Add option to create transparent Image with alpha channel when using ImageGcDrawer [Multi-Monitor][HiDPI]

[wahlbrink]

Problem
When creating an image using the new ImageGcDrawer, the image is currently initialized as opaque white image (at least on Windows) and it is not possible to create an image with alpha transparency.
Or have I overlooked something?

Request
This requests a possibility to create an image using an ImageGcDrawer comparable to this classic code:

var imageData = new ImageData(width, height, 24, new PaletteData(0xff0000, 0x00ff00, 0x0000ff));
imageData.setAlpha(0, 0, 0);
var image = new Image(display, imageData);
GC gc = new GC(image);

Additional context
This is e.g. required for a multi-monitor/HiDPI compatible solution of issue #501 (draw a mark over an existing icon while preserving its transparency).

[HeikoKlare]

Did you already see the TransparencyColorImageGcDrawer? https://github.com/eclipse-platform/eclipse.platform.swt/blob/master/bundles/org.eclipse.swt/Eclipse%20SWT/common/org/eclipse/swt/internal/TransparencyColorImageGcDrawer.java

That one allows to define a transparency color, so you could initialize the image with that exact color to make everything transparent that is not explicitly drawn.

Otherwise, do you have a complete snippet what you would like to do? If I am not mistaken, the line imageData.setAlpha(0, 0, 0); will only set an alpha value for pixel (0,0), so I have to admit that I do not understand how that is supposed to work.

[wahlbrink]

Yes, I know the TransparencyColorImageGcDrawer, IMHO this is only a limited workaround, not a solution.

If I am not mistaken, the line imageData.setAlpha(0, 0, 0); will only set an alpha value for pixel (0,0), so I have to admit that I do not understand how that is supposed to work.

No, calling imageData.setAlpha the first time initializes the alpha channel with 0 for all pixels.

[HeikoKlare]

No, calling imageData.setAlpha the first time initializes the alpha channel with 0 for all pixels.

Okay, I see, thanks for the clarification. It's kind of hack by relying on the implicit initialization of the created array with zeros.

What if you just call gc.setAlpha(0) as the first operation of the drawer? Doesn't that work either?

[wahlbrink]

What if you just call gc.setAlpha(0) as the first operation of the drawer?

The function does what it should do, it sets the transparency for the drawing operations executed afterwards. But a call of setAlpha does not change the current image.

[HeikoKlare]

@wahlbrink can you please check whether using GC style SWT.Transparent in the ImageGcDrawer solves the issue for you, as introduced with #2073?

ImageGcDrawer drawer =  new ImageGcDrawer() {
	@Override
	public void drawOn(GC gc, int imageWidth, int imageHeight) {
		...
	};

	@Override
	public int getGcStyle() {
		return SWT.TRANSPARENT;
	}
};

[wahlbrink]

Yes, the new option works.

[HeikoKlare]

Great, thank you for the feedback! Thus closing this a resolved.