BMP file data extraction

Following the BMP standard [1], file size, data size and data start, height (in pixels), width (in pixel), depth (bits) and compression were read from the header of the file. An odd behavior was noted. A file may contain extra bits at the end of each line, which is not included in width information. Hence to find offsets it is better to use (size – start) / height to find the actual length of the information of a row.

Another peculiar feature of BMP file is when height is specified in positive figure (which is the most common) the last row of the image is written first and the first row at the end. i.e. an inverted mirror image is stored. Hence to print the first row, it had to be read from (start + ( height -1) row_length )

Further, in order to support both the endiness, a bit reversal operation needed, which was implemented in a macro, instead of a for loop or function to save the additional over head that would have occurred otherwise [2]

BMP file standard [1]

 (from http://atlc.sourceforge.net/bmp.html#_toc381201084 )

Bit reversal [2]


#define RBIT(val) (((unsigned)(val) & 128) >> 7 \
| ((unsigned)(val) & 64) >> 5 \
| ((unsigned)(val) & 32) >> 3 \
| ((unsigned)(val) & 16) >> 1 \
| ((unsigned)(val) & 8) << 1 \
| ((unsigned)(val) & 4) << 3 \
| ((unsigned)(val) & 2) << 5 \
| ((unsigned)(val) & 1) << 7 )