Q:

explain ground-thruth .mat file of an image for CNN

good evening,
i'm new to coding CNN
i'v got ShanghaiTech crowd counting dataset that has (beside the images) .mat files for what i believe the ground truth for (counting) for images.
i try to print the content of one .mat file in python, here is what i get:
{'image_info': array([[array([[(array([[ 855.32345978,  590.49587357],
   [ 965.5908524 ,  472.79472415],
   [ 937.09478464,  400.93507502],
   ...,
   [  42.5852337 ,  359.87860699],
   [1017.48233659,    8.99748811],
   [1017.48233659,   23.31916643]]), array([[920]], dtype=uint16))]],
  dtype=[('location', 'O'), ('number', 'O')])]], dtype=object), '__version__': '1.0', '__header__': 'MATLAB 5.0 MAT-file, Platform: PCWIN64, Created on: Fri Nov 18 20:06:05 2016', '__globals__': []}

each .mat file corresponds to one image,
i know at some point in CNN we need to calculate the error between the network result and the ground truth we have, but i don't seem to understand the structure and the content of these .mat files.
can someone explain whats in these files and how or for what that content is used in crowd estimation.

A:

so i got the answer,
the data in the .mat presented in the question containes (or at least what we are interested in) two arrays, 
the first one:
array([[ 855.32345978,  590.49587357],
   [ 965.5908524 ,  472.79472415],
   [ 937.09478464,  400.93507502],
   ...,
   [  42.5852337 ,  359.87860699],
   [1017.48233659,    8.99748811],
   [1017.48233659,   23.31916643]])

is an N by 2 array, the 2 corresponds tho the targeted object X and Y coordinates, and the N is the number of targeted objects (ground-truth)
also, the second array contains the ground-truth 
the data of the .mat file was extracted through scipy.io.loadmat,
and the structure of the data is dictionary, now getting to the ground-trouth in that was quite tricy, but it went like that:
matContent=spy.io.loadmat(os.path.join(gtPath,gtList[1])) #var type is dictionary
gt=matContent['image_info'][0][0][0][0][1] #getting the ground-thruth number

