#FOG: meet the mysterious man

i would introduce you the the man behind the good moment. yup, he is the mr glDepthMask. he had help me to make gl_depth_test and mr foggy became friends.
actually, glDepthMask is a function i used to make the depth buffer become read-only. why? when this glDepthMask(GL_FALSE), it become read-only right? so, it will preserve the depth value, but it will not eliminate any opaque/solid object (if the opaque object is behind my billboard). so, it then blend perfectly, mr foggy and any opaque object. if i dont't use this glDepthMask, it will be the same as before (see previous pics). previously, the billboard will not blend with other billboard and opaque object. so we can see the rectangular shape of the biilboard. so, mr foggy will not as realistic as real fog.