<Swing Dev> [9] Review request for 8162350 RepaintManager shifts repainted region when the floating point UI scale is used

[Jim Graham]

Let me clarify something...

On 11/15/16 2:49 AM, Alexandr Scherbatiy wrote:
>   Let's consider the following use case:
>   scale = 1.5
>   A component calls fillRect(1, 1, 1, 1).
>   This is (1.5, 1.5, 3.0, 3.0) in the device space
>   which fills  (1, 1, 3, 3) and covers 2x2 pixels

Agreed.

>   Now the area (1, 1, 1, 1) needs to be repainted
>     create a backbuffer
>     translate(-1, -1) // move the top left corner of the area to the zero point
>     draw the component into the backbuffer:
>       fillRect(1, 1, 1, 1) -> after translation fillRect(0, 0, 1, 1) -> after scaling  (0.0, 0.0, 1.5, 1.5 ) in the
> device space
>       which fills (0, 0, 1, 1) and covers 1x1 pixels

If you did g.setTransform(identity), g.translate(-1, -1), (then restore the scale) then the analysis is as follows:

g.setTransform(identity) => [1 0 0] [0 1 0]
g.translate(-1, -1) => [1 0 -1] [0 1 -1]
g.scale(1.5, 1.5) => [1.5 0 -1] [0 1.5 -1]
g.fillRect(1, 1, 1, 1)
     => coordinates are (1.5-1, 1.5-1, 3-1, 3-1)
     => (.5, .5, 2, 2)
     => fills (0, 0, 2, 2)
     => which covers 2x2 pixels

If you did g.translate(-1, -1) on the scaled transform then the analysis is as follows:

g.transform is [1.5 0 0] [0 1.5 0]
g.translate(-1, -1) is [1.5 0 -1.5] [0 1.5 -1.5]
g.fillRect(1, 1, 1, 1)
     => coordinates are (1.5-1.5, 1.5-1.5, 3-1.5, 3-1.5)
     => (0, 0, 1.5, 1.5)
     => fill (0, 0, 1, 1)
     => covers 1x1 pixels

The second operation is what you are describing above and that would be an inappropriate way to perform damage repair 
because you used a scaled translation which did not result in an integer coordinate translation.

Please re-read my previous analysis that shows what happens when you use integer device-pixel translations which are 
translations that happen using integers on a non-scaled transform.  Note that you can add a scale *AFTER* you apply the 
integer device pixel translation and it will not affect the integer-ness of the translation.  You can see above that the 
difference in how the translate command is issues affects where the translation components of the matrix end up being 
-1,-1 or -1.5,-1.5...

			...jim