[OpenJDK 2D-Dev] Fix for drawing round endcaps on scaled lines.

Jim Graham james.graham at oracle.com
Thu Jul 8 01:03:19 UTC 2010

You can google and find a dozen sites that detail the math that shows 
that Bezier approximations to quarter circles approximate the original 
arc to within about .1%.  We use that kind of math in the geom classes 
(see ArcIterator and EllipseIterator)...


Denis Lila wrote:
> I also have two questions about computing a good bezier approximation
> to circle arcs.
> 1. Given the arc (1,0)->(cos(a),sin(a)) where 0<a<pi/2, will it result
> in a good approximation to find the control points p1, p2 by solving
> the equations imposed by the requirements B(1/3) = (cos(a/3), sin(a/3)
> and B(2/3) = (cos(2a/3), sin(2a/3)). In other words, requiring that the
> bezier curve go through two evenly spaced points in the arc.
> 2. If we have an affine transformation A which turns the circle arc in an
> ellipse arc will the transformed control points Ap0, Ap1, Ap2, Ap3 define
> a good bezier approximation to the ellipse arc?
> Thank you,
> Denis.
> ----- "Denis Lila" <dlila at redhat.com> wrote:
>> That's true.
>> Well, if we're worried about the generated paths being verbose
>> and taking long to process then the problem extends beyond just
>> drawing round end caps. As far as I can see, whenever a path is
>> drawn that doesn't consist only of straight lines (i.e. an ellipse),
>> a flattening path iterator is being used to feed Stroker. So all
>> the bezier curves are still broken down into tiny straight lines,
>> just not by Stroker itself.
>> So, my question is, given a bezier curve C and a number w, is
>> there a way of quickly computing the control points of two bezier
>> curves C1, C2 such that the stuff between C1 and C2 is the widened
>> path?
>> More formally: compute the control points of C1, C2, where
>> C1 = {(x,y) + N(x,y)*(w/2)  | (x,y) in C}
>> C1 = {(x,y) - N(x,y)*(w/2)  | (x,y) in C}, where N(x,y) is the normal
>> of C at (x,y).
>> If we could do this easily, then we can just make a new class that
>> outputs bezier curves that is similar in purpose to Stroker, but that
>> is used only when the output can handle bezier curves. This way, the
>> only use left for Stroker would be when anti-aliasing, and for
>> every thing else we wouldn't have to use a flattening path iterator.
>> Thanks,
>> Denis.
>> ----- "Jim Graham" <james.graham at oracle.com> wrote:
>>> Hi Denis,
>>> Consider the case of using BasicStroke.createStrokedShape().  How do
>>> you
>>> know how many pixels the resulting path will occupy?  You can't
>> reduce
>>> to concrete samples if you don't know the transform.
>>> So, for rendering, then you may be correct.  But for cases where the
>>> path is being asked for then beziers are the only responsible
>>> solution...
>>> 			...jim
>>> Denis Lila wrote:
>>>> Hello Jim.
>>>> I thought about checking the output and changing the behaviour
>>>> depending on whether the output is a PC2D or a LineSink, but I
>>> didn't
>>>> implement it because I thought the point was to get rid of the
>>> sampling
>>>> at this stage. However, if performance is the issue, then I guess
>>> I'll
>>>> start working on it.
>>>> Although, I wonder whether it is really worth it. I think most
>> lines
>>> drawn
>>>> won't be wider than about 5 pixels, which means that the current
>> way
>>> will
>>>> emit about 7 lines, so that's 14 coordinates. 2 bezier quarter
>>> circles will
>>>> require 12 coordinates. In terms of storage, there isn't much
>>> difference, and
>>>> for lines of width 4 or smaller the current method is more
>>> efficient.
>>>> I'm also guessing that it's harder for the rasterizer to deal with
>>> bezier
>>>> curves than with straight lines, so is it possible that replacing
>>> the
>>>> 3.14*lineWidth/2 lines generated by the current method with 2
>> bezier
>>>> quarter circles isn't worth it (for small lineWidths)?
>>>> Thanks,
>>>> Denis.
>>>> ----- "Jim Graham" <james.graham at oracle.com> wrote:
>>>>> Sigh - that makes sense.  One issue is that the resulting paths
>> it
>>>>> generates are much more "verbose" than they need to be.  This
>> would
>>>>> generally mean that it takes far more storage than it would
>>> otherwise
>>>>> need - and it means that if the result needs to be transformed
>> then
>>> it
>>>>> would take many more computations to transform each segment than
>>> the
>>>>> bezier.
>>>>> So, perhaps it would be worth having it check the type of the
>>> output
>>>>> and
>>>>> do either a bezier or a bunch of lines depending on if it is a
>> PC2D
>>> or
>>>>> a
>>>>> LineSink?
>>>>> Also, it isn't really that difficult to for Renderer to include
>>> its
>>>>> own
>>>>> Cubic/Quadratic flattening code, but it might involve more
>>>>> calculations
>>>>> than the round-cap code since it would have to be written for
>>>>> arbitrary
>>>>> beziers whereas if you know it is a quarter circle then it is
