Last modified: November 04, 2022

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Animations and Time-Varying Data

VTK offers a set of tools to create animations and visualize time-varying data. This is particularly useful in scenarios such as:

1. Keyframe Animation
2. Temporal Data Visualization
3. Animation Export

Keyframe Animation

Keyframe animation is a technique used to create smooth transitions between different states of a visualization. This approach is particularly beneficial for visual storytelling or creating explanatory visualizations, where the evolution of data over time needs to be clearly communicated.

Core Concepts

The core classes involved in keyframe animation using VTK (Visualization Toolkit) are:

• vtkAnimationCue: Represents an individual animation sequence with defined start and end times. An animation cue controls how a particular property of an object changes over a specified period.
• vtkAnimationScene: Manages multiple animation cues and orchestrates the overall animation playback. It handles the timing and sequencing of various animation cues to create a cohesive animation.

Basic Workflow

1. Set up the animation scene by creating an instance of vtkAnimationScene, and configuring its playback mode, loop setting, and frame rate.
2. Create animation cues for each property you want to animate by making an instance of vtkAnimationCue, and defining the start and end times for each cue.
3. Add the animation cues to the scene by including each vtkAnimationCue in the vtkAnimationScene.
4. Define callbacks to update the properties of the visual objects at each frame of the animation.

Example: Animating a Sphere's Radius

Below is an example demonstrating how to animate a sphere's radius using VTK. This example sets up an animation scene, creates a cue for the sphere's radius, and defines the necessary callbacks to update the radius over time.

import vtk

# Create a sphere
sphereSource = vtk.vtkSphereSource()
sphereSource.SetRadius(5)
sphereSource.SetPhiResolution(30)
sphereSource.SetThetaResolution(30)

# Create a mapper and actor
mapper = vtk.vtkPolyDataMapper()
mapper.SetInputConnection(sphereSource.GetOutputPort())
actor = vtk.vtkActor()
actor.SetMapper(mapper)

# Create a renderer, render window, and interactor
renderer = vtk.vtkRenderer()
renderWindow = vtk.vtkRenderWindow()
renderWindow.AddRenderer(renderer)
renderWindowInteractor = vtk.vtkRenderWindowInteractor()
renderWindowInteractor.SetRenderWindow(renderWindow)

# Add the actor to the scene
renderer.AddActor(actor)
renderer.SetBackground(0.1, 0.2, 0.4)  # Background color

# Set up the animation scene
animationScene = vtk.vtkAnimationScene()
animationScene.SetModeToSequence()
animationScene.SetLoop(0)  # No looping
animationScene.SetFrameRate(60)
animationScene.SetStartTime(0)
animationScene.SetEndTime(2)  # Ensuring the scene's end time matches the cue's end time

# Define the callback to update the sphere's radius
def update_radius(caller, event):
    cue = caller
    t = cue.GetAnimationTime()
    new_radius = 5 + 5 * t  # Example: linearly increase radius over time
    sphereSource.SetRadius(new_radius)
    renderWindow.Render()

# Set up an animation cue for the radius
radiusCue = vtk.vtkAnimationCue()
radiusCue.SetStartTime(0)
radiusCue.SetEndTime(2)
radiusCue.AddObserver(vtk.vtkCommand.AnimationCueTickEvent, update_radius)
animationScene.AddCue(radiusCue)

# Initialize the interactor and start the animation
renderWindow.Render()
animationScene.Play()
renderWindowInteractor.Start()

In this example:

• A sphere is created using vtkSphereSource.
• A mapper and actor are set up to render the sphere.
• An vtkAnimationScene is created and configured.
• An vtkAnimationCue is created for animating the sphere's radius over a 2-second period.
• A callback function update_radius is defined to update the sphere's radius based on the animation time.
• The animation cue is added to the animation scene.
• Finally, the animation is played, and the render window interactor is started.

Temporal Data Visualization

Temporal data visualization involves displaying data that varies over time. This technique is particularly useful for visualizing simulations, time-series data, or dynamic systems, providing insights into how data evolves and changes across different time steps.

Core Classes and Concepts

The main classes used for temporal data visualization in VTK (Visualization Toolkit) are:

• vtkTemporalDataSet: Represents a collection of datasets associated with different time steps. This class allows for organizing and managing temporal data effectively.
• vtkTemporalInterpolator: Interpolates between datasets at different time steps to create smooth transitions. This is essential for creating fluid animations and seamless visual transitions.

Basic Workflow

1. Load temporal data using appropriate readers to import the datasets.
2. Set up data interpolation (optional) to ensure smooth transitions between time steps by configuring a temporal interpolator.
3. Create a mapper and actor to visualize the data effectively.
4. Render the scene and animate through the time steps to visualize the temporal data.

Example: Loading and Visualizing a Temporal Dataset

Below is an example demonstrating how to load and visualize a temporal dataset using VTK. This example involves reading a temporal dataset, setting up a mapper and actor, and rendering the data.

import vtk

# Load temporal dataset
reader = vtk.vtkXMLMultiBlockDataReader()
reader.SetFileName("data.vtm")
reader.Update()

# Optionally set up a temporal interpolator for smooth transitions
interpolator = vtk.vtkTemporalInterpolator()
interpolator.SetInputConnection(reader.GetOutputPort())

# Create a mapper and actor
mapper = vtk.vtkCompositePolyDataMapper2()
mapper.SetInputConnection(interpolator.GetOutputPort())
actor = vtk.vtkActor()
actor.SetMapper(mapper)

