C++ bitmap editing

Question

I am trying to open a bitmap file, edit it, and then save the edited version as a new file. This is eventually to mess with using steganography. I am trying to save the bitmap information now but the saved file will not open. No errors in compilation or run time. It opens fine and the rest of the functions work.

void cBitmap::SaveBitmap(char * filename)
{
    // attempt to open the file specified
    ofstream fout;

    // attempt to open the file using binary access
    fout.open(filename, ios::binary);

    unsigned int number_of_bytes(m_info.biWidth * m_info.biHeight * 4);
    BYTE red(0), green(0), blue(0);

    if (fout.is_open())
    {
        // same as before, only outputting now
        fout.write((char *)(&m_header), sizeof(BITMAPFILEHEADER));
        fout.write((char *)(&m_info), sizeof(BITMAPINFOHEADER));

        // read off the color data in the bass ackwards MS way
        for (unsigned int index(0); index < number_of_bytes; index += 4)
        {
            red = m_rgba_data[index];
            green = m_rgba_data[index + 1];
            blue = m_rgba_data[index + 2];

            fout.write((const char *)(&blue), sizeof(blue));
            fout.write((const char *)(&green), sizeof(green));
            fout.write((const char *)(&red), sizeof(red));
        }


    }
    else
    {
        // post file not found message
        cout <<filename << " not found";
    }
    // close the file
    fout.close();
}

I suggest getting a hex editor and looking at the differences between the original file and the one your program saves. (Don't bother looking at the actual pixel data, but look at the headers, file size, and so on) — user253751, Mar 14 '16 at 01:09
I did that, the header is the same in both. Thank you though — Nicholas Tait, Mar 14 '16 at 17:03

score 0 · Answer 1 · answered Mar 14 '16 at 01:29

0

You're missing the padding bytes after each RGB row. The rows have to be a multiple of 4 bytes each.

Also, are you supposed to be writing a 24 or 32-bit bmp file? If you're writing 24-bit, you're just missing padding. If you're writing 32-bit, then you're missing each extra byte (alpha). Not enough information to fix your code sample short of writing a complete bmp writer that would support all possible options.

answered Mar 14 '16 at 01:29

Rob L

2,351
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This wouldn't stop the file from Opening.. It'd just make it look weird. His headers are wrong. – Brandon Mar 14 '16 at 03:27
No, the file would be short and any decent program would give a read error. – Rob L Mar 14 '16 at 03:34
I am trying to with with 24 bit bmp files. Can you explain padding a bit more? Thank you for your response! – Nicholas Tait Mar 14 '16 at 17:05
You might want to reference https://en.wikipedia.org/wiki/BMP_file_format#Pixel_storage. – Rob L Mar 14 '16 at 17:30

C++ bitmap editing

1 Answers1