[openjdk/jfx] 8234712: Add pivot properties for scale and rotation in Node, ScaleTransition and RotateTransition (#53)

[Michael Paus]

I don't think there are clearly right or wrong answers to all these 
questions,
so I would just like to provide a view thoughts on this.

The governing principle should be a principle of "least surprise" from a 
users
point of view. That means we should not make things more complicated as
they already are and should always keep the practical use-cases in mind.
If you need full flexibility, you can always use the explicit transforms.

We currently have the notion of the "Node's center" which is used for 
scaling
and rotation and I think we should stick to that. The documentation later
explains that "The pivot point about which the rotation occurs is the center
of the untransformed layoutBounds." which is a completely arbitrary decision
because, e.g., from a mathematically point of view the geometric center 
of the
node would seem to be more natural but is more difficult to compute for 
general
shapes. So, we should just extend this scheme in such a way that the 
user can
replace the single computed center with a single user-defined center
specified in untransformed coordinates. Introducing two separate centers
for rotation and scaling is, to my opinion, just academic without any 
practical
use-case and in the end much more complicated for the user.

I would also specify this center point as a Point3D for two reasons:
1. Three doubles always use their full space even if you don't need them.
2. Using Double.NaNs as a switch between computed and user-provided center
     is just confusing and error-prone.
     What would be the meaning of setting x to some value but leaving y 
and z as
     NaN? Would you implicitly set them to 0.0 or use the computed values or
     would you discard this user setting completely and use all the 
computed
     values if any of the components is NaN? I bet most users will 
forget to set the
     Z-value anyway if they are thinking 2D. A simple null value is much 
easier
     to handle and explain.

Just my two €ent.

--Michael

Am 16.12.19 um 03:06 schrieb Nir Lisker:
> Replying on the mailing list to the questions raised on GitHub.
>
> The state of whether to use the computed center pivot or the value of the
>> pivot attribute is implicit with no way for an application to know which it
>> is, and no way to set it back to using the computed center (i.e., the state
>> is sticky once you set it). Perhaps if you defined a null value as meaning
>> "computed center" then an app could at least reset it to the "computed
>> center" state, although there would still be no way for the app to know
>> that it was in that state.
>>
> In the JBS issue <https://bugs.openjdk.java.net/browse/JDK-8234712> I
> alluded to this in point 5. I think that null should represent the default
> (node center). However, if we use 3 doubles instead of a Point3D we might
> need to use Double.NaN for this instead, which would also be the default
> for this case. The docs will explain this.
>
> Do we need separate properties for scale pivot and center pivot?
> I say yes, otherwise the enhancement will be very limited. I think of this
> enhancement as adding pivot control to Rotate/Scale transitions, and adding
> them to Node is a necessary (and desirable) step.
>
>     ... you need to worry about what coordinate space the rotation pivot is
>> defined in. Perhaps if the rotation pivot were defined in unscaled space,
>> it might work.
>>
> Isn't it already? If I set the rotation pivot to the edge of the node, then
> scale it down, then rotate, the rotation pivot would be outside of the
> node's boundaries. In scaled space it would still be on the edge. Or did I
> misunderstand you? In any case, I don't think that there's a single correct
> answer here.
>
> Should the pivot be specified as a Point3D or 3 separate doubles? Separate
>> doubles... there would be no out-of-band null value to use
>
> See my point above about Double.NaN.
> The doubles vs Point3D is an important choice. We might want to look into
> the future even where Point3D (and 2D) could be made into an Inline class
> with Valhalla, which will help with the performance aspect. Binding to one
> of the coordinates is sill a problem there, however.
>
> On Sat, Dec 14, 2019 at 6:25 PM Kevin Rushforth <notifications at github.com>
> wrote:
>
>> This will need discussion on the openjfx-dev mailing list. Here are the
>> questions that need to be resolved:
>>
>>     1.
>>
>>     The state of whether to use the computed center pivot or the value of
>>     the pivot attribute is implicit with no way for an application to know
>>     which it is, and no way to set it back to using the computed center (i.e.,
>>     the state is sticky once you set it). Perhaps if you defined a null value
>>     as meaning "computed center" then an app could at least reset it to the
>>     "computed center" state, although there would still be no way for the app
>>     to know that it was in that state.
>>     2.
>>
>>     Do we need separate properties for scale pivot and center pivot? A
>>     single pivot property would be easier to define, but wouldn't allow you to
>>     set it from a RotationTransition and a ScaleTransition if you wanted
>>     to apply both to the same Node. With two separate properties, as you
>>     have defined it, it is more flexible, but you need to worry about what
>>     coordinate space the rotation pivot is defined in. The current transform
>>     chain looks like this:
>>
>> T(layout+translate) * T(pivot) * T(rot) * T(scale) * T(-pivot)
>>      * transform[0] * transform [1] ...
>>
>> Perhaps if the rotation pivot were defined in unscaled space, it might
>> work. The transform chain would then look like this:
>>
>> T(layout+translate) * T(pivotRot/scale) * T(rot) * T(-pivotRot/scale) * T(pivotScale) * T(scale) * T(-pivotScale)
>>      * transform[0] * transform [1] ...
>>
>> In any case, you need to think about the implications of having one of
>> scale or rotate being set explicitly and the other being the computed
>> center.
>>
>>     1. Should the pivot be specified as a Point3D or 3 separate doubles? I
>>     can see pros / cons of either approach. Separate doubles are more
>>     consistent with the the pivot values in the Rotate and Scale Transform
>>     objects, and are easier to using in binding. On the other hand, there would
>>     be no out-of-band null value to use (see issue 1 above), so you would
>>     need a boolean property for each of scale pivot and rotation pivot to
>>     indicate whether the computed value or the value of the pivot properties
>>     should be used. I don't think that the fact that the rotation axis is
>>     defined as a Point3D should have any bearing on whether the pivot
>>     should be so defined. I'd probably lean towards separate doubles.
>>
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