[OpenJDK 2D-Dev] [9] Review Request: 8167310 The graphics clip is incorrectly rounded for some fractional scales

[Jim Graham]

That looks great.  A couple of questions.

Region, line 132: why are you testing for newv > coordinate?

Test case:

Where do you get white gaps?  Is it in the line test?  If so, then consider setting the line width higher (or drawing 2 
lines per iteration as I mention in the last paragraph).  I can illustrate this with the following:

scale = 1.5
vertical line drawn with step, 0, step, SIZE covers the area:
     (step-0.5)*1.5, -0.5, (step+0.5)*1.5, SIZE+0.5
     step*1.5-.75, ..., step*1.5+0.75
     clipLoX-0.75, ..., clipHiX-0.75

Note that this will cover some pixels to the left of the clip (which should be clipped out), but it will leave a gap 
between the right side of the line and the right side of the clip, i.e. the next step (of about 0.75 pixels - which are 
likely to include a pixel center and thus leave a white pixel).  At higher scales, the gap will be higher and more 
likely to include white pixels.

A line width of 2 (actually 2*stepsize where stepsize=1 here) means that the line will cover an entire step regardless 
of the scale.  Hopefully that will remove the need for the WHITE test in the validate method.

Note that STROKE_CONTROL may come into play here as well.  The above analysis is for STROKE_PURE semantics, but the 
normalization/biasing of STROKE_NORMALIZE might increase or decrease the chance that the above will fix the issue and 
may require an even larger line width.

Another option without setting line width would be to draw two lines, one at step and another at step+1, which would 
test for gaps between adjacent lines as well as ensure that the test covers the "right half" of the line at step and the 
"left half" of the line at step+1...

				...jim

On 10/7/16 6:18 AM, Sergey Bylokhov wrote:
> Hello.
>
> Please review the fix for jdk9.
>
> This is bug which was found when the fractional scale is used in Swing. The problem is that if we save a usrClip as
> Rectangle2D then we incorrectly intersect it with device clip. The problem is in the RectangularShape.getBounds()
> method, see more details:
> http://mail.openjdk.java.net/pipermail/2d-dev/2016-July/007299.html
>
> So getBounds() produces the bigger Rectangle than is necessary and our clip became bigger as well. This means that in
> some fractional scales such clips are overlapping.
>
> The test will test fillRect, DrawImage, DrawLine methods and validates that there are no any overlapping if we set the
> clip.(The gaps between touched pixels are allowed only for lines).
>
> Note that as I understand the code this fix should affect the DrawLines, because in some situations it is possible that
> two lines are overlapped without clip. And two other cases(fillRect, drawImage) should work without clip, but only
> fillRect works. It is discussed here:
> http://mail.openjdk.java.net/pipermail/2d-dev/2016-October/007766.html
>
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8167310
> Webrev can be found at: http://cr.openjdk.java.net/~serb/8167310/webrev.00
>