MATLAB To Mathematica: How to implement imshow()

Question

I want to implement the MATLAB imshow function in Mathematica.

This is a DensityPlot[] from Mathematica

And this is imshow() image generated by MATLAB

As you can see there's a difference between the two plots. I want to obtain smooth transition in Mathematica as I can in MATLAB.

Answer 1

Blend gives smooth transition between a list of colors:

cf[v_] := Blend[{Black, Red, Yellow, White}, v]
DensityPlot[Exp[-x^2 - y^2], {x, -2, 2}, {y, -2, 2}, ColorFunction -> cf, PlotPoints -> 300]

For a clear distinction between some set of contours you can use ContourPlot:

ContourPlot[Exp[-x^2 - y^2], {x, -2, 2}, {y, -2, 2},
 ColorFunction -> cf,
 ContourStyle -> None]

If you start out with a matrix of data like you would for imshow you can use ArrayPlot:

data = Table[Exp[-x^2 - y^2], {x, -2, 2, 0.02}, {y, -2, 2, 0.02}];
ArrayPlot[data, ColorFunction -> cf, Frame -> False]

Answer 2

Alternatively:

newcolor = 
  RGBColor /@ {{0, 0, 0}, {0.2`, 0, 0}, {0.4`, 0, 0}, {0.6`, 0, 0},
  {0.8`, 0, 0}, {1, 0, 0}, {1, 0.2`, 0}, {1, 0.4`, 0},
  {1, 0.6`, 0}, {1, 0.8`, 0}, {1, 1, 0}, {1, 1, 0.2`},
  {1, 1, 0.4`}, {1, 1, 0.6`}, {1, 1, 0.8`}, {1, 1, 1}};

dat = With[{y = Range[-2, 2, .04]}, Table[Exp[-x^2 - y^2], {x, y}]];
ListDensityPlot[dat, ColorFunction -> (Blend[newcolor, #] &)]


dat = With[{y = Range[-2, 2, .01]}, Table[Exp[-x^2 - y^2], {x, y}]];
Colorize[Image[Rescale@dat], ColorFunction -> (Blend[newcolor, #] &)]

---

Note:

dat1 = Table[ Exp[-x^2 - y^2], {x, -2, 2, 0.002}, {y, -2, 2, 0.002}]; // Timing
dat2 = With[{y = Range[-2, 2, .002]},  Table[Exp[-x^2 - y^2], {x, y}]]; // Timing
dat1 == dat2
(*
{0.904806, Null}

{0.202801, Null}

True
*)

Answer 3

Your question is a bit confusing to me because imshow and DensityPlot do different things. imshow will take a matrix of values and show it as an image. The Mathematica equivalent is Image. DensityPlot will take a two-argument function and plot it in 2D.

Here's a direct comparison (using MATLink to pass the data to MATLAB, for convenience):

If you need to use an alternative colour scheme, you can map the corresponding function to the data to generate RGB values, and construct the image afterwards:

Image@Map[List @@ ColorData["Rainbow"][#] &, data, {2}]

Update: Colorize[Image[data], ColorFunction -> "Rainbow"] is better (thanks to @chyanog!)

If the input matrix has a smooth gradation of values, so will the image (provided that the colours scheme is smooth---most of them are).

Answer 4

Here is my way to simulink a Gauss beam:

 H[m_, x_] := (-1)^m*D[Exp[-x^2], {x, m}];

 Table[ArrayPlot[Table[Evaluate[H[i, x] H[j, y]],
                       {x, -3, 3, .005}, {y, -3, 3, .005}]
                ]
      , {i, 1, 3}, {j, 1, 3}] // MatrixForm