Locators in Manipulate not moving properly

Question

I thought a simple Mathematica kerning machine (for adjusting the space between characters) would be interesting, but I'm having trouble with the locators. (There are a number of other questions related to this, and I've read the answers, but as yet without finding a solution, or understanding them that well.)

 Manipulate[
  LocatorPane[Dynamic[points],
   text = "Wolfram";
   fonts = FE`Evaluate[FEPrivate`GetPopupList["MenuListFonts"]];
   Column[{
     Button["Export", Export["/tmp/t.png", g]],
     g = Graphics[{
          MapIndexed[
          Text[Style[#1, FontSize -> fontsize, FontFamily -> font], 
            points[[First[#2]]]] &, Characters[text]]}, ImageSize -> 500]
     }]],
  {points, {Table[{ x, 0}, {x, 1, Length[Characters[text]]}]}, 
    ControlType -> None},
  {fontsize , Table[x, {x, 96, 256, 12}]} ,
  {font, fonts}
 ]

(The font menu gets populated once you use it.)

I want to be able to slide the letters right or left (but not up or down), but at the moment they don't move like they're supposed to.

The solution

Thanks to the fine answers, I've got something useful working. There are some kludges and hacks too, and some problems still to be ironed out (string length needs to be dynamic, 'canvas' needs resizing, and so on), but this is excellent for now.

With[{fonts = FE`Evaluate[FEPrivate`GetPopupList["MenuListFonts"]]},
 DynamicModule[{kernedText, points},
  points = Table[{x, 0}, {x, 1, Length[Characters[text]] + 10}];
  Column[{
    (* input *)
    InputField[Dynamic[text], String],
    (* main panel *)
    Panel@LocatorPane[
      Dynamic[
       points, (points = ReplacePart[#, {{_, 2} -> 0}]) &], 
      Dynamic[kernedText = 
        Graphics[{MapIndexed[
           Text[Style[#1, FontSize -> fontsize, FontFamily -> font], 
             points[[First[#2]]]] &, Characters[text]]}, 
         PlotRangePadding -> 0, 
         PlotRange -> {{0, 1 + Length[Characters[text]]}, Automatic}, 
         Background -> None, ImageSize -> 800]], Appearance -> None],
    (* controls *)
    Row[
     {
      PopupMenu[Dynamic[fontsize], Table[x, {x, 72, 416, 12}]],
      PopupMenu[Dynamic[font], fonts],
      Button["Export", 
       Export[FileNameJoin[{$HomeDirectory, "Desktop", 
          "KernedText.png"}], kernedText], 
       ImageSize -> {Full, Automatic}]
      }]
    }, Left]]]

I realised that the locator dots didn't need to be showing - just click on the characters. The result can be compared to the unkerned version:

Text[Style[text, 192, FontFamily -> "Palatino"]]

For me, this area of Mathematica (Manipulate/Dynamic) is gradually becoming less confusing (but only gradually).

Answer 1

This is just an addendum to jVincent's answer. In order to constrain the letters/locators to a horizontal line, you need to use the second argument of Dynamic. The following modified version of jVincent's answer adds the needed constraint:

With[{text = "Wolfram", fontsize = 96, font = "Georgia"},
   DynamicModule[{chars, kernedText, points},
      chars = Length[Characters[text]];
      points = Table[{x, 0}, {x, 1, chars}]; 
      Column[{Panel@
         LocatorPane[
            Dynamic[points, (points = ReplacePart[#, {{_, 2} -> 0}]) &], 
            Dynamic[kernedText = 
               Graphics[
                  {
                  MapIndexed[
                     Text[Style[#1, FontSize -> fontsize,
                        FontFamily -> font], points[[First[#2]]]]&,
                     Characters[text]]
                  }, 
                  PlotRange -> {{0, 1 + chars}, Automatic},
                  ImageSize -> 450]]],
         Button["Export", 
            Export[FileNameJoin[
               {
               $HomeDirectory, "Desktop", "KernedText.png"
               }],
            kernedText],
            ImageSize -> {Full, Automatic}]},
         Right]]]

Edit

I've cleaned up my code and incorporated kguler's fix. The resulting kerned letters look pretty and export nicely.

Answer 2

First off, Dynamic[exp]] redraws whenever anything that appears in expression changes. Think of Manipulate as being just a Dynamic[code] with some nice shortcuts to build controllers that can change things that appear in code. In your case, you have a Dynamic[LocatorPane[(* some expression depending on points*)]]. So whenever someone changes points a new locatorpane will be created and displayed right where the old one was, so you won't notice... except that the active controller loses focus, so you stop dragging the controller. Here is a very short demonstration of the problem:

points = {{0, 0}};
Dynamic@LocatorPane[Dynamic[points], Graphics[Point /@ points, PlotRange -> 2]]

While just moving the Dynamic will cause only the graphic to be updated and solve the issue:

LocatorPane[Dynamic[points], Dynamic@Graphics[Point /@ points, PlotRange -> 2]]

So how to get around this problem In your case. I would suggest not using LocatorPane inside manipulate, and either fully using only manipulate, or going completely without it. Here is a possible solution without using Manipulate:

text = "Wolfram";
fontsize = 96;
font = "BankGothic";

 DynamicModule[{points = Table[{x, 0}, {x, 1, Length[Characters[text]]}]},
  LocatorPane[Dynamic[points],
   Column[{
     Button["Export", Export["/tmp/t.png", g]], 
     Dynamic[g = Graphics[{MapIndexed[
     Text[Style[#1, FontSize -> fontsize], points[[First[#2]]]] &,
      Characters[text]]}, ImageSize -> 500]]}]]
  ]