Re: Re: Concentric contours about the centroid, having the same length, and interior to an initial contour.

• To: mathgroup at smc.vnet.net
• Subject: [mg44032] Re: [mg43973] Re: Concentric contours about the centroid, having the same length, and interior to an initial contour.
• From: Garry Helzer <gah at math.umd.edu>
• Date: Sat, 18 Oct 2003 03:12:30 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```Gilmar,

Are you sure that the contours should be "equidistant"? The shape of
the contour will change as you shrink If further points move the same
distance as inner points. Perhaps you want the shape to remain the same
as the contour shrinks. In this case you can use a similarity
transformation. If you shift to homogeneous coordinates you can do it
using matrix multiplication.

If the centroid is at (a,b) and you want to rescale the contour toward
the centroid  using the ratio r the homogeneous scale matrix is

M={{1,0,0},{(1-r)a,r,0},{(1-r)b,0,r}}

Change the contour point to homogeneous coordinates

homcontour=Flatten[{1,#}]&/@contour

Multiply by the matrix

scaledhomcontour=M.#&/@homcontour

Change back to affine coordinates

scaledcontour=(Rest[#]/First[#])&/@scaledhomcontour

On Thursday, October 16, 2003, at 04:16  AM, Gilmar Rodr?guez
Pierluissi wrote:

