Having trouble with C++ Bitmaps - c++

Question

I am attempting to produce code that yields an image with 1 pixel set to black, the problem is that the output is 8 horizontal pixels of black when I attempt to set a single pixel to black. I also attempted to set the pixel above to black, but it resulted in a horizontal line 8 pixels above the original.
So my question is, how can I change my code so I end up working with bits instead of bytes in my image? I just want to be able to set a single pixel to black instead of a byte of pixels.
Here is my code:
#include <iostream>
#include <fstream>
#include "windows.h"
using namespace std;
#define IMAGE_SIZE 256
int main(int argc, char* argv[])
{
BITMAPFILEHEADER bmfh;
BITMAPINFOHEADER bmih;
char colorTable[8] = { 0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff };
char bits[IMAGE_SIZE][IMAGE_SIZE];
ofstream bmpOut("foo.bmp", ios::out + ios::binary);
if (!bmpOut) {
cout << "could not open file.";
return -1;
}
bmfh.bfType = 0x4d42;
bmfh.bfReserved1 = 0;
bmfh.bfReserved2 = 0;
bmfh.bfOffBits = sizeof(bmfh) + sizeof(bmih) + sizeof(colorTable);
bmfh.bfSize = bmfh.bfOffBits + sizeof(bits);
bmih.biSize = 40;
bmih.biWidth = IMAGE_SIZE;
bmih.biHeight = IMAGE_SIZE;
bmih.biPlanes = 1;
bmih.biBitCount = 1;
bmih.biCompression = 0;
bmih.biSizeImage = 0;
bmih.biXPelsPerMeter = 2835;
bmih.biYPelsPerMeter = 2835;
bmih.biClrUsed = 0;
bmih.biClrImportant = 0;
// Here I am initializing a white background for the bitmap image
for (int i = 0; i < IMAGE_SIZE; i++) {
for (int j = 0; j < IMAGE_SIZE; j++) {
bits[i][j] = 255;
}
}
// Here I attempt to set the most bottom left pixel to black and the pixel above it to black as well
bits[0][0] = 0;
bits[1][0] = 0;
char* workPtr;
workPtr = (char*)&bmfh;
bmpOut.write(workPtr, 14);
workPtr = (char*)&bmih;
bmpOut.write(workPtr, 40);
workPtr = &colorTable[0];
bmpOut.write(workPtr, 8);
workPtr = &bits[0][0];
bmpOut.write(workPtr, IMAGE_SIZE*IMAGE_SIZE);
bmpOut.close();
system("mspaint foo.bmp");
return 0;
}
Here is a link to the bitmap image produced:
Scaled up for clarity
Thanks

Answer 1

Based on BITMAPINFOHEADER documentation it looks like you are setting your bitmap to be read as monochrome by setting bmih.biBitCount = 1. This way every pixel is represented by one bit (black or white only), not byte. Thus, to set bottom-left pixel to black, you need to do bits[0][0] = 127 (127 = 0x7F = 01111111b). To set one above it, I think you would need to do bits[1][0] = 127. In this case I also believe
char bits[IMAGE_SIZE][IMAGE_SIZE/8];
...
for (int i = 0; i < IMAGE_SIZE; i++) {
for (int j = 0; j < IMAGE_SIZE/8; j++) {
bits[i][j] = 255;
}
}
...
workPtr = &bits[0][0];
bmpOut.write(workPtr, IMAGE_SIZE*(IMAGE_SIZE/8));
would be enough.
If you intended to use different color palette, you should set different biBitCount, as described in the docs I linked to at the beginning, I guess you would want bmih.biBitCount = 8 as a healthy default for your code.