by Jojon » Tue Jun 23, 2009 1:10 pm
As Nad said, though, I wouldn't muck about with transparencies and such, but just make the texture gradient fade to pure black and use "Add" blending mode for the texture layer in Blender. With additive blending, the RGB colour component values will be added to those of the stuff behind the sprite -- not only is this easier and saves you an alpha channel, but it assures the glare always brightens the image, which may not be the case with mix mode.
The maximum brightness and falloff profile of your gradient depends on the atmosphere you are looking for - if the locale has dense dusty air diffusing light, you may want to make something like a circle that kind of looks like a cone (linear) or sphere (sinus) seen from above, whereas for thin clean air, something that drops fast from the centre and slower towards the perimeter, like a "concave" pointy cone (cosine).