<Swing Dev> [10] RFR JDK-8178025:HiDPI with non-integer scale factor - SPANs in HTML are rendered overlapping each other

[Sergey Bylokhov]

On 16/10/2017 02:50, Prasanta Sadhukhan wrote:
>> Probably this is a root cause of the bug? 
> Does not seem like. If we only use floating point API 
> getTabbedTextWidth() in GlyphPainter1.getSpan() to get floating point 
> width, it calls SwingUtilities2.getFontStringWidth() which calls 
> getStringBounds() with default FRC.
> When getStringBounds() calls fm.getFontRenderContext(), it returns 
> DEFAULT_FRC which has a null AffineTransform so when we try to get the 
> transform for that through getTransform(), it creates a new 
> AffineTransform with scale 1
> public AffineTransform getTransform() {
>          return (tx == null) ? new AffineTransform() : new 
> AffineTransform(tx);
>      }

So this is a problem why default FRC is used. We should find a way to 
create a correct one instead.

>> It will apply the screens scale to the FRC even if the component is 
>> located on some other screen.
> I was of the opinion that this
> 
> GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice().getDefaultConfiguration().getDefaultTransform().getScaleX() 
> 
> 
> will give the scale factor as passed to sun.java2d.uiScale which should 
> be applicable for all screen(s), which is what I leveraged in my fix.

The "sun.java2d.uiScale" is a debug option which applies the scale on 
all screens, but in case of the common configuration it is possible to 
have different scales(example: hidpi laptop + lowdpi external screen).

GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice().getDefaultConfiguration().getDefaultTransform().getScaleX() 

Will returns default transform for default configuration of the 
main/default screen.

-- 
Best regards, Sergey.