Junk Data in a .bmp file?

[ngill]

Hi, a while back for some practice I wrote a .bmp file reader... it worked perfectly or so i thought. The thing is it works when it's reading height=2^n width=2^n, but when width=height=some odd number... it doesn't load them up right.

After some debugging I found out that

bitsPerPixel*Width*Height != (image size defined in the file)

[my 5x5 picture's size was 80 bytes, instead of 75]

but it's supposed to be!! And when I displayed the contents of the file byte by byte, i found a bunch of 0s in places... but there seemed to be a pattern for zeros... any ideas? Is there junk data in bmp image?

(I created my .bmp files using GIMP)

[DFrey]

That kind of filler data is common in many types of files. It just requires you to be strict about reading in the files as precisely as the header indicates.

[Tron]

every line has to be doubleword-aligned (4 byte), so u get 0 till 3 'junk-bytes'

example 1:
5x7 24bit rgb
needed line size: 5 * 3 = 15 bytes ( * 7 = 105)
real size: 16 bytes (1 junk byte) ( * 7 = 112)

example 2:
5x7 8bit indexed
needed line size: 5 * 1 = 5 bytes ( * 7 = 35)
real size: 8 bytes (3 junk bytes) (* 7 = 56)

speaking of palette indexed pictures: every palette entry is 4 bytes in size, not 3!
order: BLUE, GREEN, RED, UNUSED

hope that helps (: