Can the start position for ginput() crosshairs be chosen?

Hello,

I have a GUIDE function that has a button defined for picking a location in an axes. When I click the button, the pick crosshairs don't appear until I move the cursor into the axes. A user can wonder what's going on and click the button more than once, not seeing a result. Is there a way to start the ginput() IN the relevant axes?

Thanks!

Doug

Answer 1

[Voss]

There is no option for that, as far as I can tell.

However, you can make a copy of ginput.m (let's say you call it my_ginput.m so it doesn't conflict with ginput.m) and modify the copy to call updateCrossHair (a local function within ginput.m) immediately after the crosshairs are created (i.e., don't wait until mouse movement occurs to update the crosshairs).

In R2017b, in ginput.m there's a function called setupFcn, and in that function is a call to createCrossHair, so right after that you'd put this line:

updateCrossHair(fig,initialState.CrossHair)

and that should get the crosshairs to turn on immediately, which may be good enough (they'll be centered on the pushbutton or wherever the cursor is when my_ginput is called).

In addition to that, if you want to programmatically move the cursor to the center of the relevant axes, you can do that by setting the groot property 'PointerLocation', e.g.:

set(0,'PointerLocation',ax_pos([1 2])+ax_pos([3 4])/2);

but note that PointerLocation is relative to screen position, so that ax_pos has to be relative to screen position as well, so you'd have to do something like:

fig_pos = getpixelposition(fig); % figure Position in pixels
ax_pos = getpixelposition(gca(),true); % axes Position in pixels, relative to figure (taking into account any panels the axes is nested in)
set(0,'PointerLocation',fig_pos([1 2])+ax_pos([1 2])+ax_pos([3 4])/2); % center of axes, relative to screen

If you put those three lines in my_ginput.m, after the figure's WindowButtonMotionFcn is set, which is right after the call to createCrossHair mentioned above (again, at least that's where it is in R2017b), then the crosshairs will show up immediately and be centered in the current axes (and if you do that you can remove the call to updateCrossHair above, since it'll be called automatically when PointerLocation is set).

[Douglas Anderson]

Thank you very much! That sounds like exactly what I need.

[Voss]

You're welcome! Try it out and let me know if it works.

[Douglas Anderson]

I've never made a copy of a built-in function .m file. How do I do that? (Very newbie question, even though I've been using MATLAB for over a decade!)

Thanks, at your convenience!

[Voss]

Copy it to a directory you can write to, e.g., the directory where you usually put m-files you create.

Rename it with a new name, and change the name of the function inside to that new name as well. (The renaming is just so it doesn't conflict with the built-in function when you call it in MATLAB, since you probably still want to use that built-in function too.)

[Douglas Anderson]

Yes, that I understand but where IS it?! Again, feeling newbie, but I'm not sure where that file would reside.

Thanks

[Voss]

Oh, sorry about that.

The easiest way to find it is to do edit ginput on the command line. Then Save As ... that file to a new name and location.

[Voss]

@Douglas Anderson Have you been able to get this working?

[Douglas Anderson]

Yes, I have, thank you!

I had been trying to associate the crosshair with a specific axes, without success, but then figured out that all I needed to do was to find the right position within the main window!

Thanks again. Good solution.

[Voss]

You're welcome! Interesting problem. Glad it's working!