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How to detect a dark color in RGB triplet?

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I am plotting a map with many subdivision and colors.
I would like to automatically draw the borders with contrasting color with respect to the facecolor of the subdivision.
I am using the sum of the RGB triplet but I am not having perfect matchs.
Is there another way to do that?
My results until now:

Accepted Answer

Image Analyst
Image Analyst on 26 Apr 2020
I don't know what you do on the border between light and dark regions. But anyway, when you're drawing the boundary around a region get the mean RGB of that region -- summing the values is fine, or equvalently, taking the mean. Then you can say
meanRGB = mean([regionsRedValue, regionsGreenValue, regionsBlueValue]);
if meanRGB < 128
% It's dark, so draw with white line.
lineColor = 'w';
% It's bright, so draw black line.
lineColor = 'k';
plot(x, y, '-', 'Color', lineColor, 'LineWidth', 2);
David Franco
David Franco on 26 Apr 2020
For Gama 0-1 should I use 0.5 instead of 128?
David Franco
David Franco on 26 Apr 2020
I am already using
if mean(color) < 0.45 % tried with median(color) < 0.45 too
mapshow(Snew(results(ii,2)),'FaceColor',color,'EdgeColor',[1,1,1]); % white border
mapshow(Snew(results(ii,2)),'FaceColor',color,'EdgeColor',[0,0,0]); % white border
But the results aren't good enough... Sometimes light regions get white borders:

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