Skip to main content

Apply ColorFunction to an Image




is there a faster way to change the colors of an image from grayscale to something like this:


Manipulate[coltest2 = (Blend[{{a, Black}, {b, Lighter[Blue, 0.3]}, {c,Lighter[Cyan,0.3]}, {d, White}}, #] &);
Plot[0.2, {x, 0, 1}, ColorFunction -> coltest2, PlotStyle -> Directive[Thickness[1]], PlotRange -> {{0, 1}, {0, 0.5}}, Frame -> True, FrameTicks -> {True, False, None, None}, AspectRatio -> 1/8],
{{a, 0.35}, 0, b, Appearance -> "Labeled"},
{{b, 0.58}, 0, c, Appearance -> "Labeled"},
{{c, 0.7}, 0, d, Appearance -> "Labeled"},
{{d, 0.95}, 0, 1, Appearance -> "Labeled"}]

than using:



Colorize[image,ColorFunction->coltest2]

I would like to have the image in the manipulate rather than the sample of the ColorFunction, but Colorize is way to slow for that...



Answer



What you can do is, you mimic the behaviour of Blend by creating a function that interpolates linearly between colours. What you change with your parameters are the values where the color transitions take place.


Let me give you a simplified example: I use 3 colours. In the compiled function, I only work with their {r,g,b} values. As result, I want a compiled function which does the following:



  • it takes a parameter a between 0 and 1 and a pixel value between 0 and 1

  • with 3 colours c1, c2 and c3 it will colorise the pixel: from a pixel value of 0 to a it will be colorised with the transition c1 to c2. If the pixel value is greater than a it will be colorised by blending c2 and c3.

  • the compiled function should be able to work in parallel on all pixels of an image



Here is a sample implementation of a function that creates such a colorising compiled function for us:


createColorFunc[colors : {_, _, _}] :=
 Function[{c1, c2, c3},
   Compile[{{a, _Real, 0}, {value, _Real, 0}},
    If[value < a,
     c1 + ((-c1 + c2)*value)/a,
     (c3*(a - value) + c2*(-1 + value))/(-1 + a)
     ], Parallelization -> True, RuntimeAttributes -> {Listable}
    ]

   ] @@ List @@@ (ColorConvert[#, "RGB"] & /@ colors)

To test is, we load the Lena image in grayscale an build a small Manipulate:


With[{lena = ColorConvert[ExampleData[{"TestImage", "Lena"}], "Grayscale"]},
 Manipulate[
  func = createColorFunc[{c1, c2, c3}];
  Image[func[a, ImageData[lena, "Real"]]],
  {{a, .5}, 0, 1},
  {c1, Black},
  {c2, Gray},

  {c3, White}
  ]
 ]

Mathematica graphics


You task is now to extend this for more than 3 colours and one color transition position.


Comments

Post a Comment

Popular posts from this blog

plotting - Plot 4D data with color as 4th dimension

I have a list of 4D data (x position, y position, amplitude, wavelength). I want to plot x, y, and amplitude on a 3D plot and have the color of the points correspond to the wavelength. I have seen many examples using functions to define color but my wavelength cannot be expressed by an analytic function. Is there a simple way to do this? Answer Here a another possible way to visualize 4D data: data = Flatten[Table[{x, y, x^2 + y^2, Sin[x - y]}, {x, -Pi, Pi,Pi/10}, {y,-Pi,Pi, Pi/10}], 1]; You can use the function Point along with VertexColors . Now the points are places using the first three elements and the color is determined by the fourth. In this case I used Hue, but you can use whatever you prefer. Graphics3D[ Point[data[[All, 1 ;; 3]], VertexColors -> Hue /@ data[[All, 4]]], Axes -> True, BoxRatios -> {1, 1, 1/GoldenRatio}]
Post a Comment
Read more

plotting - Filling between two spheres in SphericalPlot3D

Manipulate[ SphericalPlot3D[{1, 2 - n}, {θ, 0, Pi}, {ϕ, 0, 1.5 Pi}, Mesh -> None, PlotPoints -> 15, PlotRange -> {-2.2, 2.2}], {n, 0, 1}] I cant' seem to be able to make a filling between two spheres. I've already tried the obvious Filling -> {1 -> {2}} but Mathematica doesn't seem to like that option. Is there any easy way around this or ... Answer There is no built-in filling in SphericalPlot3D . One option is to use ParametricPlot3D to draw the surfaces between the two shells: Manipulate[ Show[SphericalPlot3D[{1, 2 - n}, {θ, 0, Pi}, {ϕ, 0, 1.5 Pi}, PlotPoints -> 15, PlotRange -> {-2.2, 2.2}], ParametricPlot3D[{ r {Sin[t] Cos[1.5 Pi], Sin[t] Sin[1.5 Pi], Cos[t]}, r {Sin[t] Cos[0 Pi], Sin[t] Sin[0 Pi], Cos[t]}}, {r, 1, 2 - n}, {t, 0, Pi}, PlotStyle -> Yellow, Mesh -> {2, 15}]], {n, 0, 1}]
Post a Comment
Read more

plotting - Mathematica: 3D plot based on combined 2D graphs

I have several sigmoidal fits to 3 different datasets, with mean fit predictions plus the 95% confidence limits (not symmetrical around the mean) and the actual data. I would now like to show these different 2D plots projected in 3D as in but then using proper perspective. In the link here they give some solutions to combine the plots using isometric perspective, but I would like to use proper 3 point perspective. Any thoughts? Also any way to show the mean points per time point for each series plus or minus the standard error on the mean would be cool too, either using points+vertical bars, or using spheres plus tubes. Below are some test data and the fit function I am using. Note that I am working on a logit(proportion) scale and that the final vertical scale is Log10(percentage). (* some test data *) data = Table[Null, {i, 4}]; data[[1]] = {{1, -5.8}, {2, -5.4}, {3, -0.8}, {4, -0.2}, {5, 4.6}, {1, -6.4}, {2, -5.6}, {3, -0.7}, {4, 0.04}, {5, 1.0}, {1, -6.8}, {2, -4.7}, {3, -1....
Post a Comment
Read more