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Periodic effects are waveform effects. DirectInput defines the following waveforms.
An envelope can be applied to periodic effects. See the example in Basic Concepts of Force Feedback.
The phase of a periodic effect is the point along the waveform where the effect begins. Phase is measured in hundredths of a degree, from 0 to 35,999. The following table indicates where selected phase values (in degrees) lie along the various waveforms. Max is the top (+) or bottom ( - ) of the wave, and Mid is the midpoint, where no force is applied in either direction.
| Waveform | 0 | 90 | 180 | 270 |
|---|---|---|---|---|
| Square | +Max | +Max | - Max | - Max |
| Sine | Mid | +Max | Mid | - Max |
| Triangle | +Max | Mid | - Max | Mid |
| SawtoothUp | - Max | - Max/2 | Mid | +Max/2 (reaches +Max just before the cycle repeats) |
| SawtoothDown | +Max | +Max/2 | Mid | - Max/2 (reaches - Max just before the cycle repeats) |
A driver may round off a phase value to the nearest supported value. For example, for a sine effect some drivers support only values of 0 and 9,000 (to create a cosine); for other effects, only values of 0 and 18,000 are supported.
You can also pass any other globally unique identifier (GUID) obtained by the IDirectInputDevice8::EnumEffects method, provided the low byte of the dwEffType member of the DIEffectInfo structure (DIDFT_GETTYPE(dwEffType)) is equal to DIEFT_PERIODIC. In this way, you can use hardware-specific forces designed by the manufacturer. For example, a hardware device might support a periodic effect that rotates the stick in a small circle.
The type-specific structure for periodic effects is DIPERIODIC.
Do not confuse the period of a periodic effect (DIPERIODIC.dwPeriod) with the sample period (DIEFFECT.dwSamplePeriod). The period is the time that it takes to go through a complete wave cycle. The sample period, as for all effects, is the minimum time between actual adjustments of magnitude.