select a band of a matrix

Hello I want to select a band of a matrix with slop (S) and save them in another matrix.

a =

    [ 1    2     3      4     5     6     7    8     9;
     11    12    13    14    15    16    17    18    19;
     21    22    23    24    25    26    27    28    29;
     31    32    33    34    35    36    37    38    39;
     41    42    43    44    45    46    47    48    49;
     51    52    53    54    55    56    57    58    59]

Foe example I have a slope S1, and I want to select a band of this matrix in this way: Assume we can draw a line from array "6" to "29" and another line from "21" to "55". and remove the arrays above line 1 and below line 2. My problem is something like this but a little different. I have 2 slopes that I want to cut my matrix according with. Can you please help me to write a code for this problem?

Answer 1

[Andrei Bobrov]

[ii,jj]= ndgrid(1:size(a,1),1:size(a,2));
[I J] = find(ismember(a,[21;55;6;29]));
out = a.*(ii - I(1) > (diff(I(1:2)))/(diff(J(1:2)))*jj | ii - I(end) + 1< (diff(I(3:end)))/(diff(J(3:end)))*(jj - J(end)+1));

[Andrea]

Thank you so much.

Answer 2

[Image Analyst]

I don't know what you're trying to explain. I can get the elements fo the array going from 6 to 29 and from 21 to 55 this way:

a =...
    [ 1    2     3      4     5     6     7    8     9;
     11    12    13    14    15    16    17    18    19;
     21    22    23    24    25    26    27    28    29;
     31    32    33    34    35    36    37    38    39;
     41    42    43    44    45    46    47    48    49;
     51    52    53    54    55    56    57    58    59]

   aLinear = a'
   aLinear = aLinear(:)
   a_6_29 = aLinear(6:27)
   a_21_55 = aLinear(21:50)
   % Now what?  I have no idea.

But I don't know what to do after that. Do you want to get a slope of a line fitted through those vectors? What does this mean: "remove the arrays above line 1 and below line 2."? To me that means you want line (row) 2 and nothing else, like this:

% Remove the arrays above line 1 and below line 2.
% That would leave only line 2 itself remaining.
a_new = a(2, :);

[Andrea]

please see these images. That can help me to explain the problem better.

image 1: http://imageshack.us/photo/my-images/268/64770753.jpg/

image 2: http://imageshack.us/photo/my-images/405/52265040.jpg/

The fact is (as I explained weekly before) , I have two image matrix with both x and y of them. These two images has been taken with two different instruments (Satellite radar and ground radar) and have different spatial resolutions. I want to do somethings that really needs to have two image with exactly same geometric area. I don't want to define some specific points and change the image 1 according to image 2 because I have lots of other image from these two instruments. So I cannot define the intersection area (overlapping area) of each two images separately which may takes a lot of time. I want to write a program in Matlab which cut image 1 in the way it only has the data the image 2 already has. You can see that image 1 data has been defined in a band ( the blue band ) while image 1 has a circle form of data. I want to cut image 2 to have only this band area as image 1. I hope I explained the problem a little clearly than before.

[Andrea]

we can draw a line from array "6" to "29" and another line from "21" to "55". I want to remove arrays at the triangle made by the line 1 (from "6" to "29" a) and the the other triangle made at the left bottom corner of the matrix (from "6" to "29" ).