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; } } }