I’ve posted my question in the beginner’s section, but cannot find an answer. 
My problem is, for example, I have 4 RGB textures and 4 alpha channels. If I alpha blend the RGB textures, using different alpha channels, I’ll have to build 4*4 = 16 RGBA textures. This is a waste of memory, no?
I want to combine RGB and A without having to build 16 RGBA textures.
One way is to assign the vertex alpha value, but it is too coarse.
How can I solve my problem?
HELP ME!!! or tell me this is impossible in OpenGL. 
Try it with multitexture. That should solve your problem at least if the video card supports multitexture and if not do it in two passes.
Maybe I didn’t understanf you’re problem, but here’s an answer:
Create 4 RGBA textures.
Use multitexturing to render them.
see http://nehe.gamedev.net/tutorials/lesson22.asp
for a multitexturing tutorial.
Sorry that I couldn’t make me understood. I am not a native English speaker ;(
My problem is not multitexture or multipass.
Suppose I have 2 textures, one is grass, the other is rock. I want to draw a grassland with some rocky fields. I use grass as the basemap, then overlay rocks with blending enabled, so that the change doesn’t look too sharp. like the figure below
|---------|
| grass |
| _____ |
| |rock| |
| | | |
| ----- |
The alpha map of the left,right,upper and bottom sides and that of the upper-left, upper-right, bottom-left and bottom-right are all different.
then do I have to build 8 different RGBA textures, with the only difference in alpha map?
oops, the figure is distorted, i draw another.
'.'stands for grass
'*'stands for rock
…
…
…******…
…******…
…******…
…
…
I see your problem.
While I cannot give you much details since I’m not working on the Landscape part of my 3D engine right now, I can point you to the OutCast Game Engine description.
http://www.appeal.be/products/page1/Outcast_GDC/outcast_gdc_1.htm
I do think they use some king of ligtmap for modulating different terrains contribution to final color.
Try this way, I think it’s the good one.
I think the solution for blending two textures with an independtent alpha texure is the texture environment combine extension.
I saw a demo tha demonstated the very same effect, but I forgot the url