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MathGroup Archive 2007

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Re: locating max value on the image

  • To: mathgroup at smc.vnet.net
  • Subject: [mg75341] Re: locating max value on the image
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Wed, 25 Apr 2007 05:45:11 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <f0kbkn$que$1@smc.vnet.net>

essedra wrote:
> How can I obtain where the maximum point is located on the image?
> 
> Appreciate any comments.
> 

Out of context, your question is very imprecise.

What is the format of the image you are using? Bitmap images are not 
encoded the same way as vector graphics. Even bitmaps have different 
formats/structures (think BMP vs. JPEG).

What do you mean by /the/ maximum point?

Let's take a simple case where the image is a 2D plot created by 
Mathematica. On the given interval, the function sin has four maximums. 
Depending on numerical imprecision, the can get one, two, tree, four, or 
more "maximum" points and even in the case of just one, we do not know 
witch one is going to be returned (that is not necessarily the first one).

In[1]:=
g = Plot[Sin[x], {x, 0, 8*Pi}];

myMax[g_, eps_] := Module[{pts, max},
   pts = Cases[g[[1]], {(x_)?NumericQ, (y_)?NumericQ}, Infinity];
   max = Max[pts[[All,2]]];
   Select[pts, max - eps < #1[[2]] < max + eps & ]]

In[4]:=
Length /@ Table[myMax[g, 10^(-n)], {n, 0, 10}]

Out[4]=
{112, 79, 41, 14, 4, 2, 1, 1, 1, 1, 1}

In[5]:=
myMax[g, 10^(-4)]

Out[5]=
{{1.56881, 0.999998}, {7.84112, 0.999917},
  {14.1231, 0.999901}, {20.4209, 1.}}

Regards,
Jean-Marc


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