Questions about few flags

[Void Weaver]

1. What does
INTERPOLATEANGLES
DONTINTERPOLATE
and
SPF_INTERPOLATE flags in A_SetAngle\A_SetPitch functions?
I mean that I know in general what's 'interpolation' means, but how it work in therms of modding?

2. What's practice sense of VULNERABLE flag? I mean how taking of a some damage can affect a some non-shootable actor? o_0
EDIT:

An example of use: destructible decoration like a tech lamp. You can't break it with a gun shot, but a rocket will destroy it.

Another example: pipe bombs like in Duke Nukem.

The interest is that they don't need to have the SHOOTABLE flag, so they don't "snag" your autoaim.

Sooo, the VULNERABLE for non-shootable actors acts like SHOOTABLE but only by AoE? Then it make sense.

[Tartlman]

1. It just interpolates turning, just like you can interpolate moving. (example: instead of snapping from 30 to 60 degrees, it goes 30, 35, 40, etc...) It's probably much more noticeable with models.

2. Exactly. That way you can have actors that are destructible as collateral damage but won't get in the way of your normal combat.

[Void Weaver]

1. Oh, those flags are just for smooth movement and render with intermediate step pitch\angle values of models?

How to goes calculation of interpolation step?

In which cases it does matter for sprites too? I mean that doom perspective returns fixed angle values for sprite rendering (8 or 16 direction). Or I'm just misunderstood something?