2d map editor ?

I am creating a 2d mapeditor, I wish to be able to load bitmaps of maps and place them on the terrain so the user can use the bitmap as a tracer if you know what I mean.

I have done this, what I really want to do is make this bitmap/texture transparent via a slider control so the user can bring it in and out to their desired transparency.

How do I draw a texture then edit it’s alpha value then redraw it again in the best way?

If you are using the environment mode GL_MODULATE for your textures, you can assign an alpha to the material/color you use for your object and change that. Not sure if this is the BEST way, but it seems to me that it should be faster than changing the alpha of the texture.