[Python] How to change cairo context source color for grayscale images

I am trying to create images in greyscale using Cairo, however, I am having issues controlling the pixel intensity of the pen.

For example, in an RGB color image, I would use this code to paint the background a shade of red with a pixel intensity of 127 in the red channel:

surface = cairo.ImageSurface(cairo.FORMAT_RGB24, WIDTH, HEIGHT)
ctx = cairo.Context(surface)
ctx.set_source_rgb(.5, 0, 0)
ctx.rectangle(0, 0, WIDTH, HEIGHT)
ctx.fill()


I can't find any equivalent code for a grayscale image. I can't simply use the alpha channel because then I can't draw circles over rectangles. For example, take the below code snippet, where I would like to have:

• A grey background
• A black circle on top of the grey background

surface = cairo.ImageSurface(cairo.FORMAT_A8, WIDTH, HEIGHT)
ctx = cairo.Context(surface)
ctx.set_source_rgba(0, 0, 0, 0.5)
ctx.rectangle(0, 0, WIDTH, HEIGHT)
ctx.fill()
ctx.set_source_rgba(0, 0, 0, 0.0)
ctx.arc(WIDTH//2, HEIGHT//2, r, 0, 2*math.pi)
ctx.fill()


In the above code, the black circle won't appear because its alpha channel is lower than the gray background. How can I fix this?

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