view() not returning [az, el] ??

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Ken 2024-4-26

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编辑： Paul 2024-4-27

According to this documentation, I believe that the view function should be returning a two element vector [azimuth, elevation]. However, I observed that it actually returns a 4 x 4 projection matrix. But it won't take one! So what do I need to do to get something out of view that it will accept later, to restore the viewpoint?

Here is a command window view of actions taken at a couple of breakpoints in my program:

(Breakpoint, when graph is showing a 3D plot)

K>> a=view(graph)
a =
0.7934   -0.6088         0   -0.0923
0.3044    0.3967    0.8660   -0.7835
0.5272    0.6871   -0.5000    8.3031
0         0         0    1.0000

(Another breakpoint, when graph is showing a 2D plot)

K>> a=view(graph)
a =
1.0000         0         0   -0.5000
0    1.0000         0   -0.5000
0         0   -1.0000    9.1603
0         0         0    1.0000
K>> view(graph, a)
Error using view>ViewCore
Argument must be scalar, or two-vector.
Error in view (line 93)
ViewCore(hAxes, viewArgs{:});

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Paul 2024-4-27

编辑：Paul 2024-4-27

在 MATLAB Online 中打开

When I wrote that I thought it self-evident that the doc is required to explicitly show the cases with different numbers of outputs when those cases change what those arguments mean, which is exactly what the doc shows for FIND, SVD, and RAT.

But the doc does not (anymore) show the single output case for VIEW, which means that calling it with one output should yleld the first output of whichever documented case to which the call resolves, which is the documented behavior under Ignore Function Outputs.

In the case of view, I don't know if the implemented behavior would ever cause a problem. For example, one might try code like this to increase the azimuth angle by 10 degrees

view(view+10,30)

which will result in an error because the inner call to view returns the T matrix, and a 4x4 matrix is not a valid input to view. But what if single-output view returned a scalar? OTOH, there would be problem for code that uses view in an expression that applies an operator that returns a scalar from a matrix.

I wonder why the single output case is no longer documented.

Paul 2024-4-27

在 MATLAB Online 中打开

Yes, Stephen already pointed out the documented behavior of find and I acknowledged in my response comment that the doc page for FIND explicitly shows the difference between calling FIND with one output rather than two, so there's no question about the behavior for FIND.

The issue is what the user should expect as the return value for the not-explicitly-documented, single-output call to view. Based on the doc, the result should be the azimuth. Why do you think it's arguable?

MathWorks has had 12 years to remove that option. I wonder why they haven't already (I have my own speculation). And I wonder why they thought it wasn't a useful option for the user in the first place?

However, even though the expected behavior, based on the doc, of the single-output call to VIEW is not ambiguous IMO, the expected behavior of other functions when called with fewer outputs than documented is ambiguous. Consider this code with a call to ellip

fc = 300;
fs = 1000;
x = ellip(6,10,50,fc/(fs/2));

Should x be the return value of b as in the first documented use case, or should it be z as in the third?

I know what actually happens (x = b), but I don't know that there is a documented rule to formally answer that question.

For example, the rule could be:

a function called with fewer output arguments than any documented usage will be treated as if the function were called with the fewest number of documented output arguments and the unspecified outputs will be ignored in accordance with Ignore Function Outputs.

There may be other options as well. But AFAIK, such rule, whatever it may be, is not documented. A link to such a rule, should it exist, would be a valuable contribution to this thread.

Walter Roberson 2024-4-27

Based on the doc, the result of a single-output call to view is undefined (but the most likely result is the azimuth)

Paul 2024-4-27

编辑：Paul 2024-4-27

Why is it undefined?

According to Ignore Function Outputs, "... you can request only the first N outputs of a function (where N is less than or equal to the number of possible outputs) and ignore any remaining outputs." followed by examples to show exactly what that clause means.

The doc page for the very example on that doc page, which is fileparts, only shows a use with three outputs. Yet calling FILEPARTS with one output is not defined?

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回答（1 个）

Adam Danz 2024-4-26

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2112111-view-not-returning-az-el#answer_1448531

编辑：Adam Danz 2024-4-26

在 MATLAB Online 中打开

The azimuth and elevation outputs are returned as two separate outputs.

f = figure('visible','off'); 
ax = gca(f); 
[az, el] = view(gca)
az = 0
el = 90

You discovered a little easter egg 😏

When only 1 output is requested, view returns a view matrix in a left handed coordinate system.

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