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FW: Retrieving mouse event co-ordinates in transformed co-ordina te space.

From: Alan Kennedy <Alank@Widelearning.com>
Date: Wed, 15 Mar 2000 16:18:43 -0000
Message-ID: <412FDD0E9675D311BB7C005004F831C52F3909@MX01>
To: "'www-svg@w3.org'" <www-svg@w3.org>
Hi Folks,

I sent the attached message on xml-dev@xml.org, since an SVG thread cropped
up there, but there seemed to be no interest. I still think this is an
important point however, although I see that there is an explicit mention in
the "changes to the March 3rd WD", which states 

"Removed ambiguity regarding 'relatedNode' for clientX/clientY on mouse
events. Now clarifies that the 'relatedNode' is the **outermost** 'svg'
element, which means clientX/clientY are values in the initial coordinate
system."

I can't find any discussion of this topic on the public WWW SVG list. Has
there been any discussion on this topic? Am I wrong in thinking that mouse
events should be delivered to target objects in co-ordinates they can use
directly, without further compuation? Are there implementation issues that
make this difficult or impossible? 

I think that if the standard is not changed to deliver co-ordinates in the
local co-ordinate space, then every programmer will be forced to write a
piece of code that calculates what the event co-ordinates should be in the
local space. And the code is far from trivial. And since everyone will have
done it differently, and potentially wrongly, existing code bases may be a
barrier to further evolution of the SVG spec.

But I have to say that I think SVG is the best thing since sliced bread, and
will revolutionize the web, particularly in the areas of data visualization
and user interfacing. Keep up the great work.

Alan Kennedy.

-----Original Message-----
From:	Alan Kennedy [SMTP:Alank@Widelearning.com]
Sent:	Monday, March 13, 2000 1:10 PM
To:	'xml-dev@xml.org'
Subject:	SVG: Retrieving mouse event co-ordinates in transformed
co-ordina te space.

Hi,

Is this something missing from SVG? Or something missing from DOM2?

Hope this explanation isn't too long.

All pixel-addressable display screens have a maximum width (max-width) and
height (max-height), and I can address pixels on such displays using a
co-ordinate system where the x value ranges from 0 to max-width and the y
value ranges from 0 to max-height. Let's call this co-ordinate space SYSTEM.

When I define a SVG document, if I don't define any co-ordinate system, I
get the "initial user co-ordinate system", which is set by the user agent.
Let's call this co-ordinate space S.

<svg width="640px" height="480px">
	<!-- This rectangle will fill the visible portion of S.	-->
	<rect x="0" y="0" width="640" height="480" style="fill:red;"/>
</svg>

However, I can also define a transformation on S, using the "viewBox"
attribute, to change the mapping of co-ordinates into a new co-ordinate
space more suitable for my purposes. Let's call this transformed co-ordinate
space ST.

<svg width="640" height="480" viewBox="0 0 100 100"
preserveAspectRatio="none">
	<!-- This rectangle will fill the visible portion of ST.	-->
	<rect x="0" y="0" width="100" height="100" style="fill:red;"
onmouse="handleSTevents(event)"/>
</svg>

Next, I register event listeners on an object in ST, listening for all mouse
events, e.g. mouseover, mouseup, mousedown, etc. I do this in order to be
able to make the objects interactive, e.g. draggable.

When the event (a DOM2 UIEvent) is received by my event handler
(handleSTevents), it has four attributes on it to tell me the location of
the mouse at the time the event occurred. They are, along with definitions
given in the DOM2 spec
(http://www.w3.org/TR/DOM-Level-2/events.html#Events-UIEvent), 

screenX - The horizontal coordinate at which the event occurred relative to
the origin of the screen coordinate system.
screenY- The vertical coordinate at which the event occurred relative to the
origin of the screen coordinate system.
clientX - The horizontal coordinate at which the event occurred relative to
the DOM implementation's client area.
clientY- The vertical coordinate at which the event occurred relative to the
DOM implementation's client area.

The problem is that there seems to be no way of determining what were the
co-ordinates of the mouse event in the transformed space ST. If I want to
find out where in my graphical object the event occurred, or if I want to
change the location of my object to a new location *in space ST*, then I
cannot do so, since I can't find the location of the event in space ST.

The "screenX" and "screenY" attributes tell me the co-ordinates of the event
in the SYSTEM co-ordinate space, and the "clientX" and "clientY" attributes
tell me the co-ordinates of the event in the user agent's S co-ordinate
space.

If I want to work out the co-ordinates of the event in the space ST, then I
have to calculate them by finding all co-ordinate system transformations
that result in the formation of ST, by searching the ancestors of the
element in the document, and process "clientX" and "clientY" through them.

Or am I missing something?

Alan Kennedy.


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Received on Wednesday, 15 March 2000 11:22:03 GMT

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