Get every pixel a circle covers

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Given a Color[] TextureData representing the texture data (it's a one dimensional array), a Vector2 CircleCenter representing the center of the circle and a float R representing the circles radius; how would one go about determining every pixel that the area of the circle covers?

Posted 30-Apr-10 19:15pm

Joshua Tully

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Joshua Tully · Answer 1 · 2010-05-01T09:37:00

Well, it doesn't seem like my question was clear enough and I created my own solution.

Assuming the center of the circle is at x,y and the radius is r we can create a square with the points x-r, x+r, y-r, and y+r. This gives us a square that contains the circle. Then we can go through every pixel inside that square and determine if it's inside the circle using Pythagoras's equation. Basically it foes something like

C#

int Left = (int)centerOfExplosion.Value.X - bombRadius;
int Right = Left + bombRadius * 2;
int Top = (int)centerOfExplosion.Value.Y - bombRadius;
int Bottom = Top + bombRadius * 2;
for (int j = Top; j <= Bottom; ++j)
{
    for (int k = Left; k <= Right; ++k)
    {
        double dist = Math.Pow(centerOfExplosion.Value.X - k, 2.0) + Math.Pow(centerOfExplosion.Value.Y - j, 2.0);
        if (dist <= Math.Pow(bombRadius, 2))
        {
            Color c = MapColorData[k + j * Map01Color.Width];
            c.A = 0;
            MapCollisionData[k + j * Map01Collision.Width] = Color.White;
            MapColorData[k + j * Map01Color.Width] = c;
        }
    }
}

Rick York · Answer 2 · 2010-05-07T07:14:00

Solution 5

Josh : good idea comparing with the distance squared instead of computing a square root every time. Another performance improvement would be to compute the square of bombRadius once and save it in a temporary variable instead of computing for each point since it does not change.

Posted 7-May-10 7:14am

Rick York

The Manoj Kumar · Answer 3 · 2010-05-01T07:44:00

Solution 3

Seems that your requirement is just inverse of painting a circle. For painting we use 8-way symmetry algorithm or similar algorithms. You can use the same algorithms and in place of painting the pixed pick the pixel from the image. This link can be helpful 8-way symmetry algorithm[^]

Posted 1-May-10 7:44am

The Manoj Kumar

aasser · Answer 4 · 2010-05-01T03:14:00

Hi Josh_Jackson,
I am not sure if I have understood what you really mean or not, but it does not look a challenging problem.

Bitmap bmp = new Bitmap(100, 100);
// Change Bitmap here to image you aim
for (int i = 0; i<bmp.Width ; i++)
   for (int j = 0; j<bmp.Height ; j++)
      if((i-CenterX)*(i-CenterX)+(j-CenterY)*(j-CenterY)< R*R)
         {//Inside the Circle
            Color c = bmp.GetPixel(i, j);
            if(c==.../* My favorite color */)
            {
            //...

Did you mean that?

Saurabh.Garg · Answer 5 · 2010-05-01T03:29:00

Solution 2

This question makes no sense at all. You have 1 dimensional array containing texture data and a tuple (C, R) where C is the center and R is the radius of circle.

How does one use this information to determine every pixel circle covers?

-Saurabh

Posted 1-May-10 3:29am

Saurabh.Garg