How to create overlapping semitransparent shapes o

Question

I'm trying to create image with semitransparent shapes drawn on transparent background. For some reason instead of staying transparent, the shapes are completely covering those beneath them. My code:

from PIL import Image, ImageDraw
img = Image.new("RGBA", (256, 256), (255,0,0,127))
drawing = ImageDraw.Draw(img, "RGBA")
drawing.ellipse((127-79, 127 - 63, 127 + 47, 127 + 63), fill=(0, 255, 0, 63), outline=(0, 255, 0, 255))
drawing.ellipse((127-47, 127 - 63, 127 + 79, 127 + 63), fill=(0, 0, 255, 63), outline=(0, 0, 255, 255))
img.save("foo.png", "png")

I would expect the result to look something like (except for background not being transparent):

but it looks like:

When I try to save it as GIF with img.save("foo.gif", "gif"), result is even worse. Circles are solid, no difference between outline and fill.

Answer 1

As I mentioned in a comment, ImageDraw.Draw doesn't do blending—whatever is drawn replaces whatever pixels that were there previously. To get the effect you want requires drawing things in a two-step process. The ellipse must first be drawn on a blank transparent background, and then that must be alpha-composited with current image (bg_img) to preserve transparency.

In the code below this a has been implementing in re-usable function:

from PIL import Image, ImageDraw


def draw_transp_ellipse(img, xy, **kwargs):
    """ Draws an ellipse inside the given bounding box onto given image.
        Supports transparent colors
    """
    transp = Image.new('RGBA', img.size, (0,0,0,0))  # Temp drawing image.
    draw = ImageDraw.Draw(transp, "RGBA")
    draw.ellipse(xy, **kwargs)
    # Alpha composite two images together and replace first with result.
    img.paste(Image.alpha_composite(img, transp))


bg_img = Image.new("RGBA", (256, 256), (255, 0, 0, 127))  # Semitransparent background.

draw_transp_ellipse(bg_img, (127-79, 127-63, 127+47, 127+63),
                    fill=(0, 255, 0, 63), outline=(0, 255, 0, 255))
draw_transp_ellipse(bg_img, (127-47, 127-63, 127+79, 127+63),
                    fill=(0, 0, 255, 63), outline=(0, 0, 255, 255))

bg_img.save("foo.png")

This is the image it created viewed in my image file editor app which renders semi-transparent portions of images with a checker-board pattern. As you can see the opaque outlines are the only part that isn't.

Answer 2

All 4th dimensions-transparency must be half. Also maybe problem is with drawing. Make mask for each ellipse and then sum up images. Last option- check pil function for bending images. That will propably be most certain and easiest solution.

The function for blending is:

PIL.Image. blend (im1, im2, alpha)