Anyone knows why the UNIX bitmap-format is as it is?

I know this has not much to do with tcl/tk, but since I was using

the -bitmap feature in tk when I discovered the strange format, I got a

little curious.

I would have thought that to make a bitmap you would start at the upper left corner, taking 4 pixels at a time to make a hexadecimal number. Then you group two hexadecimal numbers together. So a picture looking like

0111 0101 1101 1110 ....

(spacing just for convinience)

would be 0x75, 0xde, ....

But instead, the format switch the two hexadecimal number, so that 0x75 becomes

0x57 and then reverses the bits in each number to get a new number.

So the first two number in the example, 0111 0101, will exchange places to

give 0101 0111 and then we reverse the order of the bits in each of them,

to end up with 1010 1110. This gives us 0xae, and not 0x75 as we started with (which I thought was more logical...)

Anyone knows why?

Cheers,

Jostein