RFR: 8238954: Improve performance of tiled snapshot rendering [v14]

[Kevin Rushforth]

On Tue, 30 Jun 2020 09:44:33 GMT, Frederic Thevenet <github.com+7450507+fthevenet at openjdk.org> wrote:

>> Issue JDK-8088198, where an exception would be thrown when trying to capture a snapshot whose final dimensions would be
>> larger than the running platform's maximum supported texture size, was addressed in openjfx14. The fix, based around
>> the idea of capturing as many tiles of the maximum possible size and re-compositing the final snapshot out of these, is
>> currently only attempted after the original, non-tiled, strategy has already failed. This was decided to avoid any risk
>> of regressions, either in terms of performances and correctness, while still offering some relief to the original
>> issue.  This follow-on issue aims to propose a fix to the original issue, that is able to correctly decide on the best
>> snapshot strategy (tiled or not) to adopt before applying it and ensure best performances possible when tiling is
>> necessary while still introducing no regressions compared to the original solution.
>
> Frederic Thevenet has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Prevent attempt to render tiles with a 0 sized dimension.

I've finished my testing on 4 platforms: Windows 10, macOS 10.15, Ubuntu 16.10 (physical), Ubuntu 20.04 (VM with
forceGPU set to true). All looks good.

The code looks good too with a couple questions below.

tests/system/src/test/java/test/javafx/scene/Snapshot2Test.java line 31:

> 30: import java.util.concurrent.ThreadLocalRandom;
> 31: import java.util.stream.IntStream;
> 32:

These are now unused imports.

modules/javafx.graphics/src/main/java/com/sun/javafx/tk/quantum/QuantumToolkit.java line 1569:

> 1568:                         }
> 1569:                         // Find out if it is possible to divide up the image in tiles of the same size
> 1570:                         int tileWidth = computeTileSize(w, maxTextureSize);

Very minor: this comment seems like a hold-over from when the method was computing an optimum size. Now the caller
doesn't know about "tiles of the same size".

modules/javafx.graphics/src/main/java/com/sun/javafx/tk/quantum/QuantumToolkit.java line 1557:

> 1556:                 // A temp QuantumImage used only as a RTT cache for rendering tiles.
> 1557:                 var tileRttCache = new QuantumImage((com.sun.prism.Image) null);
> 1558:                 try {

In the case of a snapshot that isn't tiled, you will end up creating and disposing a dummy `QuantumImage` that is never
used. Maybe it would be worth initializing it to null here, constructing the object in the `if (h > maxTextureSize ||
w > maxTextureSize)` block, and checking for null before disposing in the finally block?

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PR: https://git.openjdk.java.net/jfx/pull/112