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IndexProgramming Questions & HelpSyntax Questions › get all pixels in a certain area (slice)
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get all pixels in a certain area (slice) (Read 2442 times)
get all pixels in a certain area (slice)
Jun 14th, 2010, 10:23am

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:

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
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.

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.

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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