FPS
Frames per second sets the rate for animation playback when realtime playback is turned on. A good rule of thumb is to set the FPS for your animation once before you begin animating, because when you change the FPS, the time of the last keyframe will not match the new animation length (unless Keep Keys on Current Frames is turned on). If you want to modify the animation timing, you must do so explicitly.
Note the following FPS rates:
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NTSC video: 30
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PAL video: 25
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Film: 24
Production Tip: If you make a mistake, you can use a Field 
COP with a 
Pushup / 
Pulldown operation to fix a sequence of rendered-out images and convert them to a different FPS after the fact.
Keep Keys on Current Frames
When this option is turned on and you change the frame rate (FPS), then the frame that each key is attached to will be preserved. This changes the playback speed of the animation.
When this option is turned off, the frames that the keys are at will change in order to preserve the timing of the animation sequence. This maintains the playback speed of the animation.
Note
Starting in Houdini 19.5, when you change the FPS, Houdini will:
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Keep the start frame the same.
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Shift the key times to maintain their timing relative to the start frame.
Pre-Houdini 19.5, when you change the FPS, Houdini will:
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Modify the start frame relative to time 0.
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Shift the keys to preserve their timing relative to time 0.
To enable the pre-Houdini 19.5 behavior, set the HOUDINI_DISABLE_FPS_MAINTAIN_FSTART environment variable to 1.
For example, if you have a key on frame 1001, a key on frame 1011, a frame range of 1001-1020, and you double the FPS, the following will occur:
Settings |
Behavior |
|---|---|
Keep Keys on Current Frames = Off & HOUDINI_DISABLE_FPS_MAINTAIN_FSTART = |
(Current behavior) Keys are now on frames 1001 and 1021, and the frame range is 1001-1040. The start frame stays the same. The keys are now on different frames, but still maintain their timing relative to the start frame. The playback speed of the animation stays the same. |
Keep Keys on Current Frames = Off & HOUDINI_DISABLE_FPS_MAINTAIN_FSTART = |
(Pre-Houdini 19.5 behavior) Keys are now on frames 2001 and 2021, and the frame range is 2001-2040. The start frame has changed and the keys are now on different frames, but both maintain their timing relative to time 0. The playback speed of the animation stays the same. |
Keep Keys on Current Frames = On HOUDINI_DISABLE_FPS_MAINTAIN_FSTART is ignored |
Keys stay on frames 1001 and 1011, and the frame range stays at 1001-1020. The keys stay at the same frames, but the key times have now changed. The playback speed of the animation is modified. |
Snap Keys
Snaps all keys to integer frame boundaries. This is equivalent to chround -a.
Apply FPS
Applies the current frame rate (FPS) setting to the animation sequence.
Start / End
The Start field displays the starting frame number of the animation sequence. By default, it is set to 1.
The End field displays the final frame number of the animation sequence.
Apply
Applies the current Start and End frame settings to the animation sequence.
Restrict playback range
If this option in turned on, you will only be able to set playback frame values within the global frame range. For example, if the global frame range is 50-70, Houdini will not let you set the playback start frame to 40.
If this option is turned off, the playback frame range can be greater than the global frame range. See playbar for more information on the different frame ranges.
CHOP Motion Samples
This value affects the sampling rate for CHOP constraints: CHOP sampling rate = FPS * CHOP motion samples.
Changes to this value will propagate to the CHOP Motion Samples parameter in a CHOP constraints subnet.
For example, create a CHOP constraints subnet by creating and selecting an object, pressing TAB on the object and typing Constraints Network. The CHOP constraints subnet will be created inside the object node. CHOP Motion Samples and CHOP Rate are parameters of the constraints subnet.
All of the nodes inside the CHOP constraints subnet will have their Sample Rate parameter linked automatically to the constraints subnet CHOP Rate.
Show Range Slider
If this option is turned on, the slider widget underneath the playbar for visually manipulating the playback range is displayed.
Show Audio
Overlays the current audio track on the playbar so that you can see the audio wave.
Show Simulation Cache
If this option is turned on, the state of the cached simulation will show up as a background color on the playbar timeline.
Show Animation Toolbar
If this option is turned on, the Animation Toolbar will show up above the playbar.
Show Ticks
If this option is turned on, the playbar will show ticks representing the frame numbers.
Show Keys
Show Bookmarks
If this option is turned on, bookmarks will show up on the playbar.
Audio Wave Scale
Allows adjustment of the audio height when shown on the playbar.
Integer Frame Values
Houdini works with floating-point frame values ($FF). When this option is turned on, Houdini confines the display of frame values to integers ($F). When this option is turned off, Houdini allows you to set the current frame to any value, including fractional times that do not lie on an exact frame number.
Step
The size of a single increment in frame values. For example, entering a value of 2 plays every second frame.
Realtime Playback
Turns on realtime playback on the playbar. Two options control the behavior of the realtime toggle:
Maintain realtime playback, possibly skipping frames
This option is not recommended for playback of particle simulations, which will still have to compute any skipped frames.
Play every frame, but never faster than FPS
This option is better suited for playback of particle simulations.