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

[Jim Graham]

On 11/15/16 2:49 AM, Alexandr Scherbatiy wrote:
>> Were you seeing that happen?  Because that would be a rendering bug in fillRect...
>   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
>
>   Now the area (1, 1, 1, 1) needs to be repainted

Is that the area in device pixels or component scaled coordinates?

>     create a backbuffer
>     translate(-1, -1) // move the top left corner of the area to the zero point

Are you calling translate(-1,-1) on a scaled graphics or on a graphics that has an identity transform set instead?

>     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

That is true if you are using scaled translates that don't line up on pixel bounds.  I am suggesting that the repaint 
code use only *INTEGER DEVICE PIXEL TRANSLATES* which don't have this problem...

>   Draw the backbuffer to the window at coordinates (1, 1)

At (1, 1) in the device pixel coordinate space or the scaled component coordinate space?

>   The rectangle with size  1x1 pixels is drawn instead of the 2x2

It sounds like you were using scaled component coordinate space translations and blits which would cause the rendering 
to change.

RepaintManager should only ever be using device pixel translates and do none of its set-up work in the scaled coordinate 
space.

> The result of translation, painting to the backbuffer and painting the backbuffer from the requested point can differ
> from just drawing from this point just because drawing from some point and drawing the same thing from the zero point
> can differ for the floating point scale.

They can only differ if you make the mistake of using scaled coordinate translations to manage dirty areas and scrolling...

				...jim