| Re: Line-Mouse widget tool [message #43255 is a reply to message #43237] |
Tue, 29 March 2005 15:25  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Robert Barnett writes:
> I render a direct graphics plot onto a widget_draw.
> How do you transform a mouse click event which is specified in pixels to
> data-space when using direct graphics? I can compute it from the
> xmargin, ymargin, xtick_get, ytick_get, xsize and ysize. But, surely
> there must be an easier way?
Well, remember that every object I create and every-
thing I draw into a window has a coordinate system
associated with it. This is, effectively, the *data*
coordinate space. So, a medical image, for example,
might have a coordinate system in mm or cm, something
like that. And typically, these objects (images are
good examples, as are plots) know something about
where they are located in the display window.
So, these objects have a Pixel_To_Value method that
simply takes a window pixel X and Y value and can
return the proper "data" coordinate. (And also tell
me if I am inside or outside the object.)
So a call looks like this:
data = theObject -> Pixel_To_Value(event.x, event.y, Inside=in)
IF in THEN BEGIN
Print, 'Data Coordinates are: ', data
ENDIF ELSE Print, 'Outside of object'
The Pixel_To_Value method looks something like this (stripped
to the essentials):
PRO myObject::Pixel_To_Value, x, y, INSIDE=inside
inside = 0
self -> ApplyCoordinates ; Set up data coordinate space.
c = Convert_Coord(x, y, /Device, /To_Data)
retval = retValue = [c[0,0], c[1,0]]
IF (retValue[0] GE !X.CRange[0]) AND $
(retValue[0] LE !X.Crange[1]) AND $
(retValue[1] GE !Y.CRange[0]) AND $
(retValue[1] LE !Y.Crange[1]) THEN inside = 1
RETURN, retval
END
The "inside" test varies a little bit, depending upon the object.
What I've shown you is for some kind of plot object. If this were
an image, I would be checking to see if the point is inside the
image's "location" in the window.
The notion of a coordinate system associated with every object
is amazingly powerful. Today I built a "ruler" object for an
image, which is just a small scale that you want to display
on an image to give you some idea of the size of features
you are looking at. The ruler has it's own coordinate
system, so I can position it in a window, and it has a holder
for the parent coordinate system. It uses the parent coordinate
system to see how big it needs to be, and its own coordinate
system to display itself.
So I just popped it into an image object and it grows or retracts
in size as I zoom in and out of the image, all the while staying
in the same place in the window. Amazing!! Magic!
Let's just say there are days when you've *got* to love IDL!
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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