[x3d-public] v4 proposed MultitouchSensor

[GPU Group]

I found bugs in my first implementation and spent another 4.5 days working
through them;
Results:
http://dug9.users.sourceforge.net/web3d/tests/sensors/DragCascade_MultitouchSensorII.mp4


Method:
MultitouchSensor had output fields for rotation_changed, scale_changed,
translation_changed
When computing the translation from 2 points you might end up with
something like how Transform nodes are done - with a center point C for
where you rotate and scale:
T x C x R x S x (-C)
And so to get a summary translation with no center, you need to solve:
TxRxS = TxCxRxSx(-C)
And I wasn't doing that properly. I found there are 2 ways:
1) matrix decompose - you can chain transforms together, then decompose the
final transform

https://webdocs.cs.ualberta.ca/~graphics/books/GraphicsGems/gemsiv/polar_decomp/

- GraphicsGems IV matrix decomposer

2) least squares similarity 2D solver - which solves for 4 params:
rotation, scale, xy translation

https://sourceforge.net/p/freewrl/git/ci/develop/tree/freex3d/src/lib/input/SensInterps.c


- lines 1725 - 2063 are marked MIT Lic or equivalent, and have the matrix
solver and 2D similarity solver

I used a bit of both - least squares for computing the transform from 2
drags,

and matrix decomposer for combining with previous offsets .offset
(translation) rotationOffset, scaleOffset.

Tout = Tcomputed_from_current_drags X Toffsets

- then decomposing Tout to set the translation_changed, rotation_changed,
scale_changed fields.


A few days of my time were spent trying to diagnose freewrl-specific
issues, primarily

we 'freeze' the transform to the sensor while a button is down. For 2
buttons, what

happens if you add a 3rd button, or remove one of 2 buttons. Having a way
to update

the transforms reliably took some time, and one thing that helped was
rendering the touch/drag points

in sensor space -by making it a renderable node, and rendering the drag
ponits-

 as well as screen / viewer space. Weird motions often made sense relative
to

the touch-down points in sensor space.


Having a built-in emulator for multitouch helped a bit - didn't need to
fiddle with other devices,

- just mouse.


Good luck - hope other x3d browsers are easier.

-Doug



My experience implementing (v4 proposed) MultitouchSensor in freewrl:
>
> Results:
>
>
> http://dug9.users.sourceforge.net/web3d/tests/sensors/DragCascade_MultitouchSensor.mp4
>
>
> http://dug9.users.sourceforge.net/web3d/tests/sensors/DragCascade_MultitouchSensor.x3d
>
>
> Method:
> freewrl has had a multitouch emulator built in for serveral years.
> - a command line option turns it on, then it uses the mouse differently
> -- a RMB click creates a new drag -with a touchID
> -- a LMB click near an existing drag will grab it, and LMB drag will drag
> it
> -- another RMB click on an existing drag will delete it
> - and then whereever we are passing around mouse coordinates and button
> status, we pass an additional touchID.
> -- I verified the touch emulator approach with another emulator that puts
> touches into the windows desktop from a 2nd computer, and inhales the
> touches via windows events.
> But all we did was drag 2 separate dragsensors with 2 separate touches.
>
> This new spec node adds a node that will take 2+ touches and do something
> interesting - output a rotation and scale, along with the usual planesensor
> type translation.
> So I started by copying the Planesensor and hacking it to count active
> drags, and when it has only one, it acts like a PlaneSensor, and when it
> has 2+ it uses the first 2 to compute rotation and scale
>
> Time:
> Took 2.5 days. fiddly hard to understand code in freewrl. Hope other
> browsers are more organized.
>
> And of the 2.5 days I spent 1/2 day reviewing a least_squares option which
> I didn't use, but could be used with 3+ points to compute a best-fit affine
> (6 parameter - 2 scales, 1 shear, 1 rotation, and xy 2 translations)
>
> -Doug
>
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