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Can anyone tell me why the following keeps burning my CPU without ever producing the desired plot?

f[x_, y_] := If[Sqrt[x^2 + y^2] < 1, 1, 0];  
test[t_, z_] := N[Convolve[f[x, y], f[x, y], {x, y}, {t, z}]];  
Plot3D[test[t, z], {t, -5, 5}, {z, -5, 5}];

Thanks!

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    $\begingroup$ Two reasons: 1. you re-compute the convolution for every plotted point; 2. Mathematica actually cannot do this convolution analytically anyway. (Not even if you rewrite it as f[x_, y_] := HeavisidePi[Sqrt[x^2 + y^2]/2].) $\endgroup$
    – Oleksandr R.
    Jan 28, 2016 at 14:31
  • $\begingroup$ You may try to convolve in polar coordinates. $\endgroup$
    – Dr. belisarius
    Jan 28, 2016 at 17:55

1 Answer 1

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If you need result at some sensible time frame vs quality...

f[x_, y_] := Piecewise[{{1, Sqrt[x^2 + y^2] < 1}}]

nconv[t_, z_] := NIntegrate[f[x, y] f[x - t, y - z], 
{x, -Infinity, Infinity}, {y, -Infinity, Infinity}]

data = ParallelTable[nconv[t, z], {t, -3, 3, .2}, {z, -3, 3, .2}]; // AbsoluteTiming

{57.205376`, Null}

ListPlot3D[data, PlotRange -> All]

enter image description here

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