Indexing question for member variables of opticalFlow collected into an MxN cell array

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Erick Oberstar
Erick Oberstar 2020-6-10
编辑： Ameer Hamza 2020-6-11
I am attempting to find the maximum value over alll member variables (PxQ) for a multidimensional set of MxN opticalFlow system objects
Flow_PlaneA{TestNum,frame} loads up a single opticalFlow system object from a mat file
I would like to find max value of all something like:
VxMax = max(Flow_PlaneA{:,:}.Vx
VyMax = max(Flow_PlaneA{:,:}.Vy
Magnitude Max = max(Flow_PlaneA{:,:}.Magnitude
OrientationMax = max(Flow_PlaneA{:,:}.Orientation
This gives me an error for the VxMax equation for example:
"Expected one output from a curly brace or dot indexing expression, but there were " N "results." (where N = 802 for this data)
The only way I can seem to index is to do 2 nested loops for each member variable for example
for TestNum = 1:M
for frame = 1:N
VxMaxNew = max(max(Flow_PlaneA{TestNum,frame}.Vx )); % max(max( covers PxQ)
VxMax = max([VxMaxNew VxMax])
end
end
Is there an array type indexing based method to do this I am just missing? How do I handle the mix of opticalFlow objects (which look like structs to me) and the multidimensional cellarray indexing at the same time? Is that possible?

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回答（1 个）

Ameer Hamza
Ameer Hamza 2020-6-11
Try this
VxMax = [flow{:}];
VxMax = max([VxMax.Vx]);
or alternatively
VxMax = max(cellfun(@(x) x.Vx, flow), [], 'all');
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Ameer Hamza
Ameer Hamza 2020-6-11
编辑：Ameer Hamza 2020-6-11
The solution could have been more efficient, but some of the cells are empty. The solution requires the creation of an intermediate variable. Try the following two alternatives
maxVx = max(arrayfun(@(x) max(max(x.Vx)), [Flow_PlaneA{:}]));
or
maxVx = max(max(cellfun(@helperFun, Flow_PlaneA)));
function y = helperFun(x)
if ~isempty(x)
y = max(max(x.Vx));
else
y = -inf; % smallest value
end
end
'all' is not supported in R2018a, so two calls to max() are required.

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