If a game wants to run at a constant 60 FPS (no frameskip), the calculations for a frame need to last less than 1/60 = 16.7ms. For 30 FPS (1/2 frameskip), they need to last less than 1/30 = 33.3ms.
If calculations are too long and continue into the next frame, that frame will be skipped. For example, if calculations last 16.8ms, a whole frame will be skipped because the CPU was 0.1ms late.
Games known for having slowdowns are:
In the following graphs:
- The vertical lines are frame ticks (interrupts) given by the video controller, they always occur every 16.7ms (60Hz)
- Black lines are updated frames (different from the last one)
- Red lines are skipped frames (same as the last one)
- Cyan areas are the v-blank periods, during which the CPU updates VRAM
- Green zones are CPU calculations not causing frameskip
- Yellow zones are CPU calculations causing unwanted frameskip
60 FPS engine
If CPU calculations are always shorter than 16.7ms, each and every frame is updated.
If CPU calculations are longer than 16.7ms, the next frame is skipped. Multiple consecutive frames may be skipped (see below).
30 FPS engine
The game's engine voluntarily updates one frame out of two, making the game run at 30 FPS at best. The video controller always runs at 60 FPS.
This has the advantage of giving a more consistent framerate if the game tends to often exceed the 16.7ms limit, at the expense of animation which is less smooth overall.
If CPU calculations are always shorter than 33.3ms, one frame out of two is updated, as expected.
If CPU calculations are longer than 33.3ms, two or more frames are skipped. Multiple groups of frames can be skipped (see below).
Some game engines are smart enough to "resync" and skip only one frame if they detect that one was already accidentally skipped (doesn't rely on the LSB of a frame counter).
Skipping one or two frames from time to time is rarely perceived or felt by the player. Annoying slowdowns occur when the video appears to stall.
Perception of slowdowns depends on the spreading of updates:
- #-#-#-#-#-#-#-#- is 30 FPS, acceptable.
- ########-------- is also 30 FPS, but very bothering.
Game engine logic
Are there game engines stupid enough to try catching up on each and every frame, instead of skipping some ?