Overlay BMP image on top on another with C++ - c++

Question

I'm working on a C++ program that overlays one 8 bit depth BMP image into another one of the same format and size. The program is supposed to read both images and then for every black pixel in the overlay picture, it changes the same pixel in the original picture to white. The essentials parts of my code are shown below:
The class for a BMP picture:
class BMPImage{
int width;
int height;
int size;
unsigned char* data; //Array with all pixels
unsigned char* header; //54 byte header
void setSize();
public:
BMPImage(const char* filename);
BMPImage(unsigned char* header, unsigned char* data);
int getSize();
int getWidth();
int getHeight();
unsigned char* getData();
unsigned char* getHeader();
void createFile(char* filename);
}
Constructors to create an object from a filename or from a header and a data array:
BMPImage::BMPImage(const char* filename){
FILE* f = fopen(filename, "rb");
if(f == NULL)
throw "Argument Exception";
header= new unsigned char[54];
fread(header, sizeof(unsigned char), 54, f); // read the 54-byte header
// extract image height and width from header
setSize();
cout << endl;
cout << "Reading file " << filename << endl;
cout << " Width: " << width << endl;
cout << " Height: " << height << endl;
data = new unsigned char[size];
fread(data, sizeof(unsigned char), size, f); // read the rest of the data at once
fclose(f);
}
BMPImage::BMPImage(unsigned char* h, unsigned char* d){
header = new unsigned char[54];
for(int i=0;i<54;i++){
header[i]=h[i];
}
setSize();
data = new unsigned char[size];
for(int i=0;i<size;i++){
data[i]=d[i];
}
}
A function to create a file from a BMPImage object:
void BMPImage::createFile(char* filename){
FILE* f;
f=fopen(filename, "wb");
//write header
fwrite(header,sizeof(unsigned char), 54, f);
//write data
fwrite(data, sizeof(unsigned char), size, f);
//close file
fclose(f);
cout<<endl<<"File "<< filename <<" was successfully created."<<endl;
}
Finally, my function to overlay two images. The overlay is done so that if the pixel of "overlay" is black, the corresponding pixel in the result image will be white, else the pixel will be the same as in the "original" image.
void overlay(char* original, char* overlay, char* result){
BMPImage overlayIm(overlay);
BMPImage originalIm(original);
//Check if the pictures have the same size
if(originalIm.getHeight()!=overlayIm.getHeight() || originalIm.getWidth()!=overlayIm.getWidth())
throw "Files do not have the same size";
int size = overlayIm.getSize();
unsigned char* dataResult = new unsigned char[size];
unsigned char* dataOriginal = originalIm.getData();
unsigned char* dataOverlay = overlayIm.getData();
//If a pixel in overlay is black, change corresponding pixel in original to white
for(int i=0; i<size; i++){
if(dataOverlay[i]!=0){
dataResult[i]=dataOriginal[i];
}else{
dataResult[i]=(unsigned char)255;
}
}
//Create new file with the result
BMPImage resultIm(originalIm.getHeader(), dataResult);
resultIm.createFile(result);
cout<<endl<<"New overlay image created in "<<result <<endl;
}
Although I only check for the pixels in the overlay image, it seems that the program is checking for the pixels in both the overlay and the original image at the same time. So when a pixel is black in the original image, the pixel in the result will be white, and I just want it to be white only if the pixel in overlay image is black.
An example of my problem is shown in this picture. The pictures on the left are the arguments in the overlay function. In the right, the "desired" picture is the one that my program should generate, and the "result" is the one that my program actually generates:
I don't understand why the black pixels in the original are changed to white if I never check the pixels in my original picture when I create the result image. Can anybody point me to the mistake I'm committing so that I can solve my problem?