Photogrammetry, distance in image

I have the following image and I want to measure the size of the object on the A4 sheet of paper (Known dimensions 297*210).

I started by grayscaling the image and then applied a Sobel filter in order to get the edges.

How can I interpolate the size then ? (Automatically)

Thanks

[Image Analyst]

Interpolate what variable between what starting value and and what ending value?

[Joachim Huet]

The big rectangle is the A4 sheet of paper and I would like to measure the size of the object on top of it.

I could do it manually by measuring the length of both the object and the A4 sheet and then doing a linear interpolation knowing the real length of the A4 sheet. But is there a way in matlab to do that automatically ? (Sorry if it is not clear enough)

Answer 1

[Ramnarayan Krishnamurthy]

A possible approach would be to use the function 'regionprops' after some basic pre-processing:

I = imread('edges.png');

% Reduce the number of individual regions
I1 = double(ones(size(I))-(I));

% Label the connected components
I2 = bwlabel(I1,4);

% Find the label of the region of interest using the data cursor
imagesc(I2)

% Use regionprops to automatically evaluate the image properties (FilledArea as an example)
R = regionprops(I2,'Area');

% The component of interest is labeled 12 and the output is the number of pixels of the region
R(12).Area

% Identify the ROI visually
R_I = regionprops(I2,'Image');

% Plot the Region of Interest
imshow(R_I(12).Image);

There are additional features such as Perimeter, Filled Area, etc which may be of interest to you. The complete list of properties is available at:

https://www.mathworks.com/help/images/ref/regionprops.html

Hope this helps!

[Joachim Huet]

Thanks for your answer, I will give it a try !