Convert BMP into grayscale issue - c

Question

I started messing around with image processing and wanted to know if there is a way to make a gray scale image without using image processing libraries, I know the first three bits are given the information on the image, the format and size, from now on I want to make a gray scale image This is the code :
#include <stdio.h>
int main(){
FILE * binary = fopen("C:/Arabidopsis3.bmp", "rb");
FILE * bCopy=fopen("C:/copy.bmp","wb");
int get_char;
int i=3;
while ((get_char=fgetc(binary))!= EOF)
{
if(i>3)
{
doing the grayscale process
}
fputc(get_char, bCopy);
i++;
}
fclose(binary);
fclose(bCopy);
return 0;
}
As you can see I`m copying the bmp into copy.bmp but copy.bmp should be grayscale.
Any Suggestions?

Answer 1

To start, the BMP file format has a header file that gets stripped (not in memory). After the header, there is a region of memory that is a fixed size, but there are 7 different formats to what this next region is: http://en.wikipedia.org/wiki/BMP_file_format
You'll have to determine the format, then determine how many bytes are needed for each pixel. Then you'll need to know the pixel description format.
All of this is pretty easy if you just do a little research. I don't know that much about the file format, but you should be able to do it pretty easily if you just understand its structure.
Edit:
Well, if you know where each pixel value is located, you should be able to extract them just by walking along the pixel array with a pointer. For instance, if You know there are 8 pixels in an image, and each pixel is defined by 8 bytes, you can walk along the pixel array with an 8 byte pointer by placing an 8 byte pointer at the beginning the pixel array section and then do your Xor. I don't know exactly how to make something gray scale; I assume you just get rid of all the color values. As such, If the first byte describes the gray scale, and the rest is color data, you would just take the color data and make sure those bits are set to 0 then.

Answer 2

If this is a Windows bitmap, your unstructured approach is ill-advised. I'd suggest you read up on the Windows SDK for information on manipulating bmp files. Some keywords to point you in the right direction: BITMAPINFOHEADER, GetDIBits/SetDIBits. Also, GDI+ has some very convenient wrappers for loading and saving images.