why the plot curve is not smooth as the fitted curve

I fitted a Quadratic curve, such as

f = fit(x,y, 'ploy2');

If I plot the fitting curve plot(f, x, y)

plot(f, x, y);

the curve is smooth, but if I plot the curve like this

yy = f.p1 * x.^2 + f.p2 * x + f.p3;
plot(x, yy);

the plotted curve is not as smooth as the curve plotted using the syntax plot(f, x, y). What's the difference between this two function callings?

Answer 1

[dpb]

The supplied plot function that is fit -object aware uses an internally-determined number of points to evaluate the function that are more than those in the data set itself--I just did a test here--for a sample dataset of length 15 where I fit y=x.^2+rand(size(x))*10; for a vector x=[1:15];

>> f=fit(x',y','poly2');  % fit the above quad+noise
>> h=plot(f,x,y)          % use the builtin plot; return line handles
h =
633.0012
634.0002
>> tmp=get(h(1),'xdata');  % get the data for the raw data...
>> length(tmp)             % and show how many points were there--
ans =
  15
>> tmp=get(h(2),'xdata');  % ditto for the curve of the fit...
>> length(tmp)
ans =
      1013
>>

As you can see, it computed the fit at over 1000 points internally to draw a smooth curve whereas your manual plot is just "connecting the dots" between the fitted points since you only evaluated at the input values.