read pixel value in bmp file [closed]

Answer 1

I can't comment on the top level answer because I don't have enough stackoverflow rep yet, but I just wanted to point out one very critical bug with that implementation.

Some bitmaps can be written with a negative height, so when you try to allocate your image data buffer, your code will crash with std::bad_alloc. Bitmaps with negative height means that the image data is stored top to bottom instead of the traditional bottom to top. Therefore, a slightly better version of the top level answer is (still not including portability for systems with different endianness and size of bytes):

unsigned char* readBMP(char* filename)
{
    int i;
    FILE* f = fopen(filename, "rb");
    unsigned char info[54];
    fread(info, sizeof(unsigned char), 54, f); // read the 54-byte header

    // extract image height and width from header
    int width, height;
    memcpy(&width, info + 18, sizeof(int));
    memcpy(&height, info + 22, sizeof(int));

    int heightSign = 1;
    if (height < 0){
        heightSign = -1;
    }

    int size = 3 * width * abs(height);
    unsigned char* data = new unsigned char[size]; // allocate 3 bytes per pixel
    fread(data, sizeof(unsigned char), size, f); // read the rest of the data at once
    fclose(f);

    if(heightSign == 1){
        for(i = 0; i < size; i += 3)
        {
            //code to flip the image data here....
        }
    }
    return data;
}