What's wrong with this polyshape?

The polyshape created has nothing to do with the original polygon: what am I doing wrong?

[Geoff Hayes]

Joana - please attach or copy and paste your code to this question rather than posting screen shots of the code.

[Joana Brito]

plot([106.2620, 107.2080, 106.5060, 104.6750, 105.7130], [666.0000, 670.0000, 644.0000, 628.0000, 656.0000]);
BW=poly2mask([106.2620, 107.2080, 106.5060, 104.6750, 105.7130], [666.0000, 670.0000, 644.0000, 628.0000, 656.0000], 800, 800);
imshow(BW))

[Muhammad Usman]

Generally when you draw the polygons in MATLAB by using points, then in order to close the shape of any polygon the first point is last point too...

Here is an example:

plot([0,1,1,0,0],[0,0,1,1,0])
axis([-0.5 1.5 -0.5 1.5])

[Ameer Hamza]

Joana, the region you specified is only 3 pixels wide. It cannot be correctly made unless you increase its width.

[Joana Brito]

Muhammad, the poly2mask function should close the polygon automatically

[Joana Brito]

Ameer, how do I do that? How do I scale the x and y axis in order for the pixels to match the shape?

Answer 1

[Ameer Hamza]

Try this code. As I mentioned in my comment to your question that the polygon is just 3 pixels wide. In this code, I scaled the coordinate to make it 50 pixels wide. Now it should be visible.

x = [106.2620, 107.2080, 106.5060, 104.6750, 105.7130];
y = [666.0000, 670.0000, 644.0000, 628.0000, 656.0000];

x = (x - min(x))./(max(x)-min(x))*50+100;
BW=poly2mask(x, y, 800, 800);
imshow(BW)

[Joana Brito]

and why didn't you scale the y axis?

because I will use this binary image to take out information such as Centroid/MinorAxisLength/... and then apply it to the original polygon. Do I scale it back?

[Ameer Hamza]

I scaled the x-values, just to demonstrate the issue with making a mask from the polygon coordinate. I guess you are planning to use regionprops to find the required information. If your goal is to find the centroid of the polygon, then why not use centroid(): https://www.mathworks.com/help/matlab/ref/polyshape.centroid.html. Also, regionprops consider the region as an ellipse and returns the major and minor axis. However, your shape is a polygon. How are you defining the axes of the polygon?

[Joana Brito]

Thank you for you sugestions! But the goal is to find an ellipse which encloses the polygon and for that finding the major and minor axes with regionprops

I did it like you said and now my shape is rotated, for some reason

[Ameer Hamza]

The polygon is not rotated, it is just drawn upside down because the imshow() function makes a y-axis, which increases in the downward direction, whereas plot() make y-axis increasing in the upward direction. Apart from appearance, the vertex coordinates are the same.

If you want to do this with regionprops, then you will need to apply some type scaling before making the poly2mask and then apply reverse scaling at the end. I recommend avoiding the image processing toolbox function because it is really a geometry problem, and there are other available tools to solve this problem efficiently. For example, check this FEX submission: https://www.mathworks.com/matlabcentral/fileexchange/3215-fit_ellipse. It fit an ellipse to a set of data points and directly outputs the major and minor axes.