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# ios – The way to constrain a transferring circle in a bigger circle

This can be a lot simpler should you change the place the calculation is. As a substitute of attempting to calculate whether or not the circumference of the interior circle goes outdoors the outer circle you may scale back this to have a look at the centres.

When you’ve got an outer circle of radius `R` and an interior circle of radius `R'` then, as a result of they’re circles we will say that if the interior circle centre is larger than `(R - R')` away from the centre it will likely be outdoors the outer circle.

So, if the centre of the outer circle is at `(0,0)` then we simply have to calculate the Pythagorean distance of the centre of the interior circle.

If the coordinates of the centre of the interior circle is `(x,y)` then we will do the calculation…

``````if x*x + y*y > (R - R') * (R - R') {
// interior circle is outdoors outer circle
}
``````

If the outer circle centre will not be at (0,0) then you definitely simply have to make use of `dx` and `dy` within the calculation the place `dx = outer circle centre x - interior circle centre x` and identical for `dy`.

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