Distinguishing between two surfaces

Hi guys,

I have a simple question. I want to distinguish between two surfaces with x, y, and z values. The equations resemble like the ones here:

z = x + y; z = 2x + y;

I basically want to find out when z is different with the same values for x and y. I then want to be able to graph this 'difference'. Is it a simple thing to do? Is it as simple as subtracting the two equations?

If this is possible, then is it possible to do something similar with 3 or more surfaces with similar equations? I want to find x and y values where z is different for all equations.

I hope this makes sense.

Thank you

Answer 1

[Joseph Cheng]

I would first call the two z something different that way you don't override the other z. Yes a simple subtraction would be something you could do and then you can use the surf(x,y,z1-z2) to show the difference. to find the the points that are different for the same x and y values, the find() function for when z1-z2~=0 (z1 subtract z2 does not equal 0) will give you the index of values not equal zero. when you introduce 3+ surfaces you can use the same find but you'll have to do some round robin comparison scheme and display it.

[A]

Thanks for your response. I will try your suggestion.

What do you mean by round robin stuff?

Thanks

[Image Analyst]

He means generate the 2 z's for every possible x,y coordinate. If not in a nice evenly spaces matrix (which is probably preferable), then at least such that z1 and z2 have values for the same pairs of (x,y) coordinates.

Answer 2

[A]

Can I use this formula?

http://www.mathworks.co.uk/help/matlab/ref/setdiff.html

[Star Strider]

‘ C = setdiff(A,B) returns the data in A that is not in B. ’

The setdiff function looks for the numbers that two arrays do not share. It will not do point-by-point comparisons.

I believe Joseph meant ‘round robin comparison scheme’ to be (z1 - z2), (z3 - z1), (z3 - z2), etc. in the event you have more than two surfaces you want to compare. You would probably have to plot the (z1 - z2), (z3 - z1), (z3 - z2) surfaces each in a different plot.