Drawing on a transparent image using Java SWT

Question

How do I create an in-memory fully transparent SWT image and draw a black line on it with antialias enabled?

I expect the result to include only black color and alph

Answer 1

To scale with transparency, I've found that I have to manually set the alpha byte array as shown below. So the alpha ends up with nearest-neighbor anti aliasing.

public static Image scaleImage(Device device, Image orig, int scaledWidth, int scaledHeight) {
    Rectangle origBounds = orig.getBounds();
    if (origBounds.width == scaledWidth && origBounds.height == scaledHeight) {
        return orig;
    }

    ImageData origData = orig.getImageData();
    ImageData imData = new ImageData(scaledWidth, scaledHeight, origData.depth, origData.palette);
    if (origData.alphaData != null) {
        imData.alphaData = new byte[imData.width * imData.height];
        for (int row = 0; row < imData.height; row++) {
            for (int col = 0; col < imData.width; col++) {
                int origRow = row * origData.height / imData.height;
                int origCol = col * origData.width / imData.width;
                byte origAlpha = origData.alphaData[origRow * origData.width + origCol];
                imData.alphaData[row * imData.width + col] = origAlpha;
            }
        }
    }
    final Image scaled = new Image(device, imData);
    GC gc = new GC(scaled);
    gc.setAntialias(SWT.ON);
    gc.setInterpolation(SWT.HIGH);
    gc.setBackground(device.getSystemColor(SWT.COLOR_WHITE));
    gc.fillRectangle(0, 0, scaledWidth, scaledHeight);
    gc.drawImage(orig, 0, 0, origBounds.width, origBounds.height, 0, 0, scaledWidth, scaledHeight);
    gc.dispose();
    return scaled;
}