Finding index between two parallel lines

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Hello,

I have bathymetry data (an intuition is attached here) which is stored in a regular 10m-resolution grid with the following limits:

In the x-axis: xmin:10:xmax. This contains 3039 columns
In the y-axis: ymin:10:ymax. This forms 1195 rows
The depth is Z matrix whose size is equal to 1195x 3039;

And I have 2 parallel lines crossing this bathymetry with the following information:

Line 1 is defined as points (x1,y1) and (x2,y2)
Line 2 is defined as points (x3,y3) and (x4,y4)

My task is to find indices between these two lines. Could you please help to solve this?

Many thanks

T

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Star Strider 2019-3-10

‘My task is to find indices between these two lines.’

‘Index’ means different things in different contexts (e.g. matrix, book, etc.). How do you define it here?

wave_buoys 2019-3-10

编辑：wave_buoys 2019-3-10

Hi, Star Strider,

I have updated the intuition as an photo attached. The indices are defined based on the matrix Z (water depth) which is bounded by xmin, xmax, ymin, ymax.

The area to find indices is a parallelogram, so it might be more tricky than a square or rectangular shape. In other words, I can't use idx=find (x>= xmin & x<= xmax & y>=ymin & x<=ymax) to define the indices in the parallelogram region.

Thanks

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Answer 1

darova 2019-3-10

在 MATLAB Online 中打开

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You can use inpolygon

clc, clear
xmin = 50;
xmax = 551;
ymin = -35;
ymax = -800;
y1 = ymax + 150;
y4 = ymin - 150;
k_up = (y1 - ymin)/(xmax-xmin);
k_down = (ymax - y4)/(xmax-xmin);
cla, hold on
for x = xmin:10:xmax
    y_start = ceil(y1 - x*k_up);
    y_end = floor(ymax - x*k_down);
    for y = y_start:-10:y_end
        plot(x,y,'.r')
    end
end
hold off

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wave_buoys 2019-3-10

编辑：wave_buoys 2019-3-10

Hi darova ,

Thanks for the code.

it looks you are on the right track. Using inpolygon works for me! Here is my approach:

polx=[2908,2943,2938,2916]; % define x-coordinates for the polygon

poly=[5757,5760,5759,5756]; % define y-coordinates for the polygon

% XB, YB: two-dimensional matrices defined by meshgrid(xmin:10:xmax,ymin:10:ymax);

[in,out]=inpolygon(XB,YB,polx,poly); % this returns logical values

nums_idx = double(in); % then I converted into numbers

idxall=find(nums_idx); % find non-zero values

XC=XB(idxall); YC=YB(idxall); % this returns to indices inside the polygon!

Thanks for your help with suggesting to use inpolygon again.

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