Improve JPEG writing performance when there's transparency in the source image

[jmiettinen]

I was looking into memory usage of some of the services we run at work and there ImmutableImage#removeTransparency ended up allocating surprisingly lot.
This affect performance in two ways: needing more GC to be run and needing to create the objects (Pixel) themselves.

I first removed the creation of Pixel[] in various method and replaced it with a streaming approach but then realized that while this takes allocations down, performance still leaves something to consider: operating pixel by pixel means that to get ARGB values of the image, we'll need to, for each pixel, consult color model.

In the end, it's better to use Graphics2D which is meant for operations like this.
However, using it changes results. My understanding is that there's a difference in how alpha compositing is done in AWT and what we're doing in MutableImage#replaceTransparencyInPlace.

Tasks this PR consists of

• Write a simple test case for the affected functionality.
• Write a JMH test case for the affected functionality. JpegWriterTest uses the results from before I started as a gold standard.
• Decide if the new results of transparency removal are correct.
• Based on the above step, remove potentially vestigial parts such as PixelConsumer, PixelUtils.replaceTransparencyWithColor or whole ImmutableImageTest.

Test case "JpegWriterTest.jpeg writer replaces transparent with white" fails now because of the alpha compositing issue.

Fileresults_fyi.md records some performance effects of the changes.

[sksamuel]

However, using it changes results. My understanding is that there's a difference in how alpha compositing is done in AWT and what we're doing in MutableImage#replaceTransparencyInPlace.

What's the change ?

[jmiettinen]

As described shortly here, we get different results for ARGB -> RGB transformation.

So ARBG values of (165, 173, 0, 17) was earlier transformed into (201, 90, 101) and now into (202, 90, 101).

That's why the test is failing as it's comparing that we'll get exactly the same pixels and after JPEG compression, this change one intensity level change turns into a slightly bigger change.

My guess for the reason would be that JDK uses floating-point-based math and PixelTools.replaceTransparencyWithColor uses integer math.

[jmiettinen]

So what could I do to get this forward?

Would comparing results of transparency removal to, say, Imagemagick's results make sense?

My understanding of the difference we observe in this PR is that PixelTools.replaceTransparencyWithColor uses integer math and thus truncating "rounding" whereas JDK-method uses floating point rounding where 201.9x gets rounded to 202 when it earlier was rounded (truncated) to 201.

[sksamuel]

I think we should align with the JDK then. If you want to make a PR to change this I will make a release. @jmiettinen

[jmiettinen]

I can do it, probably in the next few days of time.

[sksamuel]

Awesome

[sksamuel]

How are things looking on this @jmiettinen

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