<Swing Dev> [9] Review request for 8156217 Selected text is shifted on HiDPI display

[Alexandr Scherbatiy]

I tried to look deeper in the code and it seems there is a rounding 
issue when float values are summed up.

Suppose a transform with scale 1.5 is used and the 'a' char advance is 
10 in a dev space.
The 'a' char has advance 10 / 1.5 = 6.666666666666667 as double value 
and 6.6666665 when it is cast to float in user space.
The width of a string which consists of 15 'a' chars is 15 * 6.6666665 
=  100.000000.
But the same width calculated as sum of each glyph advance in 
StandardGlyphVector.initPositions() method is 99.999992.
--------------
        double scale = 1.5;
         float advance = (float) (10 / scale);
         int N = 15;
         System.out.printf("%d * %f = %f\n", N, advance, N * advance);
         float sum = 0;
         for (int i = 0; i < N; i++) {
             sum += advance;
         }
         System.out.printf("sum: %f\n", sum);
--------------

Because of this a string drawn from float position 99.999998 is shifted 
one pixel left which affects the text selection code in Swing:
------------------------
         g.scale(1.5, 1.5);
         String TEXT = "aaaaaaaaaaaaaaaa";
         Rectangle2D rect = font.getStringBounds(TEXT, 0, index, 
g.getFontMetrics().getFontRenderContext());
         float selectedTextPosition = (float) rect.getWidth(); //   
99.999992
         g.drawString(TEXT.substring(0, index), x, y); // non-selected text
         g.drawString(TEXT.substring(index, TEXT.length()), x + 
selectedTextPosition, y); // selected text is shifted to one pixel left
------------------------

Thanks,
Alexandr.

On 6/2/2016 11:41 PM, Alexandr Scherbatiy wrote:
> On 5/31/2016 10:40 PM, Phil Race wrote:
>>
>> I applied this and it is *much* better but there still seem to be 
>> some tiny quirks.
>> When I drag the mouse to select text down and then up again, as I 
>> pass the
>> original mouse click point vertically, repaint seem to jiggle 
>> vertically by a pixel.
>> Perhaps a rounding issue in the repaint code's calculation of the 
>> location of
>> the target y. I think I may see the same in left/right dragging along 
>> a line too.
>> So I think this is repaint and not text related. Can you take a look.
>
>     I am able to reproduce this only using a floating point scale.
>     It looks like 2d issue. I used a test which draws a text in two 
> pieces. The second piece of the text is shifted from the first piece 
> by the floating point size of the the first piece of the text.
>   -----------
>     Rectangle2D rect = font.getStringBounds(TEXT, 0, index, 
> g.getFontMetrics().getFontRenderContext());
>     float selectedTextPosition = (float) rect.getWidth();
>     g.drawString(TEXT.substring(0, index), x, y);
>     g.drawString(TEXT.substring(index, TEXT.length()), x + 
> selectedTextPosition, y);
>   -----------
>
>   The second piece of the text can be shifted in the 2 cases:
>   a) graphics scale is 1.5 and translation is 1.
>   b) graphics scale is 2.25 without applied translation
>
>   I have filed an issue on it:
>     JDK-8158370 Text drawn from float pointing position and with float 
> pointing scale is shifted
> https://bugs.openjdk.java.net/browse/JDK-8158370
>
>   Thanks,
>   Alexandr.
>
>>
>> -phil.
>>
>>
>> On 05/06/2016 12:31 PM, Alexandr Scherbatiy wrote:
>>> Hello,
>>>
>>> Could you review the fix:
>>>   bug: https://bugs.openjdk.java.net/browse/JDK-8156217
>>>   webrev: http://cr.openjdk.java.net/~alexsch/8156217/webrev.00
>>>
>>>   This is the second part of the fix related to the fact that char 
>>> width can be fractional in user space.
>>> http://mail.openjdk.java.net/pipermail/swing-dev/2016-May/005814.html
>>>
>>>   The Font.getStringBounds(...) method is used for the fractional 
>>> string width calculation by Swing in user space.
>>>
>>>  Thanks,
>>>  Alexandr.
>>>
>>
>

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