Finding X,Y coordinates after image rotation

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Anisha Jamil
Anisha Jamil on 22 Jul 2021
Commented: Simon Chan on 22 Jul 2021
Hello, I am trying to find the values of some selected points with xy coordinates after rotating an image (90,180,270).
The following code lets me select an option for rotation. Therefore, I created a loop.
Could someone please tell me if my equations are correct?
x1,y1 is the coordinate of a point and x2,y2 is the coordinate of the same point after rotation.
[m,n]=size(image);
% Need rotate?
prompt = {'no rotation: 0; counter clockwise 90: 1, counter clockwise 180: 2,counter clockwise 270: 3'};
dlgtitle = 'Need rotate?';
definput = {'0'};
answer2 = str2double(char(inputdlg(prompt,dlgtitle,[1,80],definput)));
if answer2 == 0
moving = moving;
x2=x1;
y2=y1;
elseif answer2 == 1
moving = rot90(moving,1);
x2=y1;
y2=n-x1;
elseif answer2 == 2
moving = rot90(moving,2);
x2=m-x1;
y2=n-y1;
elseif answer2 == 3
moving = rot90(moving,3);
x2=n-y1;
y2=x1;
else
msg = 'invalid input!';
error(msg)
end

Answers (1)

Simon Chan
Simon Chan on 22 Jul 2021
The following code is going to check the coordinates and not replacing your code, so you may need to verify you code based on the result:
clear; clc;
m = 25; n = 20;
BW = zeros(m,n);
x1=9; y1=7;
BW(y1,x1)=1; % Put one white dot in the image
Result{1} = BW;
Result{2} = rot90(BW,1);
Result{3} = rot90(BW,2);
Result{4} = rot90(BW,3);
[y2, x2] = cellfun(@(x) ind2sub(size(x),find(x)),Result);
And the result shown as follows:
x2 =
9 7 12 19
% x1 y1 n-x1+1 m-y1+1
y2 =
7 12 19 9
% y1 n-x1+1 m-y1+1 x1
  2 Comments
Simon Chan
Simon Chan on 22 Jul 2021
I think you are able to verify easily by slightly modifying my previous code:
m = 25; n = 20;
BW = zeros(m,n);
x1=9; y1=7;
BW(y1,x1)=1;
Result{1} = BW;
Result{2} = fliplr(BW);
Result{3} = flipud(BW);
Result{4} = fliplr(flipud(BW));
[y2, x2] = cellfun(@(x) ind2sub(size(x),find(x)),Result)
Result:
y2 =
7 7 19 19
%% y1 y1 m-y1+1 m-y1+1
x2 =
9 12 9 12
%% x1 n-x1+1 x1 n-x1+1

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