n = near
f = far
_ZBufferParams =[(1-f/n),f/n,x/f,y/f]
z = depth buffer Z-value (f+n)/(f-n)+2*f*n/(EZ*(f-n))
EZ  = eye Z value
LZ  = depth buffer Z-value remapped to a linear [0..1] range (near plane to far plane)
LZ2 = depth buffer Z-value remapped to a linear [0..1] range (eye to far plane)

GL:
EZ  = (2*n*f)/(f+n - z * (f-n))
    = (2*n*f)/(f+n - (f+n)+2*f*n/EZ)
    = (2*n*f)/(2*f*n/EZ)
    = 1/(1/EZ)
    = EZ

LZ  = (EZ-n)/(f - n)
    = n * (z + 1.0) / (f + n - z * (f - n))

LZ2 = EZ/f
    = (2n) / (f + n - z * (f - n))

LZ2 in two instructions:
LZ2 = 1.0 / (_ZBufferParams.x * z + _ZBufferParams.y)
    = 1.0/ b
b = (1-f/n)*((f+n)/(f-n)+2*f*n/(EZ*(f-n))) + f/n

好吧，我还是没推出来