Rotate image DIY

beginners
#1

Hi all

I have an assignment to make my own rotation method for rotating an image around a point using. Problem is, I’m completely clueless and haven’t been able to find anything helpful.

I’m supposed to have the following input: a grayscale image, an angle, x and y coordinates for the point.

So far, I have made the code that allocate the space needed for the rotated image. But I can’t figure out how to rotate it :frowning:

  
  
//Stuff about the angle  
	int angle = 90;  
	float radians = (2*pi*angle)/360;  
	float sine = sin(radians);  
	float cosine = cos(radians);  
	  
	inputImg.loadImage("images/bikers.jpg");  
	  
	colorImg.allocate(inputImg.getWidth(), inputImg.getHeight());  
	colorImg.setFromPixels(inputImg.getPixels(), inputImg.getWidth(), inputImg.getHeight());  
	  
	grayImg.allocate(inputImg.getWidth(), inputImg.getHeight());  
	grayImg = colorImg;  
	  
	//Calculate the size of the rotated image canvas   
	//point0 is already known (0,0)  
	  
	float point1x = inputImg.getWidth() * cosine;  
	float point1y = inputImg.getWidth() * sine;  
	float point2x = -inputImg.getHeight() * sine;  
	float point2y = inputImg.getHeight() * cosine;  
	float point3x = inputImg.getWidth() * cosine - inputImg.getHeight() * sine;  
	float point3y = inputImg.getWidth() * sine + inputImg.getHeight() * cosine;  
	  
	float minx = min(0 , min(point1x , min(point2x , point3x)));  
	float miny = min(0 , min(point1y , min(point2y , point3y)));  
	float maxx = max(point1x , max(point2x , point3x));  
	float maxy = max(point1y , max(point2y , point3y));  
	  
	int rotWidth = floor(fabs(maxx) - minx);  
	int rotHeight = floor(fabs(maxy) - miny);  
	  
	rotatedImg.allocate(rotWidth, rotHeight);  
	  
	unsigned char* pixels  = grayImg.getPixels();  
	  
	//Actually rotate the image  
	for(int y = 0; y < rotHeight; y++)  
	{  
		for(int x = 0; x < rotWidth; x++)  
		{  
                      //WHAT TO DO HERE?  
      }  
   }

Any ideas to get me in the right direction?

#2

something like this should do the trick:

  
  
p2.x=cos(a)*(p1.x-o.x)-sin(a)*(p1.y-o.y)+o.x;  
p2.y=sin(a)*(p1.x-o.x)+cos(a)*(p1.y-o.y)+o.y;

where p2 is your target point, p1 is your source point, o is the rotation origin and a is the angle