User Guide > Animation   

Path Animation

To quickly review the motion and timing of an animated sequence, Houdini lets you create an OpenGL rendered flipbook of your viewport contents. This provides a faster method for reviewing a shot's timing without rendering out the sequence. You can also attach an audio file to test out how your shot is working with the sound.

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1. Create the Path

Switch to a top view and zoom out until you can see the plane and a large area in front of it. Select tab > Path then click five times to define the shape of the path. RMB-click to complete. To edit the position of these points you can click on a path CV point then use the transform handle to move, rotate (r), or scale (e) the handles. Here the first path CV has been rotated to align with the direction of the plane.


2. Assign an Object to the Path

Switch to a perspective view and tumble to see the plane and the path. Make sure that Select > Secure Selection is On then press tab > Follow Path. Click on the airplane then RMB-click to accept. Click on the path curve then RMB-click to accept. The plane is moved to the start point of the curve.

If you press play in the play button in the playbar the plane does not move. The follow path tool set the curve as the path for the plane but its position along the curve has not yet been keyed.

3. Keyframe the Object's Position on the Path

Press tab > Pose, click on the plane and RMB-click to accept. At first a standard transform handle appears. Press y to switch to a Path handle (or RMB-click on the handle and select Path Handle). Press k to keyframe the Roll and Position parameters on the plane.

Set the current time to 200. Drag the arrow at the center of the path handle to the end of the path. Press k to set more keyframes. Playback the animation to preview the results. You will notice that the plane speeds up and slows down depending on the length of the various curve segments. In the Parameter pane, set Parameterization to Arc Length to move more evenly along the curve.

4. Apply Bank and Roll to the Plane

You can use banking or rolling or a combination of the two to control the sideways rotation of the plane in relation to the path. The Auto-bank factor, found at the bottom of the plane's Transform tab, is set to 1 by default which produces a small amout of banking in the plane. Change this value to 5 to see a more pronounced effect especially at the path control points where the curve segments meet.

If you want to control this rotation yourself to create a smoother effect, set Auto-bank to 0. Make sure the plane is selected with the Pose tool and the Path handle is active. Move to the frame where you want some roll and drag on the round portion of the handle to rotate the plane. Press k to keyframe the Position and Roll parameters. Repeat this as many times as you want to alter the roll of the plane.

When you playback the motion you will see the plane speed up and slow down at every point where you keyed the roll. This is because the in-between position keys are preventing a smooth run along the curve. RMB-click on the Position parameter in the Parameter pane and select Scope Channels. Select all the in-between position keys and press Delete. Next, select the remaining channel and change its Function to linear (). Now the playback is smooth with the keyframed Roll parameter contributing to the animation.


3. Edit the Shape of the Path

To further control the shape of the path, turn off Secure Selection and use the Pose tool to click on any of the path CV handles.You can use the default handle to move and rotate the handle. Press e to go to the scale handle. Scale along the Z axis of the handle ( the other two axes are locked) to scale out the tangents at this point. Repeat for the other path CVs to shape the curve as desired.

Another option would be to set keyframes on the path CVs to animate the shape of the path while the object is being animated along it.