Reading a bmp image directly in C++ : Part 1

For this we need to first understand the bmp file format

As illustrated by the figure below:

The bitmap image consists of 3 main parts:


1) Header ( Including the signature and the image info data)

2) Colour Profile

3) Image data

Image source: http://en.wikipedia.org/wiki/BMP_file_format

The header data is of size 54 bytes. This can be observed by adding up the size requirements of the individual entries as written below. Also mentioned is the offset value for individual entries. An interesting observation is that the total size is not a multiple of 4 which might create several problems as in most compilers memory is read in chunks of 4 bytes.



offset size description
0 &nbsp&nbsp&nbsp 2 &nbsp&nbsp&nbsp&nbsp&nbspsignature, must be 4D42 hex
2 &nbsp&nbsp&nbsp 4 &nbsp&nbsp&nbsp&nbsp&nbsp size of BMP file in bytes (not very reliable)
6 &nbsp&nbsp&nbsp 2 &nbsp&nbsp&nbsp&nbsp&nbsp reserved, must be zero
8 &nbsp&nbsp&nbsp 2 &nbsp&nbsp&nbsp&nbsp&nbsp reserved, must be zero
10 &nbsp&nbsp 4 &nbsp&nbsp&nbsp&nbsp&nbsp offset to start of image data in bytes
14 &nbsp&nbsp 4 &nbsp&nbsp&nbsp&nbsp&nbsp size of BITMAPINFOHEADER structure, must be 40
18 &nbsp&nbsp 4 &nbsp&nbsp&nbsp&nbsp&nbsp image width in pixels
22 &nbsp&nbsp4 &nbsp&nbsp&nbsp&nbsp&nbsp image height in pixels
26 &nbsp&nbsp2 &nbsp&nbsp&nbsp&nbsp&nbsp number of planes in the image, must be 1
28 &nbsp&nbsp2 &nbsp&nbsp&nbsp&nbsp&nbsp number of bits per pixel (1, 4, 8, or 24)
30 &nbsp&nbsp4 &nbsp&nbsp&nbsp&nbsp&nbsp compression type (0=none, 1=Run Length Encoding (RLE)-8, 2=RLE-4)
34 &nbsp&nbsp4 &nbsp&nbsp&nbsp&nbsp&nbsp size of image data in bytes (including padding, to make it a multiple of 4)
38 &nbsp&nbsp4 &nbsp&nbsp&nbsp&nbsp&nbsp horizontal resolution in pixels per meter
42 &nbsp&nbsp4 &nbsp&nbsp&nbsp&nbsp&nbsp vertical resolution in pixels per meter
46 &nbsp&nbsp4 &nbsp&nbsp&nbsp&nbsp&nbsp number of colours in image, or zero
50 &nbsp&nbsp4 &nbsp&nbsp&nbsp&nbsp&nbsp number of important colours, or zero



(To be continued in next post)