Can glReadPixels get Alpha in color buffer?

Hi,all.
I used glDrawPixels to draw some pixels in GL_AUX0 ,then I want to read
these Pixels by glReadPixels. I use GL_RGBA as the format for draw and
read, but I found it seems all the pixels read has the same alpha value
of ONE. So I just wonder how to get the right alpha?

.

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