Proportional paint object behavior inconsistent and needs to change

Kevin Rushforth kevin.rushforth at
Mon Nov 18 19:39:05 PST 2013

I haven't thought through all of the implications, but my initial take 
is the same as Jim's. Namely, that using the fill bounds for 
proportional paints is the most consistent and will have the least 
degree of unpleasant surprises.

-- Kevin

Jim Graham wrote:
> Felipe mentioned recently that we encountered some issues in fixing a 
> bug with SVGPath.
> The outcome of this fix could be a significant change in how your 
> proportional gradient fills look and so we'd like to get feedback on 
> the various ideas.  You can read about them in that Jira issue, but 
> I'll also summarize below.  Discussion would probably be better on the 
> mailing list, but we eventually need to work the salient points back 
> into the Jira issue for future maintenance.
> The basic issue is that we had a disagreement between the way that the 
> shape caching code worked and the way that uncached shapes were filled 
> with proportional paints.
> First, there is the concept of tight and loose bounds.  Loose bounds 
> are very cheap to calculate, but can contain area not strictly inside 
> the shape.  Tight bounds are more careful to figure out exactly how 
> far curved sections of the shape reach, but a fair number of 
> calculations and recursions are needed to accurately calculate those 
> extents.
> Our current code will use loose bounds of the basic shape (i.e. the 
> part that is filled) to calculate the bounds for proportional paints 
> when you first paint a shape - whether you stroke it, fill it, or both.
> But, if you ran with shape mask caching (the default mode) then after 
> a rendering or 2 we would decide to cache the antialias coverage mask 
> and we would then use the node's content bounds to figure the bounds 
> for the proportional paint.  The content bounds are calculated more 
> precisely as tight bounds, though, so they didn't always agree with 
> the bounds used in those first few uncached renderings.
> The net result is that the proportional paint would shift after the 
> first couple of frames unless you animated the shape and then it would 
> revert while you were animating and then shift back when it was stable.
> There is also the Canvas object that can also render proportional 
> paints, and it does so using the same code that the shape nodes use 
> when they don't have their coverage mask cached (i.e. loose fill bounds).
> We'd like to make all of this as consistent as possible.  Here are the 
> various decisions and how they'd impact code:
> - Loose bounds are faster to calculate for shapes that aren't likely 
> to be reused.  The uncached shape rendering used them because the 
> shapes may change before we need the bounds again.  Canvas uses it 
> because it is an immediate mode API with no input on how often it may 
> see a particular shape again.
> - Tight bounds would likely be less suprising to developers and users, 
> though.  They are better for hit testing and damage management because 
> they generate fewer false positive hits.  They are also fairly fast to 
> calculate for most shapes and it is a rare shape that we'd have to 
> recurse much to get it right.  Also, for straight edges there is no 
> difference in performance to calculate tight or loose bounds.
> All in all, it would probably be better for the FX API to standardize 
> on tight bounds and treat any cases where we noticeably affect 
> performance (which should be very rare) as opportunities for tuning.  
> This may not be compatible with the first rendering of current Shape 
> nodes, but they would shift back and forth anyway so we aren't worried 
> about incompatibility with an inconsistent system.
> The other part of the decision is which bounds to use.
> Currently uncached rendering uses fill bounds, and mask-cached 
> rendering uses the content bounds which depends on whether you supply 
> a stroke or a fill paint so it could be either fill bounds or stroke 
> bounds.
> - For filled-only shapes we obviously want to use the "fill bounds".
> - For stroked and filled shapes, we have 3 choices:
> - - use fill bounds for both paints so that the geometry used for 
> proportional stroke and fill paints are similar for both parts of 
> those nodes.  This helps line up any color discontinuities or 
> highlights between the two.
> - - use stroke bounds for both which also means the two are 
> consistent, but Canvas can't really do this because it doesn't know if 
> you are filling and stroking until it sees the latter operation
> - - use fill bounds for fill and stroke bounds for stroke which means 
> the geometries of the two are different and it makes it harder to line 
> up the transitions of proportional stroke+fill shapes, but Canvas can 
> do this so Canvas vs. Shape node remain consistent
> I'm not sure which of the above is the best, but I lean towards "fill 
> bounds for both" because it allows consistent geometry for stroke+fill 
> and it allows consistent behavior between Canvas and Shape node, at 
> the possible expense of "0% on a stroke isn't at the edge of the stroke".
> Thoughts?
>             ...jim

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