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

Jim Graham james.graham at oracle.com
Mon Jun 21 20:00:29 UTC 2010

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...


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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