Gradient/ColorFunction in Disk with transparent background

Question

My goal is to make an iOS-like enable/disable switch like this:

I use a LocatorPane for the slider with so-called "jumps" to only make two settings possible:

LocatorPane[Dynamic[pt], Graphics[{LightGray, Rectangle[{-.5, -.5}, {1.5, .5}]}],
            {{0, 0}, {1, 0}, {1, 1}},
            Appearance -> ImageResize[Graphics[{Gray, Disk[]},
                                               Background -> None], Scaled[.25]]]

Now, I want make the 'slider' better looking by adding a gradient to it but i cannot make it work. ColorFunction (probably the only way to add gradients in Mathematica) seems not available to Disk. I tried using Heike's solution for gradients in this question, but pasting the graphics from her function into my code does not work: the size of these graphics is way too large and rescaling it (using ImageResize) adds a white background which I cannot remove. I tried making a disk with a gradient myself using Raster and ImageAdd, but here the same problem of a none-removable background occurs again:

ImageAdd[
Graphics[Raster[Table[i, {i, 100}, {j, 200}],
         {Scaled[{0, 0}], Scaled[{1, 1}]}, {1, 100}, ColorFunction -> "GrayTones"]],
Graphics[Disk[]]]

Can anyone give some tips about how to add gradients to disks?

Answer 1

Seems a bit clumsy, but how about this:

shiny[z_] := GrayLevel[1 - z/4];

gradient = 
  Image[DensityPlot[y^2, {x, -2, 2}, {y, -2, 2}, 
    PlotRangePadding -> 0, Frame -> None, ColorFunction -> shiny, 
    ColorFunctionScaling -> True]];
diskAlpha = 
  Image[Graphics[Disk[], Background -> None], 
   ImageSize -> ImageDimensions[gradient]];
d = SetAlphaChannel[gradient, AlphaChannel[diskAlpha]];

LocatorPane[Dynamic[pt],
 Graphics[{
   Blue, 
   Rectangle[{-.25, -.25}, {1.25, .25}, RoundingRadius -> 0.25],
   White,
   Style[Text["ON", {0.25, 0}], 64, Bold, 
    FontFamily -> "Helvetica"]}], 
 {{0, 0}, {1, 0}, {1, 1}},
 Appearance -> ImageResize[d, Scaled[.3]]]

Answer 2

Well, this is the switcher I made using the answer from cormullion. Just one thing: it sort of works, but not quite yet... After defining the function SwitcheriOS twice in one session I get a Set::setraw which I cannot repair. I am unable to figure out what is wrong...

But apart from this 'minor' issue it works like a charm!

The code:

SwitcheriOS[var_] := Module[{gradient, coord, img, rastimg},
shiny[z_] := GrayLevel[1 - z/4]; 
gradient = 
 Image[DensityPlot[y^2, {x, -.75, .75}, {y, -.25, 25}, 
   PlotRangePadding -> 0, Frame -> None, ColorFunction -> shiny, 
   ColorFunctionScaling -> True]];
coord = 
 Block[{n = 100}, 
  Table[{Cos[2 \[Pi] k/n], Sin[2 \[Pi] k/n]}, {k, 0, n - 1}]];
img = Graphics[{Texture@gradient, EdgeForm@None, 
 Polygon[coord, VertexTextureCoordinates -> (coord/2 + .5)], 
  LightGray, Thickness[.05], Circle[]}, ImageSize -> 28, 
  Background -> None];
rastimg = Rasterize[img, RasterSize -> 100, Background -> None];
LocatorPane[Dynamic[var], 
 Graphics[{Dynamic[
    Which[var[[1]] == -.75, Lighter[Gray, .8], var[[1]] == .75, 
     Lighter[Blue, .3]]], 
   Rectangle[{-1.25, -.5}, {1.25, .5}, RoundingRadius -> .5], 
   Dynamic[Which[var[[1]] == -.75, Opacity[.6, White], 
     var[[1]] == .75, Opacity[.2, White]]], 
   Rectangle[{-1, -.5}, {1, 0}, RoundingRadius -> .25], 
   Opacity[.9, White], 
   Dynamic[Text[
     Style[Which[var[[1]] == -.75, "OFF", var[[1]] == .75, "ON"], 
      Which[var[[1]] == -.75, Gray, var[[1]] == .75, White], 13, 
      FontFamily -> "DejaVu Sans", Bold], 
     Which[var[[1]] == -.75, {.5, 0}, var[[1]] == .75, {-.5, 0}]]], 
   EdgeForm[{Lighter[Gray, .3], Thickness[.025]}], Transparent, 
   Rectangle[{-1.25, -.5}, {1.25, .5}, RoundingRadius -> .5]}, 
  ImageSize -> 75], {{-.75, -0}, {.75, -0}, {1.5, 1}}, 
 Appearance -> rastimg]]