On Fri, 29 Jan 2021 00:05:57 GMT, Kevin Rushforth <[email protected]> wrote:
> As indicated in the JBS bug, using a `PixelReader` to read a scaled image in
> HiDPI mode, for example an `@2x` image, to read more than one pixel will read
> data from the wrong location in the image, usually leading to an IOOBE.
>
> The bug is in the `getPixelAccessor` method of Prism Image. The method
> correctly creates a new `Accessor` if one hasn't already been created, but
> then it unconditionally wraps the current `Accessor` in a
> `ScaledPixelAccessor` if the scale is > 1. So the second time this method is
> called, it created another `ScaledPixelAccessor` that wraps the first
> `ScaledPixelAccessor`, meaning that the indexes into the pixel array are
> multiplied by 4. This continues for each new call to this method.
>
> The fix is to only wrap an `Accessor` in a `ScaledPixelAccessor` the first
> time, when it is created.
>
> I added a test, which is run for both scaled and unscaled images, to ensure
> that we get the right value when reading a pixel, and that reading it a
> second time returns the same result. Without the fix, the new test will fail
> with IOOBE when the scale factor is two. Related to this, I initially
> commented out the wrapping in a `ScaledPixelAccessor` entirely, just to see
> what would happen, and none of the existing tests caught it. The new test
> will catch this.
modules/javafx.graphics/src/main/java/com/sun/prism/Image.java line 654:
> 652: }
> 653: if (pixelScale != 1.0f) {
> 654: pixelaccessor = new ScaledAccessor<>(pixelaccessor,
> pixelScale);
is that really correct? I think you should not overwrite save the scaled
pixelAccessor in the instance variable - if I read that correct with this
change the following happens: if I call getPixelAccessor() 3 times I get a
ScaledAccessor wrapping a ScaleAccessor wrapping a ScaledAccessor, wrapping one
of ByteAccess, ByteRgbAccess, IntAccess
-------------
PR: https://git.openjdk.java.net/jfx/pull/389
