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# If your GZ-Doom has a stutter

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I don't know if anyone else had this problem but I'm getting back into Doom (my last post here was in July because I got a job haha) and GzDoom was looking like it had a really low framerate but it said it was like 120FPS. So I gave up after searching for why after a few hours, then I turned it on today and the first option I clicked fixed it. Turn rendering interpolation on. I have no idea what it means but it's smooth as butter :) sorry if this doesn't really belong here but I wanted to tell people I'm still alive (if anyone cares)

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What interpolation means is simple.

The game logic works at a fixed clock rate of 35 Hz. When actor (such as an imp, or the imp's fireball) move, they don't really translate from one point to another, they perform short teleportations: the imp is here, and now it moves there. Monsters move every few tics, depending on how their logic is coded; for the imp it moves by up to 8 map units every three tics. So basically that means that every ~0.085 second, the imp performs a small jump of 8 map units.

What interpolation does is that it smooths things up. The movement of the imp is predicted and the imp is made to visually (it only affects rendering, since it's rendering interpolation) make much smaller jumps. If you render at 120 FPS, you'll get a minuscule adjustment to the imp's position 120 times per second, instead of a small one 11 times per second.

Likewise, other types of movement (player, doors, platforms, projectiles, etc.) are also interpolated, so everything moves smoothly and the illusion of continuous movement instead of a series of short teleportation is preserved.

It's purely visual, as the game logic continues to work the way it always did.

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FWIW, simply trying to "perk up" the game logic's speed to 70 Hz or higher in order to have actually smooth internal workings is not as trivial as altering the various timings and speeds: the movement code with its many hacks and kludges becomes VERY problematic if you simply halve movement distances and timings. Walking near a wall now feels like putting a square peg into a round hole, linedef crossings suddenly feel "funny" etc.

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Gez said:

Likewise, other types of movement (player, doors, platforms, projectiles, etc.) are also interpolated, so everything moves smoothly and the illusion of continuous movement instead of a series of short teleportation is preserved.

Does it break certain things like instantly rising floors?

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kuchitsu said:

Does it break certain things like instantly rising floors?

No. As gez said, it's a purely visual effect. Even if something moves "instantly" between two frames (directly to its final position), it might get visually interpolated and appear to just be moving very fast. For what the game's logic is concerned though, it will be in its proper place when it's supposed to be.

It's not possible to "catch" things in an inconsistent state, because the game logic is still lockstepped at 35 Hz. N.B.: the player's view may move more smoothly than the actual player's actor. At most, some actions may feel funny/inconsistent to the human eye (e.g. monsters and floors may appear to move in ways they normally cannot), but in the end everything will be consistent engine-wise.

The only possible side effect will be that an interpolated visual may fool a human player to e.g. time a jump or aim differently than without it.

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@Maes: I'm pretty sure that kuchitsu actually meant: "Does it VISUALLY break certain things like instantly rising floors?" Like when a monster lurks in an illusio pit, then the pit's floor instantly raises, but the interpolation causes the monster to look like raising gradually (although incredibly fast) rather than instantly. Or when a monster teleports across the map, the interpolation makes the monster appear gradually (although even more incredibly fast) travelling through the space between the source and destination spots. I believe both of these actually happen.

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Interpolation is specifically disabled for some types of movement, instant floors and teleporter uses are included.

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Well, if not in GZDoom, I'm pretty sure that instant floor and teleporter uses are affected by interpolation at least in some other ports. I remember reading a couple posts / threads reporting and explaining such issues with instant floors and with teleporting.

EDIT: This and this is what I'm talking about.

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Well, as I said, it's possible for some things to look "funny", like a monster moving just a tiny bit on previous course before zig-zagging to another course etc. Monsters and items rising with floors also tend to look kinda broken, sometimes moving floors may appear to have reached their final position when they are not etc.

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Cool thanks for the explanation that helps a lot, I guess I always played with it on because it was really bugging me. And I play vanilla doom as well and never noticed it being that choppy

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Well, I wouldn't describe the overall effect as "choppy", that would kinda defeat the purpose as interpolation is supposed to make things smoother. However it may look "weird" or "funny", esp. when monsters perform weird sliding movements and appear to be strafing or walking backwards while still facing you.

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That's why monster movement interpolation can be deactivated separately from general movement interpolation.

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Maes said:

Well, I wouldn't describe the overall effect as "choppy", that would kinda defeat the purpose as interpolation is supposed to make things smoother. However it may look "weird" or "funny", esp. when monsters perform weird sliding movements and appear to be strafing or walking backwards while still facing you.

No I was talking about without it it seems choppy

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