What is the color value "a" for?

[earth_walker]

I know what "r" "g" "b" means, and guessed "a" is the priority of displaying, such as (1,0,0,1) should be infront of (0,0,1,0.5), but according to the test, this is not true.

Can you tell me what does it mean?

Thank you!

[starman]

'a' is for "alpha" and is used for transparency and blending. Read the Red Book section on Blending for a full description:
http://tassadar.physics.auth.gr/~chameleon/OpenGL/The%20Red%20Book/chapter06.htm#1

[earth_walker]

Thank you!