How to make the contour smooth other way(spline) and want to color a specific part

I wnat make a contour more smooth.

The code using __fit('cubicinterp')

'cubicinterp' is 3rd interpolation by each point?. i want interpolation by all point

how to make the contour more smooth (use spline? )

and I want to fill in the only red part (x1,y1,z1)

plz help me

Thanks

x=[250	250	250	250	250	250	300	300	300	300	300	300	350	350	350	350	350	350	400	400	400	400	400	400	450	450	450	450	450	450	500	500	500	500	500	500]';
y=[-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2]';
z=[45.32387079	29.24976486	18.98522122	20.34655974	9.322369104	6.099755477	35.95644961	36.63977979	22.23230132	15.44277474	26.46045535	27.33915193	47.61434049	38.89634871	31.30817744	27.35747741	13.92859198	6.494014321	55.87879512	45.20216626	36.04598958	30.00851201	12.3192618	4.23514123	58.91006549	50.26502058	37.81663326	31.22407039	21.04293731	14.41272158	60.07610895	52.66807669	41.19547341	31.91688177	23.86284429	22.12621742]';

f = fit( [x, y],z, 'cubicinterp' )

     Piecewise cubic interpolant:
       f(x,y) = piecewise cubic surface computed from p
     Coefficients:
       p = coefficient structure

D = plot(f, 'Style', 'Contour');
set(D, 'LevelList',3:4:56,  'ShowText','on',  'Fill','off',  'LineColor', 'auto')
grid off;
set(gca,'FontSize',15)

hold on;
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
x1=[250	250	250	250	250	250	300	300	300	300	300	300	350	350	350	350	350	350	400	400	400	400	400	400	450	450	450	450	450	450	500	500	500	500	500	500]';
y1=[-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2]';
z1=[0.102819727	-0.017654204	0.026066466	-0.041190188	-0.212698548	-0.152755654	0.227364491	0.11989207	-0.062549974	0.12148117	0.708627406	1.036772875	0.224630519	0.150396798	0.109177532	-0.017412138	-0.237425239	-0.297834215	0.295633059	0.196812704	0.130896878	-0.056606636	-0.359548825	-0.44208297	0.348611924	0.23943992	0.129783908	0.057714851	-0.050285245	-0.086700291	0.372194136	0.275552906	0.150646442	0.071323847	0.055922125	0.091577766]';

f1 = fit( [x1, y1], z1, 'cubicinterp' )

     Piecewise cubic interpolant:
       f1(x,y) = piecewise cubic surface computed from p
     Coefficients:
       p = coefficient structure

d = plot(f1, 'Style', 'Contour');
set(d, 'LevelList',0:0.1:0,  'ShowText','off',  'Fill','off',  'LineColor', 'red')
grid off;

[Adam Danz]

I've edited the question to format the code and produce the outputs usng the run feature (green arrow in toolbar).

Answer 1

[Adam Danz]

> 'cubicinterp' is 3rd interpolation by each point?

Matlab documentation describes cubicinterp as Piecewise cubic interpolation. I looked for more details in the documentation to determine how this is implemented and its difference from cublicspline but didn't find much. However, these terms are general terms and you can look them up to find background info on the internet.

> how to make the contour more smooth (use spline? )

If you're just looking for a visually more smoother contour line, you could try several methods until you're satisfied. Here is a list of model for curve and surface fitting:

https://www.mathworks.com/help/curvefit/list-of-library-models-for-curve-and-surface-fitting.html

> and I want to fill in the only red part

1. Redefine the LevelList to segment your desired contour levels. Based on your data and your description, you want to segement all negative levels.
2. Fill in the remaining contour areas. To do this, I use getContourLineCoordinates from the file exchange to get the contour coordinates and then patch to fill in the areas.

% SKipping the first part of your code that plots
% the first layer of contour lines

x1=[250	250	250	250	250	250	300	300	300	300	300	300	350	350	350	350	350	350	400	400	400	400	400	400	450	450	450	450	450	450	500	500	500	500	500	500]';
y1=[-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2	-3	-2	-1	0	1	2]';
z1=[0.102819727	-0.017654204	0.026066466	-0.041190188	-0.212698548	-0.152755654	0.227364491	0.11989207	-0.062549974	0.12148117	0.708627406	1.036772875	0.224630519	0.150396798	0.109177532	-0.017412138	-0.237425239	-0.297834215	0.295633059	0.196812704	0.130896878	-0.056606636	-0.359548825	-0.44208297	0.348611924	0.23943992	0.129783908	0.057714851	-0.050285245	-0.086700291	0.372194136	0.275552906	0.150646442	0.071323847	0.055922125	0.091577766]';

f1 = fit( [x1, y1], z1, 'cubicinterp' )

d = plot(f1, 'Style', 'Contour');

% Set LevelList to segment negative contours 
set(d, 'LevelList', [d.LevelList(1), 0],  'ShowText','off', 'Fill','off', 'LineColor', 'red')

% Get contour line coordinates (from file exchange)
% https://www.mathworks.com/matlabcentral/fileexchange/74010-getcontourlinecoordinates
contourTable = getContourLineCoordinates(d); 

% Loop through each contour patch
hold on
for i = 1:max(contourTable.Group)
    rowidx = contourTable.Group == i;
    patch(contourTable.X(rowidx),contourTable.Y(rowidx), 'r', 'FaceAlpha', .5)
end
grid off;