Area Calculation Using polyarea: Figure vs PNG

I'm encountering an issue while working with the polyarea function. I've created a 2D scatter plot and used drawfreehand to select my roi and polyarea to calculate its area. However, when I save the figure as a PNG and recalculate the area from the image, the result is significantly larger. From what I can understand it has to do with how I am saving the figure to a png (?).

What I eventually want to do is have a png displaying a set of points and a figure displaying a different set of points and calculate their respective areas and compare them. The two will not be displaying the same thing necesserily so that why I need their areas to be comperable in dimensions.

I am attaching an example script.

Answer 1

[Image Analyst]

There is no spatial calibration when you save a figure, or a non-image graph in an axes on a figure. So the image you get upon saving the figure or axes is just basically a screenshot, so the area will depend on that size of the figure. If you resize the figure before saving you will get a different area.

You should be writing to a digital image and saving it with imwrite.

[Aikaterini]

Thank you for your response. I am not resizing the figure at all. I tried saving using imwrite as well as:

F = getframe(gca);

F = frame2im(F);

[A,map] = rgb2ind(F, 256);

imwrite(A, map, 'File.png');

Still, when I calculate the area of a 1x1 square from the png I get something around 30000.

[Image Analyst]

So 30,000 pixels, or sqrt(30000) pixels per unit. So your calibration factor is

unitsPerPixel = 1/sqrt(30000);

Multiply linear pixel distances by that and areas by the square of that to get quantities in real world units.

See attached demo.