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Re: Cursor position on images

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  • Subject: [mg129364] Re: Cursor position on images
  • From: Alexei Boulbitch <Alexei.Boulbitch at>
  • Date: Mon, 7 Jan 2013 23:07:00 -0500 (EST)
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I am sure this is straightforward but I cannot quite see how to do this. I have an image (jpg file, but could be any format if that matters). I want to be able to left click on the image and mark the point with a dot of some sort and collect the coordinates of the point (the coordinate system is fairly arbitrary). I then want to be able to repeat this as many times as required and end up with a list of the coordinates of the points clicked on for further manipulation. Clickpane should do the trick but I can't work out how.

Any suggestions, thanks.


Hi, Peter,

It is not exactly but very close to what you ask about. Below you will find a function along with its description. I use it to digitalize  functions only available as curves on graphics, like taken from images. This might be what you have in mind. To try it just copy-paste any image into Mathematica, give it a name and apply the copyCurve function to it. Then act as it is described below. As the result you will get a list of points coordinates.

Have fun, Alexei


The function copyCurve enables one to get the coordinates of curve points from a plot found on an image and memorises them in a list entitled ?listOfPoints?

image is any image. It should have Head=Image, but may be also a Graphics object, since no specific functions of image have been used. I typically wrap it in Image[] statement.


The Checkbox ?whiteLocatorRing? defines, if the locators are shown by a single color ring (unchecked), or with two rings, the outer having a color defined by the ColorSlider (see below), the innder being white. This may be helpful, if working with a too dark image.

size controls the size of the image. The default value is 450. This slider is used to adjust the size to the one to enable the most comfortable work with the image plot.

opacity controls the opacity of the line connecting the locators

thickness controls the thickness of the double ring that forms each locator.

lineThickness controls the thickness of the line connecting the locators

color is the color slider that controls the color of the outer ring forming the locator and the line connecting them. The inned locator ring is always white.

radius controls the radius of the locators.

InputFields: should be supplied by the reference points x1 and x2 at the axis x, as well as y1 and y2 at the axis y.

Buttons: The buttons ?Memorize scale X? and ?Memorize scale Y? should be pressed after the first two locators are placed on the corresponding reference points (presumably, located at the x or y axes). Upon pressing the reference points are memorized. The button ?Make list of the curve points? should be pressed at the end of the session. Upon its pressing the actual list of points representing the points of the curve is assigned to the global variable ?listOfPoints?

Step 1: Execute the function with the name of the image containing the curve to be digitalized.

Step 2: Enter the reference points at the plot axes into the input fields. Press Enter.

Step3: Alt+Click on the point with x-coordinate x1. This brings up the first locator visible as a circle. Alt+Click on that with x2 which gives rise to the second locator. Adjust the locators, if necessary. Press the button ?Memorize scale X?.

Step 4: Move the two already existing locators to the points with the coordinates y1 and y2. Press the button ?Memorize scale Y?. Now the both scales are captured.

Step 5: Move the two already existing locators to the first two points of the curve to be captured. Alt+Click on other points of the curve. Each Alt+Click will generate an additional locator. Adjust locators, if necessary. To remove,  Alt+Click on unnecessary locators.

Step 6: Press the button ?Make the list...?. This assigns the captured list to the variable ?listOfPoints?. Done.

The ?listOfPoints? is a global variable. It can be addressed everywhere in the notebook.

The function copyCurve


copyCurve[image_] :=

   DynamicModule[{pts = {}, x1 = Null, x2 = Null, y1 = Null,
     y2 = Null, X1, X2, Y1, Y2, \[CapitalDelta]X, \[CapitalDelta]Y, g,

    myRound[x_] := Round[1000.*x]/1000. // N;

    (* Begins the column with all the content of the manipulate *)
      (* Begin LocatorPane*)
         Show[{ReplacePart[image, {4, 2} -> size],
           Graphics[{color, AbsoluteThickness[lineThickness],
             Opacity[opacity], Line[Union[pts]]}]
               }], LocatorAutoCreate -> True,
        (* Begin Locator appearance *)
        Appearance -> If[whiteLocatorRing,

