MATLAB: How to detect a dark color in RGB triplet

colorgeoshowmapshowMATLABplottingrgb

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:

Best Answer

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';
else
    % It's bright, so draw black line.
    lineColor = 'k';
end
plot(x, y, '-', 'Color', lineColor, 'LineWidth', 2);
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