If an image has RGBA encoding and all pixels are opaque, then
`SwingFXUtils.fromFXImage(image, null)` selects TYPE_INT_ARGB_PRE pixel format
but the JPEG ImageIO writer cannot encode this alpha-bearing image pixel format
since it has no support for it and returns false; so an empty byte array is
created.
Issue is when `bimg` (used to store the returned pixel data from `fromFXImage`)
is null`, fromFXImage` uses the JavaFX PixelReader’s storage format, not the
alpha values of individual pixels.
For an RGBA PNG, JavaFX commonly decodes the pixels into premultiplied
ARGB/BGRA, so `fxFormat.getType()` is one of INT_ARGB_PRE/BYTE_BGRA_PRE so
`getBestBufferedImageType` returns `BufferedImage.TYPE_INT_ARGB_PRE` because
the pixel format has an alpha component. It does not inspect whether every
alpha value happens to be 255.
The existing `fromFXImage` code only calls `checkFXImageOpaque()` when the
caller supplies a non-null `bImg` BufferedImage.
If `bImg `is null, all-opaque RGBA PNG produces an alpha-capable BufferedImage
i.e., BufferedImage.TYPE_INT_ARGB_PRE
if (bimg == null) {
bimg = new BufferedImage(iw, ih, prefBimgType);
}
and since JPEG has no standard alpha channel so it cannot store transparency,
so when `ImageIO.write(image, "jpg", out)` runs, ImageIO looks for a registered
JPEG writer which can encode that pre-multiplied-alpha image type,
but the JPEG writer rejects an alpha-bearing BufferedImage, so `ImageIO.write`
finds no suitable writer and returns false
so OutputStream is not written into and have 0 bytes
[Basically the JPEG writer does not check whether the alpha values are all 255;
it only sees that the input image has an alpha channel and declines to write it]
The proposed JavaFX change avoids the rejection for an RGBA-formatted but fully
opaque image by returning TYPE_INT_RGB, which JPEG can encode.
ie., for a JavaFX image with an alpha-capable format but only opaque pixels,
`fromFXImage(image, null)` is made to choose RGB format rather than ARGB.
Additionally, checkFXImageOpaque is improved to scan one row at a time instead
of costly full-image scan so that unnecessary Color object for each pixels is
not created just to inspect alpha.
A regression subtest is added to existing testcase
---------
- [x] I confirm that I make this contribution in accordance with the [OpenJDK
Interim AI Policy](https://openjdk.org/legal/ai).
-------------
Commit messages:
- 8388450: ImageIO.write(SwingFXUtils.fromFXImage()) creates 0 length JPEG for
some images
Changes: https://git.openjdk.org/jfx/pull/2254/files
Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2254&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8388450
Stats: 36 lines in 2 files changed: 25 ins; 4 del; 7 mod
Patch: https://git.openjdk.org/jfx/pull/2254.diff
Fetch: git fetch https://git.openjdk.org/jfx.git pull/2254/head:pull/2254
PR: https://git.openjdk.org/jfx/pull/2254
