OFImage & pixel processing

beginners
#1

Hi I a trying to write a simple program that for a given x,y position in an image , loops through all the pixels in x, y’s vicinity (i.e. for a given radius) and swaps them with the identical pixels in another image.

When I run this program with a 20x20 image and the x,p positions as 10,10 and a radius of 5 I should have a square in the center of the destination image - but instead I get lines swapped out. I’m think I’m not understanding how to process the pixels correctly. Help!

    ofPixels &srcPixels = img2.getPixels();
ofPixels &destPixels = img1.getPixels();

int h = img1.getHeight();
int w = img1.getWidth();


//set the radius for the area we want to swap pixelsaround eye gaze position
int radius = 5;
int diameter = (radius * 2)  ; //diameter to change is double radius * 3 indices per pixel
posX = 10;
posY = 10;
posX = posX - radius;
posY = posY - radius;

//make sure we don't go off edge of image
if (posX < 0) {
	//		cout << "posx < 0";
	posX = 0;
}
if (posY < 0) {
	posY = 0;
	//		cout << "posx < 0";
}

// using the new adjusted PosX and PosY loop through to swap pixels out row by row
int index = (posY * w) + posX; // calculate first insertion point in array of pixels
for (int i = 0; i < diameter; i++) { // loop on Y coords
	
	int temp = posX;

	for (int j = 0; j < diameter; j++ ) { // loop on x coords

		if (posX < w) {
			//					cout << "i " << i << "posX " << posX << "posY " << posY << "index " << index << ".\n";
			destPixels[index] = srcPixels[index];
			destPixels[index+1] = srcPixels[index+1];
			destPixels[index+2] = srcPixels[index+2];
			index = index + 2;
			posX++;
		}
	}
	if (posY < h) {
		posY++;
		posX = temp; //set x back to starting value
		int index = (posY * w) + posX; // calculate first insertion point in array of pixels


	}
}
#2

Does the “Three-Channel (RGB) Images” chapter of http://openframeworks.cc/ofBook/chapters/image_processing_computer_vision.html help?

index = index + 2 doesn’t sound right if you are working with an RGB image. I find the loops a bit hard to understand because the loop variables i,j are not used. It might be a bit easier if you did something like

for(int x=posX-radius; x<posX+radius, x++) {
  for(int y=posY-radius; y<posY+radius, y++) {
    // here use x and y to calculate pixel indices
    // and also check that you are not out of bounds
  }
}

to avoid having if statements and the extra variables.