# Create a renderer, render window, and interactor
renderer = vtk.vtkRenderer()
renderWindow = vtk.vtkRenderWindow()
renderWindow.AddRenderer(renderer)
renderWindowInteractor = vtk.vtkRenderWindowInteractor()
renderWindowInteractor.SetRenderWindow(renderWindow)

# Add the actor to the scene
renderer.AddActor(actor)
renderer.SetBackground(0.1, 0.2, 0.4)  # Background color

# Initialize and start the render window interactor
renderWindow.Render()
renderWindowInteractor.Initialize()

# Set up animation for the temporal data
animationScene = vtk.vtkAnimationScene()
animationScene.SetModeToSequence()
animationScene.SetLoop(1)  # Loop the animation
animationScene.SetFrameRate(24)  # 24 frames per second

# Define the callback to update the time step
def update_time_step(caller, event):
    time_cue = caller
    time_step = int(time_cue.GetAnimationTime() * reader.GetNumberOfTimeSteps())
    reader.SetTimeStep(time_step)
    renderWindow.Render()

# Set up an animation cue for the time steps
timeCue = vtk.vtkAnimationCue()
timeCue.SetStartTime(0)
timeCue.SetEndTime(reader.GetNumberOfTimeSteps() / 24)
timeCue.AddObserver(vtk.vtkCommand.AnimationCueTickEvent, update_time_step)
animationScene.AddCue(timeCue)

# Play the animation
animationScene.Play()
renderWindowInteractor.Start()

In this example:

• A temporal dataset is loaded using vtkXMLMultiBlockDataReader.
• An optional vtkTemporalInterpolator is set up to create smooth transitions between time steps.
• A mapper and actor are created to visualize the data.
• A renderer, render window, and interactor are initialized.
• An animation scene is set up to handle the temporal data animation.
• A callback function update_time_step is defined to update the dataset based on the current time step.
• An animation cue is created to manage the animation timing.
• The animation is played, and the render window interactor is started.

Animation Export

Animation export involves saving an animation as a video or image sequence. This capability is crucial for sharing animations with others or for viewing them offline. It allows for easy distribution and playback of visualizations created in VTK (Visualization Toolkit).

Core Classes and Concepts

The main classes used for exporting animations in VTK are:

• vtkWindowToImageFilter: Captures the contents of a render window as an image. This class is essential for converting the rendered scene into a format suitable for saving.
• vtkAVIWriter, vtkOggTheoraWriter, vtkFFMPEGWriter: These classes save image sequences as video files in different formats. They provide the functionality to encode and write video files.

Basic Workflow

1. Capture the render window using vtkWindowToImageFilter to save the current state of the render window as an image.
2. Set up a writer by choosing an appropriate one (vtkAVIWriter, vtkOggTheoraWriter, vtkFFMPEGWriter, etc.) to save the captured images as a video file.
3. Write the images or compile them into a video file using the chosen writer.

Example: Capturing a Render Window as an Image

Below is an example demonstrating how to capture the current render window as an image and save it using vtkPNGWriter.

import vtk

# Set up the render window and renderer
renderWindow = vtk.vtkRenderWindow()
renderer = vtk.vtkRenderer()
renderWindow.AddRenderer(renderer)

# Add an example actor to the scene
sphereSource = vtk.vtkSphereSource()
sphereMapper = vtk.vtkPolyDataMapper()
sphereMapper.SetInputConnection(sphereSource.GetOutputPort())
sphereActor = vtk.vtkActor()
sphereActor.SetMapper(sphereMapper)
renderer.AddActor(sphereActor)
renderer.SetBackground(0.1, 0.2, 0.4)  # Background color

# Render the scene
renderWindow.Render()

# Set up window to image filter
windowToImageFilter = vtk.vtkWindowToImageFilter()
windowToImageFilter.SetInput(renderWindow)
windowToImageFilter.Update()

# Write the image to a file
writer = vtk.vtkPNGWriter()
writer.SetFileName("screenshot.png")
writer.SetInputConnection(windowToImageFilter.GetOutputPort())
writer.Write()

Example: Exporting an Animation to a Video File

Below is an example demonstrating how to capture a series of frames and export them as a video file using vtkFFMPEGWriter.

import vtk

# Set up the render window and renderer
renderWindow = vtk.vtkRenderWindow()
renderer = vtk.vtkRenderer()
renderWindow.AddRenderer(renderer)

# Add an example actor to the scene
sphereSource = vtk.vtkSphereSource()
sphereMapper = vtk.vtkPolyDataMapper()
sphereMapper.SetInputConnection(sphereSource.GetOutputPort())
sphereActor = vtk.vtkActor()
sphereActor.SetMapper(sphereMapper)
renderer.AddActor(sphereActor)
renderer.SetBackground(0.1, 0.2, 0.4)  # Background color

# Render the scene
renderWindow.Render()

# Set up window to image filter
windowToImageFilter = vtk.vtkWindowToImageFilter()
windowToImageFilter.SetInput(renderWindow)

# Set up video writer
videoWriter = vtk.vtkFFMPEGWriter()
videoWriter.SetFileName("animation.mp4")
videoWriter.SetInputConnection(windowToImageFilter.GetOutputPort())
videoWriter.Start()

# Animate the scene and capture frames
for i in range(100):
    # Update the scene (example: rotate the sphere)
    sphereActor.RotateX(1)
    sphereActor.RotateY(1)
    renderWindow.Render()
    windowToImageFilter.Modified()
    videoWriter.Write()

# Finalize the video file
videoWriter.End()

In these examples:

• The render window's contents are captured and saved as a PNG image.
• A series of frames are captured while animating the scene, and these frames are compiled into an MP4 video file using vtkFFMPEGWriter.