MATLAB: Get children of a figure

figureplot

I have a code below that highlights a defined region in a plot:
x = 1:24;
a = 1;
b = 50;
y = a + (b-1).*rand(length(x),1);
plot(x,y);
h = area([18 20],[max(y) max(y)]);
set(h,'FaceColor',[.5,.5,.5]);
set(h,'EdgeColor',[.5,.5,.5]);
h1 = get(h,'children');
set(h1,'FaceAlpha',0.3);
hold on;
plot(x,y);
hold off;
axis([min(x) max(x) min(y) max(y)]);
Although this works as intended, I am unsure about what get(h,'children') does. Could anyone clarify for me what it does and what is the numeric value it returns?

Best Answer

The return value of area() is a handle (an arbitrary double precision value with no inherent significance to the value, but which is used by MATLAB to refer to individual graphics objects.)
area() returns the handle to an areaseries object.
In current implementations, areaseries objects have a single child object that is the handle of a patch object, with the patch being in the shape of the area being filled.
The code takes advantage of the fact that patch objects happen to support transparency (when the OpenGL renderer is used), and sets the patch's transparency to 0.3 which is "fairly transparent" (the range being 0 as completely transparent, to 1 being completely opaque.)
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