_at_Rodents.Montreal.QC.CA>
Date: Sun Mar 14 13:48:43 2004

>>> [Sparkle] 1701> bmptoppm < keyboard1.bmp > keyboard1.ppm
>>> bmptoppm: warning: offBits is 54, expected 67108918
> 54 looks fine - offbits is the byte offset after the file header
> where the image data begins. [...]
> An expected offset over 60MB in size seems extremely wrong :)

67108918 is 0x4000036, while 54 is 0x36; I assumed, absent
documentation, that the low three bytes are the offset and the high
byte is some kind of flags, which is getting rolled into the same
variable by the code.

>> I managed to view them fine with xv under solaris, is there a
>> problem with bmptoppm ?

Yes, there obviously is something wrong with bmptoppm; even if it can't
work, it shouldn't actually core.

However, it's the only bmp-reading tool I have, and I have no
documentation on BMP format to either fix it or write another.

I may see if I can deduce enough from the code to fix it; this
discussion has gotten me curious.

> Run hexdump -C on the file:

> Bytes 0 and 1 should have the ascii characters 'BM'.

Check.

> Bytes 2,3,4,5 give the size in 4 byte units of the file.

Mismatch. Bytes 2,3,4,5 give the size in bytes (little-endian; you
didn't specify which byte sex).

> Bytes 6,7,8,9 are reserved - I've only ever seen these set to zero.

Check. The bmptoppm code implies these are a hot-spot location.

> Bytes 10,11,12,13 give the offbits value, least significant byte
> first. It's found 54, so is obviously reading that correctly.

"36 00 00 00" - yes.

> Bytes 14,15,16,17 are within the image header itself and give the
> size of the image header - 12 bytes for an OS/2 bmp file, 40 bytes
> for a Windows bmp, and no expected other values.

"28 00 00 00" - that's 40.

> I haven't got a copy of the zipfile that Lyos sent to try otherwise
> I'd take a look...

I still have it. Shall we take this offlist?

/~\ The ASCII der Mouse
\ / Ribbon Campaign
 X Against HTML mouse_at_rodents.montreal.qc.ca
/ \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
Received on Sun Mar 14 2004 - 13:48:43 GMT