Adding transparency to PNGCanvas images

I've been playing around with PNGCanvas, and the one thing I can't figure out is how to add transparency to images. I'v开发者_运维百科e been trying to vary the alpha channel values, but it only adjusts the intensity of the color. Am I barking up the wrong tree using this library, or is there hope?

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You're barking up the wrong tree. Here's a quote from PNGCanvas.dump:

# 8-bit image represented as RGB tuples
# simple transparency, alpha is pure white

Looking further through its code, when you provide alpha it's just blending it with the background, and the final background is just white.