Loop through a array in circle shape without repeat indexes - algorithm

Question

I need to loop through a array in circle in arc shape with a small radius (like draw a circle pixel by pixel), but all algorithm i tried, checks duplicate indexes of array (it's got the same x and y several times).
I have a radius of 3, with a circle form of 28 elements (not filled), but the algorithm iterate 360 times. I can check if x or y change before i do something, but it's lame.
My code now:
for (int radius = 1; radius < 6; radius++)
{
for (double i = 0; i < 360; i += 1)
{
double angle = i * System.Math.PI / 180;
int x = (int)(radius * System.Math.Cos(angle)) + centerX;
int y = (int)(radius * System.Math.Sin(angle)) + centerY;
// do something
// if (array[x, y]) ....
}
}
PS: I can't use midpoint circle, because i need to increment radius starting from 2 until 6, and not every index is obtained, because his circle it's not real (according trigonometry)
EDIT:
What i really need, is scan a full circle edge by edge, starting by center.
360 steps (it's get all coordinates):
Full scan
for (int radius = 2; radius <= 7; radius++)
{
for (double i = 0; i <= 360; i += 1)
{
double angle = i * System.Math.PI / 180;
int x = (int)(radius * System.Math.Cos(angle));
int y = (int)(radius * System.Math.Sin(angle));
print(x, y, "X");
}
}
Using Midpoint Circle or other algorithm skipping steps (missing coordinates):
Midpoint Circle Algorithm
for (int radius = 2; radius <= 7; radius++)
{
int x = radius;
int y = 0;
int err = 0;
while (x >= y)
{
print(x, y, "X");
print(y, x, "X");
print(-y, x, "X");
print(-y, x, "X");
print(-x, y, "X");
print(-x, -y, "X");
print(-y, -x, "X");
print(y, -x, "X");
print(x, -y, "X");
y += 1;
err += 1 + 2 * y;
if (2 * (err - x) + 1 > 0)
{
x -= 1;
err += 1 - 2 * x;
}
}
}

Answer 1

There are two algorithmic ideas in play here: one is rasterizing a circle. The OP code presents a couple opportunities for improvement on that front: (a) one needn't sample the entire 360 degree circle, realizing that a circle is symmetric across both axes. (x,y) can be reflected in the other three quadrants as (-x,y), (-x,-y), and (x,-y). (b) the step on the loop should be related to the curvature. A simple heuristic is to use the radius as the step. So...
let step = MIN(radius, 90)
for (double i=0; i<90; i += step) {
add (x,y) to results
reflect into quadrants 2,3,4 and add to results
}
With these couple improvements, you may no longer care about duplicate samples being generated. If you still do, then the second idea, independent of the circle, is how to hash a pair of ints. There's a good article about that here: Mapping two integers to one, in a unique and deterministic way.
In a nutshell, we compute an int from our x,y pair that's guaranteed to map uniquely, and then check that for duplicates...
cantor(x, y) = 1/2(x + y)(x + y + 1) + y
This works only for positive values of x,y, which is just what you need since we're only computing (and then reflecting) in the first quadrant. For each pair, check that they are unique
let s = an empty set
int step = MIN(radius, 90)
for (double i=0; i<90; i += step) {
generate (x,y)
let c = cantor(x,y)
if (not(s contains c)) {
add (x,y) to results
reflect into quadrants 2,3,4 and add to results
add c to s
}
}

Answer 2

Got it!
It's not beautiful, but work for me.
int maxRadius = 7;
for (int radius = 1; radius <= maxRadius; radius++)
{
x = position.X - radius;
y = position.Y - radius;
x2 = position.X + radius;
y2 = position.Y + radius;
for (int i = 0; i <= radius * 2; i++)
{
if (InCircle(position.X, position.Y, x + i, y, maxRadius)) // Top X
myArray[position, x + i, y]; // check array
if (InCircle(position.X, position.Y, x + i, y2, maxRadius)) // Bottom X
myArray[position, x + i, y2]; // check array
if (i > 0 && i < radius * 2)
{
if (InCircle(position.X, position.Y, x, y + i, maxRadius)) // Left Y
myArray[position, x, y + i]; // check array
if (InCircle(position.X, position.Y, x2, y + i, maxRadius)) // Right Y
myArray[position, x2, y + i]; // check array
}
}
}
public static bool InCircle(int originX, int originY, int x, int y, int radius)
{
int dx = Math.Abs(x - originX);
if (dx > radius) return false;
int dy = Math.Abs(y - originY);
if (dy > radius) return false;
if (dx + dy <= radius) return true;
return (dx * dx + dy * dy <= radius * radius);
}