How to Determine Value at Discontinuity Points in VTC Diagram (VDD/2) in Digital Electronics

Hi guys, in idea VTC diagram in digital microelectronics, there's a "discontinuity points" which it's in my case VDD/2 .. what's confusing me, what's the value on that point?!! because the graph is like step function so ..how do I determine the value on that point? if we aren't determining..so is  a term "discontinuity points" in digital electronics  something that we can deal with it over all subjects in micro-digital-electronics?!

I'm alot confusing about "discontinuity points" term..can someone explain it to me to be more sense-able to deal with it over my calculations?!

Voltage transfer characteristics usually don't have discontinuities.  For example, here's the transfer characteristic of a typical CMOS inverter:At the transition point the transfer characteristic is steep, but it's continuous.  If you're doing digital design you can usually pretend it's discontinuous provided that the input voltage is either a solid logic 1 or a solid logic 0.  If you can't make that assumption or if you're using the CMOS inverter as an analog amplifier, you'll want to consider the continuous characteristic.