Given a quadrilateral image, not necessarily a rectangle, how to find its top left point's x-y coordinate

Hello,

Given a quadrilateral image, not necessarily a rectangle, how to find its top left point's x-y coordinate? white spaces are NaN values and (x,y) is about (93,122) in this image.How to return it? Thanks!

Answer 1

[William Rose]

@Yuhong,

Since you did not include an image, I will make one. Then I search along the diagonals for a Non-NaN pixel, starting at the top left corner: (1,1). When I find a non-NaN pixel, I stop and display the pixel coordinates.

%% make image with a NaN border along top and left sides
Nr=300; Nc=400; 
im1=ones(Nr,Nc);            % red layer
im1(:,:,2)=0.8*ones(Nr,Nc); % green layer
im1(:,:,3)=0.2*ones(Nr,Nc); % blue layer
im1(1:15,:,:)=NaN;          % NaNs for rows 1-15
im1(:,1:25,:)=NaN;          % NaNs for columns 1-25
imshow(im1)                 % display image

%% search for top left corner pixel that is not NaN
row=1; col=1;                   % start point
while isnan(im1(row,col,1))
    if row==1
        row=col+1; col=1;       % reset to lower left corner of a new diagonal
    else
        row=row-1; col=col+1;   % move up and right along the diagonal
    end
end
fprintf('Top left non-NaN pixel at row %d, column %d.\n',row,col)

Top left non-NaN pixel at row 16, column 26.

Try it. Good luck.

[Yuhong]

Hi it worked, thank you! answer accepted!

[William Rose]

@Yuhong, you're welcome.

Answer 2

[Matt J]

You can use pgonCorners from

https://www.mathworks.com/matlabcentral/fileexchange/74181-find-vertices-in-image-of-convex-polygon