surf indices reversed?
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Quick question: It seems surf and surfl have indices reversed. Is this correct? For example, if I type
x=0:pi/10:pi
y = 0:pi/10:2*pi
for i = 1:11
for j = 1:21
z(i,j) = sin(x(i))*sin(y(j))
end
end
surfl(x,y,z)
I get the standard: Error using surfl (line 94)
The lengths of X and Y must match the size of Z.
error. If I use z' in place of z it runs. But shouldn't the first indiex be the x and the second the y? This is completely unintuitive to me.
4 个评论
Image Analyst
2021-12-30
编辑:Image Analyst
2021-12-30
Craig, if the axis direction defaults don't do it the way you'd like, you can flip the direction of the y axis with
axis ij
axis xy
Use whichever does it the way you want.
采纳的回答
Matt J
2021-12-29
编辑:Matt J
2021-12-29
Yes, the x-axis traverses the rows of z and the y-axis traverses the columns.
For some reason, many people prefer their x-axis to be horizontal.
3 个评论
Matt J
2021-12-29
编辑:Matt J
2021-12-30
As a matrix, x should traverse the rows (the first index). But it's not.
When I say "traverse the rows", I mean you move along a row as x varies.
but anyone familiar with plotting matrix data figures out that you rotate 90 degrees from matrix to plot.
I'm not sure what language requires that you rotate 90 degrees. Regardless, here you have a very similar situation, except that you are required to transpose instead of rotate. The need to do either one is unappealing, IMHO.
The bottom line is, your conjecture is right. The surf() routines view z(i,j) such that y varies with i and x varies with j. The same is true for interp2(), meshgrid(), and nearly all Image Processing Toolbox functions.
更多回答(2 个)
Sean de Wolski
2021-12-30
I think you're seeing the difference between meshgrid and ndgrid. Meshgrid is used for plotting, ndgrid for matrix/tensor work
[rr,cc] = ndgrid(1:3,1:4)
[xx,yy] = meshgrid(1:3,1:4)
When I was heavily involved in 3d image processing in grad school I tried to be very very consistent and always use row/col as: the convention, notation, and variable names.
2 个评论
Image Analyst
2021-12-30
Oh, so that's the difference. I never knew. I just always used meshgrid() for everything since I knew that one and didn't want to learn another function. But I can see how ndgrid() could be useful in some situations.
Image Analyst
2021-12-29
Craig, you do know that matrices in MATLAB are indexed (row, column), right? Apparently not since if I
choose better names for your loop iterators we get this:
x = 0:pi/10:pi
y = 0:pi/10:2*pi
for xIndex = 1:11
for yIndex = 1:21
z(xIndex,yIndex) = sin(x(xIndex))*sin(y(yIndex))
end
end
Why are you indexing z like that? Well since row is y, M(row, column) is M(y, x). Matrices are not indexed like M(x, y). You should have z(yIndex, xIndex). The obvious solution is to just label the axes:
xlabel('X', 'FontSize', 25);
ylabel('y', 'FontSize', 25);
zlabel('Z', 'FontSize', 25);
axis equal
But make sure you do it right. You can also use meshgrid(), which you should. I don't have surfl() so I used surf():
x = 0:pi/10:pi;
y = 0:pi/10:2*pi;
for xIndex = 1: length(x)
for yIndex = 1: length(y)
z(yIndex,xIndex) = sin(x(xIndex))*sin(y(yIndex));
end
end
[X, Y] = meshgrid(x, y);
surf(X, Y, z);
% Label the first argument to surf(), which is the columns or x values.
xlabel('X', 'FontSize', 25);
% Label the second argument to surf(), which is the rows or y values.
ylabel('Y', 'FontSize', 25);
zlabel('Z', 'FontSize', 25);
axis equal
g = gcf;
g.WindowState = 'maximized';
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