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Index › Programming Questions & Help › Syntax Questions › get all pixels in a certain area (slice)
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get all pixels in a certain area (slice) (Read 2442 times)
monsieurBlutbad
get all pixels in a certain area (slice)
Jun 14th, 2010, 10:23am  
hello,

i want to access the color information of pixels in a slice of a circle, i.e. from 30° to 90°.

here is what i tried:
Code:

startAngle = 30:
endAngle  = 90;

for (float a = startAngle; a < endAngle; a+=0.1) {  
     for (int i = 0; i < radius; i++) {
	  x = width / 2 + round ( i * cos( (a) * PI / 180) );
	  y = height/ 2 + round ( i * sin( (a) * PI / 180) );
	  test = pixels[y * width + x];
	}
    }


this works, but it is very unelegant, because i calculate multiple pixels (especially those near to the circle center) more than once.

does anyone know of a more effecient way to get the pixels in a slice?

thank you
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Quark
Re: get all pixels in a certain area (slice)
Reply #1 - Jun 14th, 2010, 1:02pm  
You also risk missing pixels near the circumference if the radius is large.

Try it the other way round iterate over all the pixels in the image calculate the equivalent x, y position from the pixel number then see if it is in the slice.
Code:

int x, y, l;
int cx = width/2;
int cy = height/2;

float ang;

for(int i = 0; i < pixels.length; i++){
  y = i / width - cy; 
  x = i % width - cx;
  l = dist(x, y, cx, cy);
  if(l < radius){
    ang = degrees(atan2(y,x));
  }
  if(ang >= 30 && ang <=90){
    // inside slice
  }
}


This is rough and ready so there maybe the odd syntax/logic error but it might get you started.
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monsieurBlutbad
Re: get all pixels in a certain area (slice)
Reply #2 - Jun 18th, 2010, 2:11am  
thank you very much quark, this works very nice!

i used your code and added a part that builds a minimum size rectangle over the circle slice, and i only check the pixels within that rectangle, to save some resources by not checking every pixel on the screen each frame.
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