> gilmar.rodriguez at nwfwmd.state.fl.us (Gilmar Rodr?guez Pierluissi)
> wrote in message news:<bmj2mv\$prl\$1 at smc.vnet.net>...
>> I have a basic contour defined by:
>>
>> In[1]:
>>
>> contour = {{10545, 28012}, {10232, 28166}, {10027, 28218},
>> {9714, 28115}, {9354, 28166}, {9042, 28115}, {8785, 28064},
>> {8575, 27858}, {8160, 27858}, {7954, 27546}, {8108, 27238},
>> {8421, 27238}, {8836, 27079}, {9042, 26874}, {9247, 26510},
>> {9406, 26197}, {9611, 25992}, {9662, 25627}, {9560, 25268},
>> {9196, 25007}, {8939, 24750}, {8678, 24750}, {8421, 24386},
>> {8216, 24125}, {8160, 23919}, {8006, 23607}, {8006, 23401},
>> {7852, 23037}, {7749, 22832}, {7539, 22678}, {7179, 22678},
>> {6918, 22570}, {6713, 22468}, {6451, 22365}, {6036, 22365},
>> {5728, 22468}, {5677, 22104}, {5625, 21693}, {5625, 21483},
>> {5364, 21483}, {5159, 21950}, {5000, 22211}, {4846, 22519},
>> {4538, 22570}, {4641, 22160}, {4949, 21950}, {4949, 21642},
>> {5159, 21278}, {5313, 21068}, {5569, 20862}, {5569, 20708},
>> {5518, 20447}, {5415, 20190}, {5415, 19929}, {5313, 19565},
>> {5000, 19308}, {4743, 19308}, {4589, 19411}, {4431, 19565},
>> {4225, 19672}, {4020, 19621}, {3758, 19621}, {3758, 19359},
>> {4020, 19154}, {4328, 18944}, {4482, 18585}, {4897, 18739},
>> {5210, 18841}, {5364, 18426}, {5467, 18118}, {5625, 17754},
>> {5831, 17339}, {5625, 17026}, {5364, 16821}, {5056, 16667},
>> {4897, 16200}, {5000, 15887}, {5159, 15579}, {5107, 15318},
>> {4897, 15005}, {4538, 15005}, {4174, 15215}, {4020, 15420},
>> {4071, 15112}, {4277, 14851}, {4589, 14697}, {4897, 14594},
>> {4897, 14179}, {4897, 13866}, {4743, 13507}, {4795, 13143},
>> {4795, 12830}, {4692, 12363}, {4589, 12107}, {4589, 11794},
>> {4328, 11486}, {3964, 11327}, {3861, 10968}, {4020, 10604},
>> {3861, 10086}, {3810, 9670}, {3707, 9255}, {3446, 8737},
>> {3292, 8219}, {3343, 7804}, {3502, 7337}, {3189, 6977},
>> {2928, 6926}, {2620, 6870}, {2466, 6613}, {2256, 6147},
>> {2050, 6352}, {1948, 6613}, {1686, 6767}, {1481, 6716},
>> {1117, 6716}, {804, 6926}, {650, 6819}, {547, 6562},
>> {445, 6352}, {393, 5993}, {235, 5941}, {81, 5629},
>> {286, 5475}, {701, 5423}, {911, 5372}, {963, 5111},
>> {860, 4905}, {650, 4695}, {650, 4280}, {1014, 4126},
>> {1271, 3869}, {1686, 3659}, {2050, 3505}, {2466, 3402},
>> {2774, 3351}, {3086, 3454}, {3446, 3659}, {3861, 3710},
>> {4174, 3659}, {4589, 3710}, {5056, 3608}, {5313, 3244},
>> {5467, 2726}, {5677, 2469}, {5933, 2418}, {6195, 2208},
>> {6451, 1900}, {6503, 1484}, {6713, 1171}, {6815, 812},
>> {7179, 653}, {7488, 551}, {7800, 345}, {8057, 551},
>> {8267, 397}, {8575, 187}, {8836, 33}, {8836, 238},
>> {8626, 499}, {8524, 705}, {8472, 966}, {8626, 1171},
>> {8267, 1274}, {8216, 1069}, {8057, 966}, {7800, 1069},
>> {7539, 1171}, {7385, 1330}, {7282, 1690}, {7590, 1741},
>> {7642, 1900}, {7642, 2208}, {7800, 2469}, {7590, 2674},
>> {7282, 2572}, {7021, 2418}, {6815, 2418}, {6815, 2833},
>> {6918, 3038}, {7123, 3090}, {6918, 3244}, {6713, 3454},
>> {6605, 3556}, {6451, 3869}, {6713, 3972}, {7021, 4126},
>> {7385, 4228}, {6918, 4331}, {6713, 4438}, {6451, 4592},
>> {6195, 4228}, {5882, 4387}, {5728, 4695}, {5728, 5008},
>> {5625, 5475}, {5728, 5890}, {6143, 5890}, {6661, 5890},
>> {7072, 5941}, {6815, 6198}, {6400, 6198}, {6087, 6147},
>> {6246, 6511}, {6451, 6819}, {6713, 7080}, {6969, 7495},
>> {7231, 7495}, {7800, 7598}, {7436, 7752}, {7179, 7855},
>> {7072, 8168}, {6764, 8480}, {6713, 9050}, {6918, 9465},
>> {7123, 9824}, {7282, 10240}, {7590, 10968}, {8057, 11379},
>> {8318, 11794}, {8785, 12312}, {8990, 12727}, {9406, 13091},
>> {9457, 13661}, {9714, 14333}, {10027, 14748}, {10339, 15420},
>> {10909, 15579}, {11632, 15733}, {11945, 15630}, {12355, 15318},
