More efficient way than a loop for this particular example
显示 更早的评论
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
采纳的回答
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.
4 个评论
Osman Kavcar
2022-2-15
Osman Kavcar
2022-2-15
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))
% collect color table, permute, expand (faster)
b = timeit(@() testB(spatialFrequency_px,screenHeight_px))
% modified original with direct output assignment (slower)
c = timeit(@() testC(spatialFrequency_px,screenHeight_px))
% time ratios
[a/b a/c]
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
2022-2-26
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