Distorted output when saving an image under Linux

[bertholddaum]

Describe the bug
Under some cirumstances saving an image under Linux through org.eclipse.swt.graphics.ImageLoader will distort the image.

To Reproduce
Create an image with a width that is not a multiple of the scanLinePad, e.g.

new ImageData(326, 224, 8, palette); (using the default scanLinePad of 4).

Fill this image with some drawings.
Save the image using ImageLoader.save()

Expected behavior
When the saved image is opened with a viewer, the image appears distorted.

** Diagnosis **
The class org.eclipse.swt.internal.NativeImageLoader ignores padding bytes in the source image.
Lines 316-317 and lines 331-332 should read

for (int y = 0, new_offset = 0, alphaIndex = 0; y < height; y++) {
    for (int x = 0, offset=y*imgData.bytesPerLine; x < width; x++, offset += n_channels, new_offset += bytes_per_pixel) {

instead of

for (int y = 0, offset = 0, new_offset = 0, alphaIndex = 0; y < height; y++) {
    for (int x = 0; x < width; x++, offset += n_channels, new_offset += bytes_per_pixel) {

Environment:

1. Select the platform(s) on which the behavior is seen:

• • [] All OS
• • Windows
• • [ x] Linux
• • macOS

2. Additional OS info (e.g. OS version, Linux Desktop, etc)
   Linux desktop, GTK 3

3. JRE/JDK version

Version since
Eclipse or SWT version since when the behavior is seen [e.g. 4.23]

Workaround (or) Additional context
Add any other context about the problem here.
Any known workarounds for the problem?

[iloveeclipse]

@bertholddaum : do you want to contribute a fix? Ideally you could also add a simple standalone SWT snippet that would demonstrate the problem.

[bertholddaum]

The fix actually is contained in the previous post. I find the bug quite obvious when looking at the code, but if you need a demonstration you could use the following snippet:

	Display display = Display.getDefault();
	PaletteData palette = new PaletteData(0xFF, 0xFF00, 0xFF0000);
	ImageData data = new ImageData(326, 224, 8, palette); // each image line will have two padding bytes
	Image image = new Image(display, data);
	GC gc = new GC(image);
	gc.setForeground(display.getSystemColor(SWT.COLOR_BLACK));
	gc.drawLine(163, 0, 163, 224); // draw vertical line
	gc.dispose();
	ImageLoader imageLoader = new ImageLoader();
	imageLoader.data = new ImageData[] { image.getImageData() };
	try (FileOutputStream out = new FileOutputStream("someFilePath")) {
		imageLoader.save(out, SWT.IMAGE_JPEG);
	} catch (IOException e) {
		e.printStackTrace();
	}