>> {12981, 15215}, {13443, 14800}, {13653, 14333}, {13756, 13918},
>> {13910, 13451}, {14222, 13091}, {14586, 12830}, {14689, 12415},
>> {14638, 11948}, {14535, 11430}, {14274, 11071}, {13961, 10707},
>> {13807, 10655}, {13602, 10394}, {13961, 10394}, {14274, 10553},
>> {14428, 10342}, {14376, 9983}, {14535, 9773}, {14843, 9824},
>> {14946, 9568}, {15104, 9358}, {15258, 9101}, {15361, 8788},
>> {15566, 8840}, {15982, 8947}, {16243, 8891}, {16603, 8788},
>> {16864, 8532}, {17279, 8373}, {17587, 8270}, {17849, 7911},
>> {18208, 7649}, {18315, 7080}, {18469, 6767}, {18782, 6665},
>> {18936, 6408}, {19193, 6044}, {19557, 5834}, {19972, 6044},
>> {20178, 6352}, {20644, 6511}, {20747, 6870}, {20798, 7234},
>> {20542, 7547}, {20388, 8014}, {20229, 8322}, {20075, 8634},
>> {19921, 9152}, {20229, 9465}, {20490, 9722}, {20747, 9876},
>> {20490, 9876}, {20178, 9824}, {19972, 9619}, {19660, 9619},
>> {19608, 9876}, {19300, 10086}, {18885, 10137}, {18675, 10342},
>> {18623, 10604}, {18782, 10809}, {18936, 11019}, {18572, 11173},
>> {18469, 10912}, {18315, 10655}, {18208, 10291}, {17900, 10394},
>> {17587, 10394}, {17177, 10342}, {16761, 10394}, {16551, 10450},
>> {16295, 10604}, {15879, 10707}, {15566, 11019}, {15622, 11379},
>> {15828, 11794}, {16033, 12107}, {15725, 12522}, {15622, 12779},
>> {15515, 13143}, {15776, 13451}, {15776, 13764}, {15464, 14025},
>> {15258, 14333}, {15048, 14543}, {14894, 14748}, {14843, 14902},
>> {14689, 15164}, {14689, 15420}, {14586, 15836}, {14376, 16200},
>> {14222, 16615}, {14120, 16872}, {13910, 16923}, {13756, 17026},
>> {13499, 17236}, {13186, 17236}, {12981, 17390}, {12668, 17493},
>> {12304, 17651}, {12047, 17703}, {11786, 17600}, {11427, 17544},
>> {11011, 17600}, {10493, 17600}, {9924, 17651}, {9611, 17908},
>> {9457, 18169}, {9714, 18585}, {10181, 18944}, {10391, 19462},
>> {10339, 19878}, {10232, 20293}, {10181, 20657}, {10699, 20862},
>> {11063, 21278}, {11375, 21847}, {11735, 22160}, {12201, 22468},
>> {12514, 22314}, {12719, 21898}, {13032, 21586}, {13084, 22052},
>> {13238, 22314}, {13238, 22519}, {12873, 22570}, {12719, 22883},
>> {12719, 23350}, {13032, 23765}, {12925, 24125}, {12719, 24437},
>> {12463, 24801}, {12355, 25058}, {12047, 25525}, {11837, 25940},
>> {11632, 26253}, {11319, 26253}, {11011, 26253}, {10750, 26458},
>> {10596, 26822}, {10596, 27182}, {10596, 27494}, {10545, 28012}};
>>
>>
>> The length of the contour is given by:
>>
>> In[2]:
>>
>> Length[contour]
>>
>> Out[2]:
>>
>> 375
>>
>> A plot of this contour is given by:
>>
>> In[3]:
>>
>> plt1 = ListPlot[contour, PlotStyle -> {Hue[0], PointSize[.01]}]
>>
>> Out[3]: graphics
>>
>> The centroid of the contour is given by:
>>
>> In[4]:
>>
>> centroid = N[Plus @@ contour/Length[contour]]
>>
>> Out[4]: {9426.41, 12877.6}
>>
>> My question is:
>>
>> How can I generate (say) five contours, so that:
>>
>> (1.) Each contour (set) have the same length as the original contour
>>
>>  (in this case, each contour should contain  375 {x,y} points).
>>
>> (2.) Each contour is equidistant, and concentric (with respect to
>>
>>  the centroid) to the previous contour.
>>
>> (3.) The five contours are inside the original contour defined above?
>>
>> Thank you for your help!
>
> ***********************************************************************
> It is important to add the following:
>
> We are shrinking the initial contour toward its centroid, ao that
> the five contours are "snapshots" of the initial contour as it
> shrinks.
>
> Thank you!
>
>
Garry Helzer
Department of  Mathematics
University of Maryland
1303 Math Bldg
College Park, MD 20742-4015

```

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