>>> easier
>>>>> to
>>>>> know how far to subdivide...  :-(
>>>>> 			...jim
>>>>> Denis Lila wrote:
>>>>>> So, I have been thinking about this, and I can't see a good
>>>>>> way to do it that wouldn't involve heavy changes to Pisces.
>>>>>> In order for Stroker to generate Bezier quarter circles, it
>> would
>>>>>> have to implement a curveTo method, which means Stroker should
>>>>>> start implementing PathConsumer2D and instead of using a
>> LineSink
>>>>>> output it would have to use a PathConsumer2D output (either
>> that,
>>>>> or
>>>>>> LineSink should include a curveTo method, but then there won't
>>>>> really
>>>>>> be any difference between a LineSink and a PathConsumer2D. By
>> the
>>>>> way,
>>>>>> LineSink doesn't have any implemented methods, so why is it an
>>>>> abstract
>>>>>> class as opposed to an interface?)
>>>>>> Stroker is used in 3 ways:
>>>>>> 1. As an implementation of BasicStroke's createStrokedShape
>>> method.
>>>>> This
>>>>>> uses a Path2D object as output.
>>>>>> 2. As a way of feeding a PathConsumer2D without calling
>>>>> createStrokedShape
>>>>>> to generate an intermediate Shape. This uses a PathConsumer2D
>>>>> output.
>>>>>> 3. As a way of feeding lines to a Renderer object, which
>>> generates
>>>>> alpha
>>>>>> tiles used for anti-aliasing that are fed to a cache and
>>> extracted
>>>>> as needed
>>>>>> by an AATileGenerator. Obviously, Stroker's output here is a
>>>>> Renderer.
>>>>>> 1 and 2 aren't problems, because the underlying output objects
>>>>> support
>>>>>> Bezier curves. 3, however, doesn't, and it seems like
>> implementing
>>> a
>>>>>> curveTo method for Renderer would be very difficult because the
>>> way
>>>>> it
>>>>>> generates alpha tiles is by scanning the drawn edges with
>>>>> horizontal
>>>>>> scan lines, and for each scan line finding the x-intersections
>> of
>>>>> the scan
>>>>>> lines and the edges. Then it determines the alpha values (I'm
>> not
>>>>> too sure
>>>>>> how it does this).
>>>>>> In order to implement Bezier curves in Renderer, we would have
>> to
>>>>> have
>>>>>> a quick way of computing, for each scan line, all its
>>> intersections
>>>>> with
>>>>>> however many Bezier curves are being drawn.
>>>>>> I haven't given much thought to how this could be done, as I am
>>> not
>>>>> very
>>>>>> familiar with Bezier curves, but it doesn't seem easy enough to
>>>>> justify
>>>>>> fixing such a small bug.
>>>>>> ----- Original Message -----
>>>>>> From: "Jim Graham" <james.graham at oracle.com>
>>>>>> To: "Denis Lila" <dlila at redhat.com>
>>>>>> Cc: 2d-dev at openjdk.java.net
>>>>>> Sent: Wednesday, June 9, 2010 7:42:33 PM GMT -05:00 US/Canada
>>>>> Eastern
>>>>>> Subject: Re: [OpenJDK 2D-Dev] Fix for drawing round endcaps on
>>>>> scaled lines.
>>>>>> I don't understand - why do we generate sample points based on
>>> the
>>>>> size
>>>>>> of the cap?  Why not generate a pair of bezier quarter-circles
>>> and
>>>>> let
>>>>>> the rasterizer deal with sampling?
>>>>>> 			...jim
>>>>>> Denis Lila wrote:
>>>>>>> Hello.
>>>>>>> I think I have a fix for this bug:
>>>>>>> http://icedtea.classpath.org/bugzilla/show_bug.cgi?id=506
>>>>>>> Basically, the problem is that if there is a magnifying affine
>>>>> transformation set on the graphics object and one tries to draw a
>>> line
>>>>> with small thickness and round end caps, the end caps appear
>>> jagged.
>>>>> This is because the computation of the length of the array that
>>>>> contains the points on the "pen" with which the decoration is
>>> drawn
>>>>> does not take into account the size of the pen after the
>>> magnification
>>>>> of the affine transformation. So, for example, if the line length
>>> was
>>>>> set to 1, and the transformation was a scaling by 10, the
>>> resulting
>>>>> pen would have a diameter of 10, but only 3 pen points would be
>>>>> computed (pi*untransformedLineWidth), so the end cap looks like a
>>>>> triangle.
>>>>>>> My fix computes an approximation of the circumference of the
>>>>> transformed pen (which is an ellipse) and uses that as the number
>>> of
>>>>> points on the pen. The approximation is crude, but it is simple,
>>>>> faster than alternatives
>>>>> (http://en.wikipedia.org/wiki/Ellipse#Circumference), and I can
>>> say
>>>>> from observations that it works fairly well.
>>>>>>> There is also icing on the cake, in the form of slight
>>> improvements
>>>>> in performance when the scaling is a zooming out. Example: if the
>>>>> original line width was 100, but g2d.scale(0.1,0.1) was set, then
>>> the
>>>>> resulting line would have a width of 10, so only ~31 points are
>>>>> necessary for the decoration to look like a circle, but without
>>> this
>>>>> patch, about 314 points are computed (and a line is emitted to
>>> each
>>>>> one of them).
>>>>>>> I appreciate any feedback.
>>>>>>> Regards,
>>>>>>> Denis Lila.

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