Had a minor revelation this morning. About fundamental physics. Yes, I'm weird this way. Thanks for noticing.
Photon energy (best known as wavelength or frequency) times proper time (local flow of time) is a constant. As gravity increases, proper time decreases, and photons gain a proportional amount of energy. We call this blue shift.
As gravity decreases, proper time increases, and photons lose a proportional amount of energy. We call this red shift.
Interesting. It's almost as if gravity has no influence on photon energy - just our perception of it.
Relative velocity still does alter photon energy. A photon emitted in the direction of travel still has more energy than a photon emitted in the opposite direction, from the standpoint a stationary particle used as a target. Of course, that's probably because of the energy differential caused by the velocity deformation around the emitting particle.
So, the only way to tell the actual energy of a photon, is to intercept one emitted at exactly 90 degrees to the path of travel of the emitting particle, along an equal gravitational gradient, by a stationary particle, or one traveling at ninety degrees to the angle of interception. In that (and only that) circumstance, the energy delivered by the photon will be exactly equal to the energy it was emitted with.