[ianking]

18/11/2012 at 4:49 pm

Can anyone tell me why a 10 X 10 monochrome bmp file is 102 bytes in size. (I’m writing a unit on standard file formats) A txt file 15 characters long takes up 15 bytes exactly which makes sense. A 100 X 100 monochrom bitmap file is 1.62Kb instead of 1.22Kb which is better but not right. (or should I be avoiding using mspaint?)

[Mark J Tennant]

18/11/2012 at 5:59 pm

I assume it will be due to additional header information that’s stored with the bitmap image.

Mark.

[chalove]

18/11/2012 at 9:57 pm

Yes Ian,

14 bytes for the Bitmap header of the image.
40 bytes for the DIB header (I’m assuming your using Windows XP or later versions of windows – BITMAPINFOHEADER is probably used).
Pixel array would be next (its padded out to at least 4 bytes per row of pixels)
10 rows of pixels, each row padded to 4 bytes is
40 bytes for the image data
So 94 bytes so far
The last 8 bytes is for the colour palette as 4 bytes are used to define black and 4 bytes used to define white.

14 + 40 + 40 + 8 = 102 bytes

Hope that helps,

Charlie

P.S. You can read Wikipedia’s excellent breakdown of the file format here: http://en.wikipedia.org/wiki/BMP_file_format.

[ianking]

18/11/2012 at 10:29 pm

Many thanks Charlie. A truly comprehensive explanation.