Developing controls based on Canvas?

[Felix Bembrick]

Hi Jonathan,

Thanks for your reply.

I am a little restricted in exactly what I can reveal about my plans for
this control but I can say that it is one in which its very appearance
could change quite significantly in a dynamic way at runtime.  The control
also needs to support panning and zooming in a very performant way.  There
will likely be a lot of graphics being draw in a manner which (to me at
least) fits the immediate mode of rendering better than a scenegraph
because their positions and attributes are most readily defined
programmatically rather than declaratively.

Though not directly analogous, consider your typical spreadsheet
application like Excel where the user is able to pan to the right
effectively without limits and that grid lines are constantly being
rendered as the panning takes place.  Given that screens can be very large
these days it is conceivable that a complex spreadsheet will be displaying
hundreds of lines to define the cells in a grid at any one time and that it
is way more concise to programmatically define how this grid is rendered
rather than having a scenegraph containing a node for each line etc.  Also,
the panning and zooming responses are much simpler to implement
programmatically than continually fiddling around with the scenegraph.
 Then there's the whole issue of virtualisation and keeping the actual
number of "active" nodes to a minimum if it were to be done using a
scenegraph.

I am just not convinced that the costs of memory usage and processing
cycles to maintain a retained mode representation of a visual structure
like this can be justified or made performant when all you really need is a
simple algorithm that draws lines on the screen according to the properties
of your cell model.  Then consider that the *actual* control will be
significantly more complex and graphically rich than this simplistic
spreadsheet analogy.

To me this is the sort of control that really lends itself to an immediate
mode rendering component such as Canvas but there just seem to be so many
impediments in the way of actually building a professional control with
Canvas at its base.  If we continue with the spreadsheet analogy then
obviously cells would contain other controls etc. and we have discussed the
limitations on "embedding" other controls within a Canvas that seems to
suggest it's not practical.

I think in the end it gets down to the nature of the control itself and
whether its appearance is best defined by data or by an algorithm just like
the way certain types of knowledge are best defined by nouns whereas others
are best defined procedurally.

At this stage it is looking very much as though JavaFX really only supports
controls most appropriately defined in a static, structured way like a
scenegraph.  This in turns limits its applicability to a whole range of
software applications.

Felix


On 6 August 2013 10:10, Jonathan Giles <jonathan.giles at oracle.com> wrote:

> I think it would pay to take a step back and understand why you think a
> 'traditional' scenegraph-based (or retained mode) control is not sufficient
> for your needs?
> Unfortunately you've not detailed your use case, so it is hard to give any
> specific advice. Are you able to give any details about what it is you're
> trying to build and why you think the normal approach to building controls
> is not sufficient?
>
> We've built some fairly complex controls using this approach, and if
> implemented wisely, there is very little that a scenegraph-based approach
> can't do. Specifically, do you think your control will render all of the
> 'thousands of nodes' at once, or will many of these nodes be off screen or
> otherwise not visible at any one time? For things like the TableView we
> only render the nodes that are visible. This means that regardless of
> whether there are 100 or 1,000,000 rows of data, we only have visual nodes
> for the 20 visible rows, for example. Keeping your scenegraph as minimal as
> possible is always a very wise idea, if performance is a concern.
>
> As you note, the other problem is that you will run into issues if you
> want to mix canvas rendering with the scenegraph-based controls like
> Button. The best you're likely to achieve (having not tried it personally)
> is to position the control on top of the canvas, rather than attempting to
> render the control inside the canvas (and having to then deal with event
> handling, etc). This will likely prove to be finicky, and more cumbersome
> than simply using an entirely canvas-based or entirely scenegraph-based
> approach.
>
> -- Jonathan
>
>
> On 5/08/2013 10:11 p.m., Felix Bembrick wrote:
>
>> I am investigating the feasibility of developing a JavaFX 8 control based
>> on Canvas.  I have chosen Canvas as the base class as this control is of a
>> very dynamic nature and would not be easy to implement with a retained
>> mode
>> style ancestor (at least as far as I can tell).
>>
>> So is this feasible?  While I can readily see how to render the visual
>> aspects of the control, I am not sure how to best "embed" other controls
>> within it should that become necessary (and almost certainly will).
>>
>> For example, how would I go about embedding a Button within my control?
>>  It
>> looks to me like I would need to create an actual Button node somewhere
>> else in the scenegraph and then perhaps render it within my control using
>> gc.drawImage() passing in a snapshot of the Button node.  That's OK but
>> then I have to somehow handle events and I am not sure how best to do
>> that.
>>
>> Another issue I see is that there seems to be no way to apply effects to
>> individual graphic elements within the Canvas as the applyEffect() method
>> applies to the entire Canvas.
>>
>> Finally, a significant obstacle is this issue:
>>
>> https://javafx-jira.kenai.com/**browse/RT-23822<https://javafx-jira.kenai.com/browse/RT-23822>
>>
>> This issue relates to the lack of support for LCD font smoothing within
>> Canvas.  This may not sound that serious but the difference between LCD
>> font-smoothed text in other controls and the grey-scale text in Canvas is
>> so distinct on my current machine that a control based on Canvas would
>> really stick out like a sore thumb and appear significantly less appealing
>> than a "standard" control.
>>
>> So, am I wasting my time?
>> Are there any other issues I am likely to face?
>> Are there other ways to develop dynamic controls which may involve
>> thousands of nodes (such as lines, curves etc.)?
>>
>> Thanks,
>>
>> Felix
>>
>
>