Shade and calculate specific area

问题

I tried to change the code in a way so that only the first area is shaded grey. How can I set the horizontal line in a way that it only appears under the area I want to shade?

Furthermore I want to calculate the area of ONE region. How do I achieve that? I know it is trapz but I am not sure how to set the boundaries. Thanks!

x = 0:.01:4*pi;  %// x data
y = sin(x);      %// y data
level = 0.5;     %// level
plot(x, y)
hold on
area(x, max(y, level), level, 'EdgeColor', 'none', 'FaceColor', [.7 .7 .7])

Curve:-

回答1:

you can try also this simple option:

x = 0:.01:4*pi;  %// x data
y = sin(x);      %// y data
level = 0.5;     %// level
lineStart = find(y>=level,1);
lineEnd = find(y(lineStart:end)<=level,1)+lineStart;
plot(x,y)
hold all
area(x(lineStart:lineEnd),y(lineStart:lineEnd),...
     level,'EdgeColor', 'none', 'FaceColor', [.7 .7 .7],'ShowBaseLine','off')
line([x(lineStart),x(lineEnd)],[level level ])
hold off

without defining areas of interest a-priory:

And don't forget to hold off...

To calaulate the area: A = trapz(x(lineStart:lineEnd),y(lineStart:lineEnd))

回答2:

You can limit the range of your x axis in the area plot to the range of interest, e.g. from 0 to 4 and then calculate the resulting values of the function in this range. For the base line: you can hide it in the area command and add it manually using the line command.

x = 0:.01:4*pi;  %// x data
y = sin(x);      %// y data
level = 0.5;     %// level
plot(x, y)
hold on

x_interest = 0:.01:4;
y_interest = sin(x_interest);
area(x_interest, max(y_interest, level), level, ...
    'EdgeColor', 'none', 'FaceColor', [.7 .7 .7], ...
    'ShowBaseLine', 'off');
line( [ min(x_interest) max(x_interest) ], [ level level ] )

来源：https://stackoverflow.com/questions/38420623/shade-and-calculate-specific-area