Surface plot positive output and corresponding combination of variables only

I have a function with two variables over a range and wish to find all possible combination of the variables that will return positive output (>0).

Say the function is 2x-3y, and x and y is defined as

length_L = 0.0001:0.0001:0.005;
length_R = 0.0001:0.0001:0.005;

and all combinations of length_L and length_R is apply to the function by

[X, Y] = meshgrid(length_L, length_R);
F = 2*X-3Y;

which will return a matrix F which is the result of all combination of length_L and length_R, which can be visualize with

surf(length_L, length_R, F)
% or
surf(X, Y, F)

However, I only want to show the positive F value and the combination of two variables that will result it.

I had tried some logical indexing that I am certain are incorrect like

posF = F(F>0);
posX = X(F>0);
posY = Y(F>0);
surf(posX, posY, posF)

But does not work at all as posF, posX and posY are all vector and surf(X, Y, Z) the Z need to be in matrix.

Answer 1

[Cris LaPierre]

What about setting the F values that are <=0 to NaN?

length_L = 0.0001:0.0001:0.005;
length_R = 0.0001:0.0001:0.005;
[X, Y] = meshgrid(length_L, length_R);
F = 2*X-3*Y;
F(F<=0) = nan;
surf(X, Y, F)

[Kai Jie See]

That works for me, but what if I have another function 2y-3x and the combination of the variables need to return positive output for both of the function, i.e. plot only surface that overlay each other. I know the hold function can plot both surface in same graph.

[Walter Roberson]

mask = F<0 | G<0;
F(mask) = nan;
G(mask) = nan;

[Kai Jie See]

Thanks for the help guys.