read pixel value in bmp file

Question

How can I read the color value of 24bit BMP images at all the pixel [h*w] in C or C++ on Windows [better without any 3rd party library]. I got Dev-C++
A working code will be really appreciated as I've never worked on Image reading & have come to SO after Googling [if you can google better than me, plz provide a link].

Answer 1

The following code snippet is not complete, and contains lots of hidden assumptions and bugs. I wrote it from scratch for a university course project from mere observation, where it minimally fulfilled all the requirements. I didn't work on it any more, because there must be libraries that would do the job way better.

Here are the conditions where it worked okay (some assumptions are pointed out in the comments):

1. It ran on Windows, I'm not sure about other platforms
2. It works for 24-bit color BMP images
3. It assumes that the width of the image is a multiple of 4, so it doesn't handle the padding bytes in case it's not
4. It decodes the image width and height as 32-bit little endian integers
5. It returns a pointer to dynamically allocated memory, it may cause memory leak if it's not released by the caller

Other answers have covered some of these issues.

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You can try this one:

unsigned char* readBMP(char* filename)
{
    int i;
    FILE* f = fopen(filename, "rb");
    unsigned char info[54];

    // read the 54-byte header
    fread(info, sizeof(unsigned char), 54, f); 

    // extract image height and width from header
    int width = *(int*)&info[18];
    int height = *(int*)&info[22];

    // allocate 3 bytes per pixel
    int size = 3 * width * height;
    unsigned char* data = new unsigned char[size];

    // read the rest of the data at once
    fread(data, sizeof(unsigned char), size, f); 
    fclose(f);

    for(i = 0; i < size; i += 3)
    {
            // flip the order of every 3 bytes
            unsigned char tmp = data[i];
            data[i] = data[i+2];
            data[i+2] = tmp;
    }

    return data;
}

Now data should contain the (R, G, B) values of the pixels. The color of pixel (i, j) is stored at data[3 * (i * width + j)], data[3 * (i * width + j) + 1] and data[3 * (i * width + j) + 2].

In the last part, the swap between every first and third pixel is done because I found that the color values are stored as (B, G, R) triples, not (R, G, B).