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

I was able to make this work, although it feels a bit hacky:

Display display = Display.getDefault();

int width = 10;
int height = 10;

Image canvas = new Image(display, width, height);

GC gc = new GC(canvas);

gc.setAntialias(SWT.ON);

// This sets the alpha on the entire canvas to transparent
gc.setAlpha(0);
gc.fillRectangle(0, 0, width, height);

// Reset our alpha and draw a line
gc.setAlpha(255);
gc.setForeground(display.getSystemColor(SWT.COLOR_BLACK));
gc.drawLine(0, 0, width, height);

// We're done with the GC, so dispose of it
gc.dispose();

ImageData canvasData = canvas.getImageData();
canvasData.alphaData = new byte[width * height];

// This is the hacky bit that is making assumptions about
// the underlying ImageData.  In my case it is 32 bit data
// so every 4th byte in the data array is the alpha for that
// pixel...
for (int idx = 0; idx < (width * height); idx++) {
    int coord = (idx * 4) + 3;
    canvasData.alphaData[idx] = canvasData.data[coord];
}

// Now that we've set the alphaData, we can create our
// final image
Image finalImage = new Image(canvasData);

// And get rid of the canvas
canvas.dispose();

After this, finalImage can be drawn into a GC with drawImage and the transparent parts will be respected.