speed up 3 nested for loops
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Hi,
I have a matrix, called old_matrix, that is of size width = 500, length = 400, height = 50 and at each point in old_matrix (width,length,height) there is an intensity from 0-255.
I have to remap each point in the old matrix to a new matrix of size new_width = 500, new_length = 400, new_height = 400 * cosd(50) +10.
the function for mapping specific point of old_matrix(width,length,height) => new_matrix(width, length*cosd(height), length*sind(height))
This is the code I have so far
new_z = round((400*cosd(50)+10));
new_matrix = zeros(500,400,new_z);
for height = 50:-1:1
for width = 1:500
for length = 1:400
y_new = round(length*cosd(height));
if (y_new == 0)
y_new = 1;
end
z_new = round(length*sind(height));
if (z_new == 0)
z_new = 1;
end
current_val = old_matrix(width,length,height);
new_matrix(width,y_new,z_new) = current_val;
end
end
end
It works in terms of mapping the values however, it is very slow. Is it possible to vectorize this code to improve the speed for mapping? The intensities do not change, only the position of the intensity changes.
Thank you
采纳的回答
Roger Stafford
2014-3-10
I think you have the nested for-loops in the wrong order for maximum efficiency. You are having to repeat the same computation on 'y_new' and 'z_new' 500 times for each combination of 'height' and 'length' indices. Do it this way instead:
for height = 50:-1:1
for length = 1:400
%Compute y_new & z_new here only once per 'height', 'length' comb.
new_matrix(:,y_new,z_new) = old_matrix(:,length,height);
end
end
Note: The word 'length' is a reserved word in matlab and shouldn't be used for a variable name. It is the name of one of its functions.
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