More efficient way than a loop for this particular example

I code but I am never trained for it, so every once in a while I get suspicious of the possibility to do something more efficiently. Bottleneck there is the ConvertColors in-house function I use, but I think there is a better way to do the j loop. Thanks in advance.

EDIT: for some reason the editor won't indent, so see screenshot for better readibility.

referenceAchromaticGrating=zeros(screenHeight_px,spatialFrequency_px,3);
for i=0:spatialFrequency_px-1
    luminance = meanLuminance+4*sin(2*pi*(1/spatialFrequency_px)*i);
    RGB_array = ConvertColors('MWLRGB',[0,0,luminance],colorInfo);
    for j=1:3
        referenceAchromaticGrating(:,i+1,j)=RGB_array(j);
    end
end

Answer 1

[DGM]

What size is RGB_array? I'm guessing it's a 1x3 tuple, and the LHS of the output assignment is a Mx1x3 region in the output array.

I doubt that trying to expand RGB_array to allow direct output assignment without the per-channel loop would really save more time.

However, it might be better to have the outer loop collect a Nx3 matrix of all values of RGB_array. At that point, once you're outside the loop, this array can be reshaped into the output.

RGB_array = zeros(spatialFrequency_px,3);
for i = 0:spatialFrequency_px-1
    luminance = meanLuminance+4*sin(2*pi*(1/spatialFrequency_px)*i);
    RGB_array(i+1,:) = ConvertColors('MWLRGB',[0,0,luminance],colorInfo);
end

referenceAchromaticGrating = repmat(permute(RGB_array,[3 1 2]),screenHeight_px,1);

This also avoids the need to preallocate the output array

I don't know if this is any faster than what you had. That's something you'll need to test.

EDIT:

I timed it (with ConvertColors() replaced with rand(1,3)). The above method is approximately twice as fast for a 1000x1000 output. It's slightly faster yet for smaller outputs. Including the functions that I omitted will only reduce the effective difference between the two.

[Osman Kavcar]

Both of your initial assumptions are correct. I ran the code, it works just as well, but to my surprise (as yours looks tidier and more proper to me) it ran slightly slower. However, I learned two functions that I have never used before (and certainly not in conjunction with each other).

[Osman Kavcar]

You were absolutely correct when rand is used (at least within my system).

Elapsed time is 0.010750 seconds. (yours)

Elapsed time is 0.012222 seconds. (mine)

So I tried with the ConvertColors function multiple times. My code fluctuated wildly (anywhere from 0.4 to 0.7 seconds) and yours remained consistent at around 0.5. This is so interesting.

[DGM]

Take the results from tic/toc with a grain of salt -- the profiler even more.

This is what I used, though even version and environment differences can sway things.

spatialFrequency_px = 1000;
screenHeight_px = 1000;

% original method
a = timeit(@() testA(spatialFrequency_px,screenHeight_px))

a = 0.0050

% collect color table, permute, expand (faster)
b = timeit(@() testB(spatialFrequency_px,screenHeight_px))

b = 0.0022

% modified original with direct output assignment (slower)
c = timeit(@() testC(spatialFrequency_px,screenHeight_px))

c = 0.0150

% time ratios
[a/b a/c]

ans = 1×2

2.2055 0.3299

function testA(spatialFrequency_px,screenHeight_px)
    referenceAchromaticGrating=zeros(screenHeight_px,spatialFrequency_px,3);
    for i=0:spatialFrequency_px-1
        RGB_array = rand(1,3);
        for j=1:3
            referenceAchromaticGrating(:,i+1,j)=RGB_array(j);
        end
    end
end

function testB(spatialFrequency_px,screenHeight_px)
    RGB_array = zeros(spatialFrequency_px,3);
    for i = 0:spatialFrequency_px-1
        RGB_array(i+1,:) = rand(1,3);
    end
    referenceAchromaticGrating = repmat(permute(RGB_array,[3 1 2]),screenHeight_px,1);
end

function testC(spatialFrequency_px,screenHeight_px)
    referenceAchromaticGrating=zeros(screenHeight_px,spatialFrequency_px,3);
    for i=0:spatialFrequency_px-1
        RGB_array = rand(1,3);
        for j=1:3
            referenceAchromaticGrating(:,i+1,:)= repmat(permute(RGB_array,[3 1 2]),[screenHeight_px 1]);
        end
    end
end

If ConvertColors() can be vectorized, there maybe more time to shave off.

[Osman Kavcar]

Thank you, this is marvelous!