Hi! I’m an artist working on a 2D mobile platformer Unity game, and recently created a jumping sprite sheet of 6 frames, meant to be run in 12 fps. The programmer in the team sent feedback that the sprite sheet was not smooth, and should be meant to run in 30 fps (mobile) or 60 fps (PC). 

I’ve never heard of sprite sheet animations done in 60 fps, so I did quite a bit of research on the net, which states some games use variable frame rates, with sprite sheets running at a lower frame rate than the game itself. 

1. Just wanted to check if this holds true for most games, or if I misunderstood something? 

2. Is 12 fps not used to animate sprite sheets in 2D mobile platform games?

Any help is much appreciated, thank you so much!

I'm responding from the point of view of programming rather than art, so I don't know if this'll be useful. That said:

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I’ve never heard of sprite sheet animations done in 60 fps, so I did quite a bit of research on the net, which states some games use variable frame rates, with sprite sheets running at a lower frame rate than the game itself. 

1. Just wanted to check if this holds true for most games, or if I misunderstood something?

I think that's probably true, generally speaking. Different games handle timing differently, but frame rates for sprite animation can certainly be independent of the game's frame rate (and probably usually are, I'd imagine). Also, I'd guess sprite animation frame rates are probably often less than the game frame rate.

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2. Is 12 fps not used to animate sprite sheets in 2D mobile platform games?

I don't think there's anything particular special about platform games here, or maybe even about mobile games. That said, 12 FPS certainly seems reasonable for certain art styles. Maybe artists can weigh in on that part.

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The programmer in the team sent feedback that the sprite sheet was not smooth, and should be meant to run in 30 fps (mobile) or 60 fps (PC).

It might be worth asking for some clarification on this. 'The sprite sheet was not smooth' is a bit ambiguous (that could, for example, mean that the graphics are too pixelated), but based on the rest of the discussion I assume they're talking about smoothness of animation.

With respect to that, for me at least it's not immediately clear what 'meant to run at 30 or 60 FPS' means. If you haven't already, maybe you could just ask the developers what they want the frame rate of the animation to be (that is, how many frames per second), and what its duration should be.

Again, I'm answering as a programmer. Maybe some artists can give you a better answer.

It might actually help if you posted the sprite sheet or, better yet, an animated GIF of what you are talking about. However, many games run at 60/30 fps, but feature sprite art with lower frame rate animations.

Edit: There is definitely some miscommunication and/or inexperience happening between you and the programmer though.

It is totally ok to have a sprite animated at a different rate than the game's framerate. There is no restriction here. For a 12 FPS sprite you will see a new sprite frame every 83.3ms .. If the game is animating other stuff at 60 FPS (e.g. scrolling background) then those elements will change look on the screen every 16.6ms.

The result of combining such differences is not new. Many modern 2D games do it, so you can see how it looks and feels. Now, 12 FPS sprites won't look smooth against 30/60 FPS animations of course.. but this doesn't mean it's a mistake that must be fixed. Disney has been animating characters at lower frame rates than the background since dawn of cartoons ? It is just a question of style and feel and whether that fits with the theme of the game and the art/animation direction you decide to take.

That being said, 12 FPS sprites will show better on a 60 FPS game than a 30 FPS game. This is because 60/12=5 .. an integer number. So every 5 frames @60 FPS you increment your sprite frame index by 1. Whereas on 30FPS you can't do that. Meaning some sprite frames will stay longer on screen than others, hence a feeling of jittering animation as an end result.. so probably find a sprite anim frame rate that fits with both 30 and 60 FPS if you plan to have the game support both rates. For example, you can either go 10 FPS sprite anims or 15 FPS. The former being less work for you, and the latter more work ?

Good luck!