MATLAB: How to efficiently obtain a slice of a matrix / Can I obtain a reference to a slice

matrixmatrix arrayoptimization

Using a mex file, I create a matrix with approximate dimensions 150x150x150x15000. It is essentially 15,000 3D masks. I am currently trying to access each mask using:

for i=1:15000
  mask = matrix(:,:,:,i);
end

but this takes (what I feel to be) an excessive amount of time (approx 50s), presumably because it is creating a new matrix from each slice. Is it possible to obtain a reference to each slice instead?

I have tried creating an array of structs each with their own reference to a matrix and access like so:

for i=1:15000
  mask = array(i).matrix
end

but then the creation takes even longer due to all of the memory allocations!

Any help would be much appreciated.

Best Answer

See James Tursa's recent https://www.mathworks.com/matlabcentral/fileexchange/65842-sharedchild-creates-a-shared-data-copy-of-a-contiguous-subsection-of-an-existing-variable

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MATLAB: How to extract data from a slice

Already you did it...The code you gave has the way to extract the data you want.

hsp = surf(linspace(-2,2,20),linspace(-2,2,20),zeros(20)+1);
rotate(hsp,[1,-1,1],30)
xd = get(hsp,'XData');
yd = get(hsp,'YData');
zd = get(hsp,'ZData');
delete(hsp)
[x,y,z] = meshgrid(-2:.2:2,-2:.25:2,-2:.16:2);
v = x.*exp(-x.^2-y.^2-z.^2);
hss = slice(x,y,z,v,xd,yd,zd);
xs = get(hss,'XData');
ys = get(hss,'YData');
zs = get(hss,'ZData');
cs = get(hss,'CData');
figure
surf(xs,ys,zs,cs) ;

You have the data xs,ys,zs,cs in your hand now.

MATLAB: Best way to do a dynamic bit slice

Simulink has limited bit-manipulation capabilities; it's designed to address a higher level of abstraction, for the most part. That said, with the appropriate settings, your MATLAB function generates pretty good Verilog code. While the file is 53 lines long, the functional always block is only 6 lines, including two ends and range checking of the input:

always @(data, bitsel) begin
  if (((bitsel + 8'd1) >= 8'd1) && ((bitsel + 8'd1) <= 8'd31)) begin
    bit_idx = (bitsel + 8'd1) - 8'd1;
    MATLAB_Function_out1 = data[bit_idx];
  end
end

The array indexing issue does remain, due to Verilog being 0-based and MATLAB being 1-based, as you mentioned. The calculation of bit_idx will generate no logic in practice. To get this code all I needed to adjust was to deselect the saturation setting on the MATLAB code. To adjust this, open the file in the MATLAB editor and choose "Edit Data" from the Simulink section of the Edit ribbon.

The flattening of MATLAB Function Blocks into their parent subsystems is controlled globally for a design in HDL Coder. Open the Configuration Parameters dialog and choose the HDL Code Generation pane. Navigate to Global Settings > Coding style. In the "RTL customizations" section, select "Inline MATLAB Function block code".

Best Answer

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MATLAB: How to extract data from a slice

MATLAB: Best way to do a dynamic bit slice

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