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7.1 - Introducting video stacks

Starting with Rvision version 0.7.0, users can create video stacks. A video stack (VideoStack) is essentially a list of videos with similar properties (in particular, the same dimensions) that behaves almost entirely like a single continuous Video object. For that reason, VideoStack objects are particularly useful for applications requiring the combination of multiple video files and/or Video objects. For instance, video stacks can greatly simplify the merging of two separate video files into a single one, or the processing of a continuous sequence that was split into multiple files during recording.

In this vignette, we will briefly describe how to create and manipulate VideoStack objects. Since their operation is very similar to that of Video objects, using VideoStack objects should not require much additional learning for a regular user of Rvision.

7.2 - Creating video stacks

You can create a video stack by passing to the videoStack function a combination of existing Video objects and/or character strings pointing toward the location of video files on your computer. For instance:

# Find the path to the Balloon.mp4 video provided with Rvision
path_to_video <- system.file("sample_vid/Balloon.mp4", package = "Rvision")

# Open the video file stream
my_video <- video(filename = path_to_video)

# Create a video stack from a character string and a Video object
my_video_stack <- videoStack(path_to_video, my_video)

The paths to video files and the existing Video objects can be passed as separate arguments, or as a list or a vector. If a path is provided, the corresponding video will be opened before being added to the stack. If a Video object is provided, a pointer to the original Video object will be added to the stack (i.e., they will share the same reader head and any modification to the position of the reader head in the stack will be applied to the original object, and vice versa).

A video stack can contain as many videos as desired, with the only restriction that they should all have the same dimensions.

Once a VideoStack object is not required anymore, it can be released from memory as follows:


7.3 - Using video stacks

Once a VideoStack object is created, it can be used as a Video object with the same number of frames as the sum of the frames in the Video objects that it contains. For instance, to read a specific frame of the stack, you can use the readNext() and readFrame() functions, as you would normally do with Video objects:

# Capture the next available frame from the my_video_stack object created earlier
my_image <- readNext(my_video_stack)

# Capture frame 100 from the my_video_stack object created earlier
my_image <- readFrame(my_video_stack, 100)

At any time, you can check the state of the video stack as follows:

# What is the current number of videos in the stack? 

# What is the number of frames in the stack? 

# What is the index of the next frame available? 

# What are the dimensions of the queue? 

# What are the frame rates of the videos in the stack?

# What are the codecs of the videos in the stack?

Finally, you can access individually, add, or replace videos in the stack as follows:

# Access the first video in the stack individually

# Replace the first video in the stack 
my_video_stack[[1]] <- my_video

# Add a video at the end of the stack 
my_video_stack[[length(my_video_stack) + 1]] <- my_video

# Remove a video from the stack
my_video_stack[[2]] <- NULL