On Mon, 10 Aug 2026 08:54:30 GMT, Prasanta Sadhukhan <[email protected]>
wrote:
> 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).
modules/javafx.swing/src/main/java/javafx/embed/swing/SwingFXUtils.java line
209:
> 207: int[] pixels = new int[iw];
> 208: WritablePixelFormat<IntBuffer> format =
> 209: PixelFormat.getIntArgbPreInstance();
just curious: why is this line broken? it fits in 120 columns just fine. time
to update the formatting rules?
modules/javafx.swing/src/main/java/javafx/embed/swing/SwingFXUtils.java line
213:
> 211: pr.getPixels(0, y, iw, 1, format, pixels, 0, iw);
> 212: for (int pixel : pixels) {
> 213: if ((pixel >>> 24) != 0xff) {
I would have done
`((pixel & 0xff000000) != 0xff000000)`
but I think there is no difference in performance whatsoever
modules/javafx.swing/src/main/java/javafx/embed/swing/SwingFXUtils.java line
259:
> 257: PixelFormat<?> fxFormat = pr.getPixelFormat();
> 258: boolean srcPixelsAreOpaque = false;
> 259: boolean opacityMatters = bimg == null ||
minor: this is calculated even when it's not needed. could it be moved to L267?
modules/javafx.swing/src/main/java/javafx/embed/swing/SwingFXUtils.java line
266:
> 264: case INT_ARGB:
> 265: case BYTE_BGRA_PRE:
> 266: case BYTE_INDEXED:
question: this switch statement is missing `BYTE_BGRA`. is this a problem?
tests/system/src/test/java/test/javafx/embed/swing/SwingFXUtilsTest.java line
89:
> 87: @Test
> 88: public void testOpaqueArgbImageCanBeWrittenAsJpeg() throws Exception {
> 89: WritableImage image = new WritableImage(2, 1);
would it make sense to iterate over every `PixelFormat.Type` using
`WritableImage(PixelBuffer)` constructor to make sure we are getting a
meaningful result in each case?
-------------
PR Review Comment: https://git.openjdk.org/jfx/pull/2254#discussion_r3753259163
PR Review Comment: https://git.openjdk.org/jfx/pull/2254#discussion_r3753246858
PR Review Comment: https://git.openjdk.org/jfx/pull/2254#discussion_r3753276434
PR Review Comment: https://git.openjdk.org/jfx/pull/2254#discussion_r3753288191
PR Review Comment: https://git.openjdk.org/jfx/pull/2254#discussion_r3753383492
