how to find 3 points in binary image?

I want to find P1,P3 points and mark it with circle symbol.

Answer 1

[Image Analyst]

I'd find the narrowest part first, then scan down line-by-line to get the first widest part. Not the widest part because there seems to be some noise near the bottom

% First smooth the mask
mask = conv2(double(mask), ones(7)/49, 'same') > 0.5;
[rows, columns] = size(mask)
leftBoundary = zeros(rows, 1);
rightBoundary = zeros(rows, 1);
for row = 1 : rows
    thisRow = mask(row, :);
    col = find(thisRow, 2, 'first')
    if ~isempty(col)
        leftBoundary = col(1);
        rightBoundary = col(2);
    end
end

Now see where the left boundary is greatest (most to the right), and the right boundary is least (most to the left) in the top 3/4 or the rows. This will find the narrow part of the leg above the ankle. See if you can do it. Then scan from there down until the boundary reverses direction. When it first reverses direction, that will be the first widest part, which is the ankle. Then you will have the two ankle points and the average of the x and y coordinates will be the mid point.

xMid = (xLeft + xRight) / 2;
yMid = (yLeft + yRight) / 2;

Please give it a try as I don't have time to do the complete project for you.

[wongsathorn pinwihok]

I appreciate for your time to help sir. I will try my best THANKS.

[wongsathorn pinwihok]

Do I have to do the boundary extraction?

[Image Analyst]

I don't think it's necessary. But you have to come up with some kind of rule that will find P3, because I don't know. For P2, you can do

[rows, cols] = find(mask);
% Find last row/line that has a white point in it.
P2y = max(rows);
% Extract just that lowest, bottom-most line.
lastRow = mask(P3y);
% Find average of points in that last row.
columns = find(lastRow); % Vector of indexes where there are white pixels.
P2x = mean(columns); % Average of all the x locations of all the white pixels in the last row.

[wongsathorn pinwihok]

Why did I get this point instead when I use your code to find P1 ?

[Image Analyst]

Attach the original binary image along with your script.

[wongsathorn pinwihok]

this one,Thanks

[wongsathorn pinwihok]

this one, Thanks

[Image Analyst]

How are you deciding where to put the lower point, the point on the heel?

[wongsathorn pinwihok]

you mean P3 ? It's a 1/3 distance between P1 and P2.

[wongsathorn pinwihok]

did you figure it out ?