Slowdown: Difference between revisions
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Slowdowns are perceived during gameplay as slow picture updates (frameskip), this | Slowdowns are perceived during gameplay as slow picture updates (frameskip). On the NeoGeo, this happens when the [[68k|CPU]] is not able to update the [[VRAM]] often enough to animate sprites smoothly. | ||
If a game wants to run at a constant [[framerate|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 a game wants to run at a constant [[framerate|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 | 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: | Games known for having slowdowns are: | ||
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*[[Blazing Star]] | *[[Blazing Star]] | ||
*[[Metal_Slug_2_-_Super_Vehicle-001/II|Metal Slug 2]] | *[[Metal_Slug_2_-_Super_Vehicle-001/II|Metal Slug 2]] | ||
*[[Shock Troopers]] | *[[Shock Troopers]] | ||
*[[Twinkle Star Sprites]] | *[[Twinkle Star Sprites]]... | ||
In the following graphs: | In the following graphs: | ||
*The vertical lines are frame ticks given by the | *The vertical lines are frame ticks ([[68k interrupts|interrupts]]) given by the video controller, they always occur every 16.7ms (60Hz) | ||
**'''Black lines''' are updated frames (different from the last one) | **'''Black lines''' are updated frames (different from the last one) | ||
**<span style="color:#7F0000">'''Red lines'''</span> are skipped frames (same as the last one) | **<span style="color:#7F0000">'''Red lines'''</span> are skipped frames (same as the last one) | ||
*<span style="color:#007F7F">'''Cyan areas'''</span> are the | *<span style="color:#007F7F">'''Cyan areas'''</span> are the v-blank periods, during which the CPU updates VRAM | ||
*<span style="color:#007F00">'''Green zones'''</span> are CPU calculations not causing frameskip | *<span style="color:#007F00">'''Green zones'''</span> are CPU calculations not causing frameskip | ||
*<span style="color:#7F7F00">'''Yellow zones'''</span> are CPU calculations causing unwanted frameskip | *<span style="color:#7F7F00">'''Yellow zones'''</span> are CPU calculations causing unwanted frameskip | ||
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[[File:Lag60fs.png]] | [[File:Lag60fs.png]] | ||
If CPU calculations are longer than 16.7ms, the next frame is skipped. Multiple consecutive frames | If CPU calculations are longer than 16.7ms, the next frame is skipped. Multiple consecutive frames may be skipped (see below). | ||
=30 FPS engine= | =30 FPS engine= | ||
The game's engine voluntarily updates one frame out of two, making the game run at 30 FPS at best. | 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. | |||
==No slowdown== | ==No slowdown== | ||
[[File:Lag30.png]] | [[File:Lag30.png]] | ||
If CPU calculations are always shorter than 33.3ms, one frame out of two is updated. | If CPU calculations are always shorter than 33.3ms, one frame out of two is updated, as expected. | ||
==Slowdowns== | ==Slowdowns== | ||
[[File:Lag30fs.png]] | [[File:Lag30fs.png]] | ||
If CPU calculations are longer than 33.3ms, two or more frames are skipped. Multiple groups of frames can be skipped (see | 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 | 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). | ||
=Perceived slowdown= | =Perceived slowdown= | ||
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Are there game engines stupid enough to try catching up on each and every frame, instead of skipping some ? | Are there game engines stupid enough to try catching up on each and every frame, instead of skipping some ? | ||
[[Category: | [[Category:Code]] | ||
[[Category:Video system]] | [[Category:Video system]] | ||
Latest revision as of 06:17, 24 November 2018
Slowdowns are perceived during gameplay as slow picture updates (frameskip). On the NeoGeo, this happens when the CPU is not able to update the VRAM often enough to animate sprites smoothly.
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
No slowdown
If CPU calculations are always shorter than 16.7ms, each and every frame is updated.
Slowdowns
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.
No slowdown
If CPU calculations are always shorter than 33.3ms, one frame out of two is updated, as expected.
Slowdowns
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).
Perceived slowdown
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 ?