如何使用PIL在半透明背景上创建重叠的半透明形状

问题描述：

我正在尝试创建具有在透明背景上绘制的半透明形状的图像.由于某种原因而不是保持透明，这些形状完全覆盖了它们下面的形状.我的代码:

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")

我希望结果看起来像(背景不是透明的除外):

，但看起来像:

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

but it looks like:

当我尝试使用img.save("foo.gif", "gif")将其另存为GIF时，结果甚至更糟.圆圈是实心的，轮廓和填充之间没有区别.

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.

答

正如我在评论中所提到的，ImageDraw.Draw不会进行混合-绘制的内容将替换以前存在的任何像素.要获得所需的效果，需要分两步进行绘制.椭圆必须首先在空白的透明背景上绘制，然后必须为由alpha合成与当前图像(bg_img)保持透明.

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.

在下面的代码中，a已在可重用功能中实现:

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")

这是在我的图像文件编辑器应用程序中查看的它创建的图像，该图像使用棋盘图案渲染图像的半透明部分.如您所见，不透明的轮廓不是唯一的部分.

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.

如何使用PIL在半透明背景上创建重叠的半透明形状

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