[OpenJDK 2D-Dev] RFR: 8177393: Result of RescaleOp for 4BYTE_ABGR images may be 25% black

Phil Race philip.race at oracle.com
Wed May 17 17:44:45 UTC 2017

Bug: https://bugs.openjdk.java.net/browse/JDK-8177393
Webrev: http://cr.openjdk.java.net/~prr/8177393/

I have updated the bug report with a long evaluation and some additional
explanation of the proposed fix. I don't intend to duplicate all that here
so please go read it there. The following is a much briefer summary.

We are ending up in medialib native code that cannot interpret the Java 
2D child raster
it is handed. The child raster was an attempt to prevent rescaling of 
the alpha channel.

The proposed fix avoids creating that child raster and instead creates a 
which has an explicit lookup table for the alpha channel that does the 
identity lookup.

The fix has a couple of other updates.
(1) the "canUseLookupOp()" method now checks that the DataBuffer is
something that the medialib code used by LookupOp can handle.
This would have fixed the bug by itself although at the expense of always
falling back to slow Java code in RescaleOp. However it is still needed in
case the application itself supplies a child raster.

(2) The slow path in RescaleOp turns out to have the same bug that I think
was seen in some other bug report or fix. It is always re-scaling alpha as
well as colour components. I fixed that too.

Since the fix is entirely confined to RescaleOp any consequences - good 
or bad - are limited.

I ran the few regression tests we have that cover RescaleOp and also 
checked Java2Ddemo.

The commented out cases in the new regression test are to be handled 
under a different bug ID.

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