Eye glow on dark?

Hello everyone! So, I got blender on steam some time ago and I have 152 hours in it and I was wondering:
Is there any way to make an eye (or other object) glow only when it’s dark? Like:
When it’s dark, it will appear a glow, like a specular somewhere in the eye/object, which you can see in the dark.

Yes of course. You can for example make the eye incandescent with an emission shader. Or you can render the eyes in another renderlayer and play with it in post prod. You can also assign ID (object or material) to get the mask of it and make something in the compositor. It depend also of what result you want.

Also, don’t forget that movies consist of shots, each one of which is set-up and rendered separately, albeit (maybe!) from the same models, and from fixed camera-positions, all of which can be shared among shots using Blender’s linked-assets system.

You might well have a “light emitter” as part of the eye which is enabled only for the night shots. (Where, in fact, the other parts of the eye may as well be left out because they’re geometry you can’t see.) The night-eye might even just consist of a very small disc. Most of the model might be switched-off, or substituted, because “if you can’t really see it, the computer doesn’t need to waste your time on it.”

The best approach is to first shoot shots, using simple OpenGL Preview renders. “Cut the film together” at this point, maybe with “night is day.” Decide at this point exactly where the cat-eyes will appear, and, from this, exactly what you need to pull-it-off convincingly. Experiment with this, once again using OpenGL, until you’re satisfied.

Editing is the step in which a lot of footage hits the floor, as you work for nuance and timing. That footage may as well be cheap. [i]Quickly[/i shoot anything you think you might need. (“Say, what if we tried this?” Try it.) Throw most of it on the floor with abandon. When one second’s worth of film took about one second to produce, you can be very creative.

Now, you’ve got an exact model shot, in OpenGL, that you simply need to replicate. This shot, like its prototype, will use exactly the same (linked) models, camera setup, lighting, but it will be rendered and composited to suit … rendering only the specific frames that, you now know, will be required. The “final” for this shot, when dropped-in where its OGL proxy used to be, will of course match it exactly.

(And that “OGL proxy” might actually be the foundation of a finished shot, upon which you use other rendering techniques merely to elaborate. OGL is pretty sophisticated these days. In the name of saving time to get something that is good enough, anything goes.)