What creates the cut-off in a an automotive reflector headlight

Are the beam cut-off and cut-off shape (ramps, steps, or straight) created by the reflector itself or by the bulb shield? Or is it done differently altogether?

It is a combination of the bulb shield, reflector geomatry and the glas shape
In old cars the reflector was simple and most was done by the glas lens

as moders cars have smooth glas its all done by reflector geomatry and bulb shield

The bulb shield isn't to create a cutoff. It's to prevent direct viewing of the filament.

Also note that sharpness of cutoff is not an extremely good indicator of a lamp's performance, and that as cutoff sharpness improves, a headlamp is both more easily aimable and more sensitive to misaim. Severely sharp cutoffs also can produce some annoyance for the driver, and a feeling that they're not seeing as well because there's so little visible above the cutoff. For other drivers it can result in obnoxious dim/OUCH! cycles with the first car's motion on the road (bumps, undulations etc).

With the older glass style lenses, which often included fresnel lenses, it was obvious that the headlight "lens" was part of the optics that created the beam. With modern clear, usually made from untextured lexan "lenses" I guess it's just the reflector that creates the beam and cut-off pattern.
I didn't mean to imply that a well-defined or sharp cut-off in itself implies better performance. This would be another topic altogether.

nocte-diem said:

Are the beam cut-off and cut-off shape (ramps, steps, or straight) created by the reflector itself or by the bulb shield? Or is it done differently altogether?

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You might want to clarify if you actually have reflector type or projector type headlights (projectors look similar to "eye balls").
In a projector there is a strip of metal blocking part of the light beam, that strip of metal has a step on it.
A "bixenon" projector is used for both low beam and high beam. The strip of metal moves out of the way to allow the high beam performance and eliminates the cutoff. This is accomplished by the metal strip being hinged and moved out of the way using a solenoid. If the projector is only used for low beam, the strip is fixed in place, not movable.

The black tip of a bulb is for blocking direct glare. But if you look at the shield alongside the low beam filament in an H4 bulb, you will see it has something to do with beam pattern. And if you light that low beam bulb in an H4 lamp that is missing its lens, you will still see a cutoff on the wall.

Hamilton Felix said:

The black tip of a bulb is for blocking direct glare. But if you look at the shield alongside the low beam filament in an H4 bulb, you will see it has something to do with beam pattern.

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I assumed the question was asked referring to the bulb shield in a reflector lamp assembly, not the small shield in an H4. That's not really a "bulb shield" (in the shower this morning, it dawned on me that DUH it's a filament shield), since it doesn't shield the bulb from direct view.

You're right. Those internal items are filament shields. I wasn't thinking. I've certainly seen a number of different filament shields, even in sealed beams.