how to find the corners of an irregular binary shape

my work is irregular shape (image) matching. i have to extract the points( topleftmost, top right most, the same ay botom. i have to draw a line from top left to bottom right. again a line from top right to bottom left. i have to find the angle between two lines.

Answer 1

[Image Analyst]

Usually when people ask for image processing advice, they attach an image. I'd guess that you can just find all the rows and columns and take the min and max.

[rows, columns] = find(binaryImage);
topRow = min(rows);
bottomRow = max(rows);
leftColumn = min(columns);
rightColumn = max(columns);
% Draw horizontal lines at top and bottom.
line([leftColumn, rightColumn], [topLine, topLine], 'Color', 'y', 'LineWidth', 3);
line([leftColumn, rightColumn], [bottomRow, bottomRow], 'Color', 'y', 'LineWidth', 3);

Another way is to get the vertical and horizontal profiles and use find() to find the first and last non-zero elements. Not sure which is faster - you can try them both.

[Dhandapani.S]

Thank you. i get the border line for top and bottom. my work is irregular shape (image) matching. i have to extract the points( topleftmost, top right most, the same ay botom. i have to draw a line from top left to bottom right. again a line from top right to bottom left. i have to find the angle between two lines.

[Dhandapani.S]

i can check whether the shape has any deformity. please help

[Image Analyst]

So just draw the diagonals - it's a simple modification

% Draw diagonal line from top left to bottom right.
line([leftColumn, rightColumn], [topRow, bottomRow], 'Color', 'y', 'LineWidth', 3);
% Draw diagonal line from top right to bottom left.
line([rightColumn, leftColumn], [topRow, bottomRow], 'Color', 'y', 'LineWidth', 3);

And the angle between the lines is simple trigonometry

angle = 2 * atan2d((bottomRow-topRow)/(rightColumn-leftColumn));

[Dhandapani.S]

sir this also gave me an angle. angle = atan2d((bottomRow-topRow),(rightColumn-leftColumn)); is this correct? . thank you very much. i think i have not let you know the complete problem clearly. my image is a binary one. so min and max are 0 and 1. leftColumn = min(columns); which may give many 0's. please help me to get the coordinates.left most point right most point top and bottom.

</matlabcentral/answers/uploaded_files/23491/ang.jpg> i have to extract the circled points. points are not exact. points in the four corners have to be extracted for binary image. thank you in advance

[Image Analyst]

The mins and maxes are not 0 and 1. They can't be, because if you do this:

[rows, columns] = find(binaryImage);

rows and columns will be pixel numbers, not 0 and 1. They should be 1-D vectors. So when you take the min or max of rows or columns, you will NOT get 0 and 1, nor arrays of 0 and/or 1.

[Marlon Damaceno]

in the above example two diagonal lines were drawn in the image, how do you insert a vertical and a horizontal one passing through the center of the image?

[Image Analyst]

You can write into the image like

grayImage(:, 32) = 255; % Draw white line at column 32

or just use plot() to plot into the overlay.