RFR: 8303904: [macos]Transparent Windows Paint Wrong (Opaque, Pixelated) w/o Volatile Buffering

[Jeremy]

On Tue, 4 Feb 2025 00:07:48 GMT, anass baya <duke at openjdk.org> wrote:

> The PR includes two fix : 
> 1 - The first fix targets proper rendering of the transparent image.
> 2 - The second fix ensures the image is painted at the correct resolution to avoid pixelation.

src/java.desktop/macosx/classes/sun/java2d/metal/MTLGraphicsConfig.java line 246:

> 244:         int fullResWidth = Math.round( scaleFactor * width);
> 245:         int fullResHeight = Math.round( scaleFactor * height);
> 246:         WritableRaster wr = model.createCompatibleWritableRaster(fullResWidth, fullResHeight);

Thanks for working on this. :)

I have a couple of questions:

1. This math looks great for the simple case of a 200% high-res monitor; is the rounding safe for non-integer resolutions?

For ex if width is 33 and scaleFactor is 1.25, then we'll call `round(41.25)`, so `fullResWidth = 41`.

Will the native windowing system allocate 41px for our Window? (Because if so: great. We have a perfect fit.) Or could it allocate 42px? (In which case we may have a thin border on the right/bottom that becomes unpaintable...)

2. I'm assuming if `fullResWidth`/`fullResHeight` is ever zero we'll get a RTE. I don't think it's stated, but the preexisting code suggests `width` and `height` were never zero. Is it possible `scaleFactor` can ever be less than .5? (I've never observed that in my day-to-day life, but I'm just thinking about edge cases and RTE's...)

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PR Review Comment: https://git.openjdk.org/jdk/pull/23430#discussion_r1940637680