Area of Scouts

brahim · December 29, 2016, 2:24am

Hey
I wonder how BST computes the area of the scouts, the value that is displayed in the scout panel. Because I am not getting the same results. I use the bst function “tess_area.m” to compute the area of each face and then I sum areas of faces inside each region. I get smaller values than the ones displayed in scout panel.
Does brainstorm add extra area when the Euler’s number does not equal to 2?

Thank you in advance

Sylvain · January 2, 2017, 8:37am

Hi:
I think the way BST does it is by assigning a third of the triangle surface area to each vertex. As a scout is defined as a set of vertices, the corresponding surface area is the sum of the fractional surface area assigned to each vertex from the set.

brahim · January 3, 2017, 5:50am

So this means that you BST consider that all regions have Euler’s number equal to 2 (e.g the case of a regions with faces + edges that do not construct faces is not included in BST).
Your reply answers my question thank you.

Francois · January 5, 2017, 9:50am

Hi Brahim,

The area associated with a vertex in surface is the second returned value of tess_area. As Sylvain replied, it corresponds to 1/3 of all the faces attached this vertex.

github.com

brainstorm-tools/brainstorm3/blob/master/toolbox/anatomy/tess_area.m#L40


% Compute the triangle area only if needed

if (nargout >= 2)

% Build vertex-face connectivity matrix, with the area information

nFaces = size(Faces,1);

rowno = double([Faces(:,1); Faces(:,2); Faces(:,3)]);

colno = [1:nFaces, 1:nFaces, 1:nFaces]';

data  = [FaceArea; FaceArea; FaceArea];

VertFacesArea = sparse(rowno,colno,data);



% Compute the vertex area: 1/3 of each triangle involving this vertex

VertArea = 1/3 * full(sum(VertFacesArea,2));

end

The “vertex area” is computed for all the vertices of the surface, before defining any scout. The surface you are using as you cortex surface is ideally closed, so most vertices should have at least 2 or 3 faces attached to them.
The area of a scout is the sum of this “vertex area” for all the vertices included in the scout.

github.com

brainstorm-tools/brainstorm3/blob/master/toolbox/gui/panel_scout.m#L862


                end

            end

            % Use the maximum of 0.03cm3 and the compute volume of the patch

            minVol = 0.01 * length(sScouts(i).Vertices);

            if (minVol > patchVol)

                patchVol = minVol;

            end

            % Sum with the other scouts

            totalVol = totalVol + patchVol;

        end

        strArea = sprintf('Volume: %1.2f cm3  ', totalVol);

        

    % Surface: Compute the total area (cm2)

    else

        totalArea = sum(sSurf.VertArea(allVertices)) * 100 * 100;

        strArea = sprintf('Area: %1.2f cm2  ', totalArea);

    end

else

    strSize = '  No scout selected';

    strArea = ' ';

end

Any remaining doubts?

Cheers
Francois