cannot mix two alpha images

[jijung]

Dear all,
Hello.

Now I am trying to mix two alpha images.
The images are created by videotestsrc and videobox.

gst-launch-1.0 videotestsrc pattern=10 ! video/x-raw, width=640, 
height=480 ! videobox border-alpha=0 left=-640 ! videomixer name=mix 
background=3 ! xvimagesink videotestsrc ! video/x-raw, width=640, 
height=480 ! videobox border-alpha=0 right=-640 ! mix.

I expected this pipeline merges two video patterns into one, but it just 
shows the second pattern image.

If I set an alpha value to each sink of videomixer, the pipeline shows 
two merged images.
(I don't want to use alpha blending.)

gst-launch-1.0 videotestsrc pattern=10 ! video/x-raw, width=640, 
height=480 ! videobox border-alpha=0 left=-640 ! videomixer name=mix 
background=3 sink_0::alpha=0.5 sink_1::alpha=0.5  ! xvimagesink 
videotestsrc ! video/x-raw, width=640, height=480 ! videobox 
border-alpha=0 right=-640 ! mix.


Do you have any idea about this?

Thank you.

Best regards,
Jae-Il Jung