How to overlay one image on top of another? C++ - c++

Question

I have two RGB images (ppm format), and I want to be able to overlay any pixel that's not purely black of the top image onto the bottom image.
I can successfully load imaged, save images, copy images... but I'm not able to create an image out of the two images in the manner I've described above.
I'm not going to include all the code I have, but the important parts to achieve this are:
struct Pixel
{
unsigned int r;
unsigned int g;
unsigned int b;
}
I overloaded its == operator for easier comparison:
bool Pixel::operator==(const Pixel& other)
{
if(r != other.r)
{
return true;
}
else if(g != other.g)
{
return true;
}
else if(b != other.b)
{
return true;
}
else
{
return false;
}
}
In my Pic class I have this method:
Pic Pic::overlay(const Pic& top, Pixel mask)
{
for(int h = 0; h < height; h++)
{
for(int w = 0; w < width; w++)
{
if(!(top.pixels[h][w] == mask))
{
pixels[h][w] = top.pixels[h][w]; // pixels[][] is a Pixel array
}
}
}
return *this;
}
My main file has this:
Pic top;
Pic bot;
Pic overlay;
Pixel mask:
mask.r = 0;
mask.g = 0;
mask.b = 0;
top.loadimage("top.ppm"); //loadimage() loads the image in and all the data
bot.loadimage("bot.ppm"); //samme thing
overlay = bot.overlay(bot, mask);
overlay.saveimage("overlay.ppm");
The = operator is overloaded for the Pic class, obviously.
The kind of problems I have are these:
In the overlay method, if I leave this if statement as described above, top image will be displayed in the saved file. If I make it without !() part, it'll display the bottom image.
If I get rid of that if() statement completely, and simply try to alter the individual pixels, ex:
pixels[h][w].r = pixels[h][w].r - 50;
The saved image will be altered, all wacky looking, for obvious reasons.
However... .b and .g have no effect on the image.
I'm out of ideas... I've been playing with this for 2 days and I can't figure out what's wrong. Everything works as needed in my program, except this overlay method.
EDIT: So, I found one of the problems in my code and it went back to how I loaded the images with PPM P6 format. Instead of individually loading each pixel as 1 byte, I tried to load them all together, so it created that cushion stuff that happens with structures and binary reading in from compaction... Now I'm able to put the overlay of top image onto the bottom image, but not all colors are showing. Still, better than before.
Here's what I modified my overlay's nested for() loop to look like:
for(int h = 0; h < height; h++)
{
for(int w = 0; w < width; w++)
{
if(top.pixels[h][w].r != mask.r &&
top.pixels[h][w].g != mask.g &&
top.pixels[h][w].b != mask.b )
{
pixels[h][w].r = top.pixels[h][w].r;
pixels[h][w].g = top.pixels[h][w].g;
pixels[h][w].b = top.pixels[h][w].b;
}
}
}
Obviously it still requires work.

Answer 1

This line looks wrong:
overlay = bot.overlay(bot, mask);
Shouldn't it be:
overlay = bot.overlay(top, mask);
And if you want a shorter way to write your equality test then you might like this:
bool Pixel::operator==(const Pixel& other)
{
return (r==other.r && g==other.g && b==other.b);
}
Finally, since you've got an equality operator, then why not do add and assignment ('=') to keep your coder as neat as poss