          Graphics[{{color, AbsoluteThickness[thickness],
             Circle[{0, 0}, radius + thickness/2]}, {White,
             AbsoluteThickness[thickness], Circle[{0, 0}, radius]}},
           ImageSize -> 10]
          Graphics[{{color, AbsoluteThickness[thickness],
             Circle[{0, 0}, radius + thickness/2]}},
           ImageSize -> 10]](* End Locator appearance *)
                              ],(* End LocatorPane*)

      (* Begin of the block of InputFields *)
      , Row[{ Style["\!\(\*SubscriptBox[\(x\), \(1\)]\):"],
         FieldHint -> "Type  \!\(\*SubscriptBox[\(x\), \(1\)]\)",
         FieldSize -> 7, FieldHintStyle -> {Red}],
        Spacer[20], Style["   \!\(\*SubscriptBox[\(y\), \(1\)]\):"],
         FieldHint -> "Type  \!\(\*SubscriptBox[\(y\), \(1\)]\)",
         FieldSize -> 7, FieldHintStyle -> {Red}]
      Row[{ Style["\!\(\*SubscriptBox[\(x\), \(2\)]\):"],
         FieldHint -> "Type \!\(\*SubscriptBox[\(x\), \(2\)]\)",
         FieldSize -> 7, FieldHintStyle -> {Red}],
        Spacer[20], Style["   \!\(\*SubscriptBox[\(y\), \(2\)]\):"],
         FieldHint ->
          "Type  \!\(\*SubscriptBox[\(y\), \(2\)]\)+Enter",
         FieldSize -> 7, FieldHintStyle -> {Red}]
      (* End of the block of InputFields *)
      (* Begin the buttons row *)
        (* Begin button "Memorize scale X" *)
        Button["Memorize scale X",
         X1 = Min[Transpose[myRound /@ Union[pts]][[1]]];
         X2 = Max[Transpose[myRound /@ Union[pts]][[1]]];
         \[CapitalDelta]X = X2 - X1;
         ],(* End of button "Memorize scale X" *)
        (* Begin button "Memorize scale Y" *)
        Button["Memorize scale Y",
         Y1 = Min[Transpose[myRound /@ Union[pts]][[2]]];
         Y2 = Max[Transpose[myRound /@ Union[pts]][[2]]];
         \[CapitalDelta]Y = Y2 - Y1;
         ](* End of button "Memorize scale Y" *)

        }],(* End the buttons row *)

      (* Begin button "Make the list of the curve's points" *)
      Button[Style["Make the list of the curve's points" , Bold],
       g[{a_, b_}] := {(x1*X2 - x2*X1)/\[CapitalDelta]X +
          a/\[CapitalDelta]X*Abs[x2 - x1], (
          y1*Y2 - y2*Y1)/\[CapitalDelta]Y +
          b/\[CapitalDelta]Y*Abs[y2 - y1]};
       listOfPoints = Map[myRound, Map[g, pts]]
                   ](* End of button "Make the list..." *)

      }, Alignment -> Center](*
    End of column with all the content of the manipulate *)
    ],(* End of the DynamicModule *)

   (* The massive of sliders  begins *)
   Column[{Row[{Control[{whiteLocatorRing, {True, False}}],
     Row[{Spacer[32.35], Control[{{size, 450}, 300, 800}],
       Spacer[38.5`], Control[{{opacity, 0.5}, 0, 1}]}],
     Row[{Spacer[10.], Control[{{thickness, 1}, 0.5, 5}],
       Spacer[13.65], Control[{{lineThickness, 1}, 0, 10}] }],
     Row[{Spacer[22.8], Control[{color, Red}], Spacer[59.3],
       Control[{{radius, 0.5}, 0, 3}]}]
     }, Alignment -> Center],(* The massive of sliders ends *)

   (* Definitions of sliders *)
   ControlType -> {Checkbox, Slider, Slider, Slider, Slider,
     ColorSlider, Slider},
   ControlPlacement -> Top, SaveDefinitions -> True
(* End of the function *)

Alexei BOULBITCH, Dr., habil.
ZAE Weiergewan,
11, rue Edmond Reuter,
L-5326 Contern, LUXEMBOURG

Office phone :  +352-2454-2566
Office fax:       +352-2454-3566
mobile phone:  +49 151 52 40 66 44

e-mail: alexei.boulbitch at

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