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Consider two figures: a rectangle with coordinates $(x_{min},y_{min}), (x_{min}+dx,y_{min}+dy)$, and a circle. The latter has a radius $R$ and is located at the center of the coordinate frame. How to find the intersection points between these figures?

My attempt is the following (given in terms of some parameters):

ROffAxisArb[\[Theta]_, ztodet_] = ztodet*Tan[\[Theta]];
rec[xtodet_, ytodet_, dxdet_, dydet_] := 
 Rectangle[{xtodet, ytodet}, {xtodet + dxdet, ytodet + dydet}]
circ[\[Theta]_, ztodet_] := 
 Circle[{0, 0}, ROffAxisArb[\[Theta], ztodet]]
solEasy[xtodet_, ytodet_, ztodet_, dxdet_, dydet_, \[Theta]_] := 
 Solve[{x, y} \[Element] 
    rec[xtodet, ytodet, dxdet, dydet] && {x, y} \[Element] 
    circ[\[Theta], ztodet], {x, y}]
(*Longitudinal distance to detector*)
ZtoDet = 8.7;
(*Transverse distance to the beginning of detector - y*)
YtoDet = 2;
(*Transverse distance to the beginning of detector - X*)
XtoDet = -1;
(*Transverse size of the detector - width*)
DxDet = 8;
(*Transverse size of the detector - height*)
DyDet = 2;
solEasy[XtoDet, YtoDet, ZtoDet, DxDet, DyDet, 0.6]

It gives some expression $y = f(x)$ for some domain of $x$, which looks meaningless:

{{y -> ConditionalExpression[
    0.000283439 Sqrt[4.40966*10^8 - 1.24475*10^7 x^2], 
    4.40751 <= x <= 5.60591]}}

In contrast, I would expect either two points $\{x_{1},y_{1}\},\{x_{2},y_{2}\}$, or one point, or zero. For instance, graphically for $\theta = 0.6$ I get

lineStyle1 = {Thick, Blue};
lineStyle2 = {Thick, Blue};
lineStyle3 = {Thick, Darker@Darker@Green, Dashing[0.02]};
line1 = Line[{{XtoDet, YtoDet}, {XtoDet, YtoDet + DyDet}}];
line2 = Line[{{XtoDet + DxDet, YtoDet}, {XtoDet + DxDet, 
     YtoDet + DyDet}}];
Plot[{f[x, ROffAxisArb[0.6, ZtoDet]], 
  If[XtoDet < x < XtoDet + DxDet, YtoDet, -10], 
  If[XtoDet < x < XtoDet + DxDet, 
   YtoDet + DyDet, -10]}, {x, -1.2 (Abs[XtoDet] + Abs[DxDet]), 
  1.2 (Abs[XtoDet] + DxDet)}, 
 PlotRange -> {{-1.2 (Abs[XtoDet] + DxDet), 
    1.2 (Abs[XtoDet] + DxDet)}, {0, 1.2 (YtoDet + DyDet)}}, 
 Frame -> True, FrameStyle -> Directive[Black, 25], PlotRange -> All, 
 PlotStyle -> {{Thick, Red}, {Thick, Blue}, {Thick, Blue}}, 
 ImageSize -> Large, 
 FrameLabel -> {"x", 
   "y"},(*PlotLegends\[Rule]Placed[Style[#,24]&/@{"Subscript[\[Theta],\
 min]","Subscript[\[Theta], max]","Subscript[\[Theta], \
max,1]","Subscript[\[Theta], max,2]"},Right],*)AspectRatio -> 1, 
 Epilog -> {Directive[lineStyle1], line1, Directive[lineStyle2], 
   line2}]

enter image description here

Could you please tell me how to get just two points (or one point, or nothing) as a solution?

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You should generally try to use Mathematica's built-in functions as much as possible. For example, if you let

rectangle = Rectangle[{1, 1}, {2, 2}];
circle = Circle[{0, 0}, 2];

Then

intersection = RegionBoundary@RegionIntersection[rectangle, circle]
(*Circle[{0, 0}, 2, {\[Pi]/6, \[Pi]/3}]*)

is an arc segment, as expected. The intersection points are at its endpoints, which you can extract with (didn't find a built-in function for this one):

ArcEndpoints[Circle[{x_, y_}, r_, {a1_, a2_}]] := Point/@{{x + r Cos@a1, y + r Sin@a1}, {x + r Cos@a2, y + r Sin@a2}}

To visualize the result, use for example

Show@Graphics@{rectangle, circle, Red, ArcEndpoints@intersection}

to get

rectanglecircleintersection

Note: I'm probably overlooking multiple edge-cases (one-point intersection, no-intersection, etc), but you get the idea...

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