sig.extract {imagefx}

Extract Time-Series Signal from Video Data

Description

This function investigates time variations of pixel values in video frames via both a simple stack and also a cross correlation analysis. Both methods attempt to highlight signals present within video data. See Details.

Usage

sig.extract(img.list, fps, base.vec, ...)

Arguments

img.list

A series of images saved in a list. Each list element is a matrix that represents pixel values of an image, the dimensions of which correspond to the rows and columns of the matrix.

fps

Sample rate of video in frames per second (FPS). Each list element of img.list should be separated by 1/fps seconds. Defaults to 30.

base.vec

Vector to correlate all pixel signals with. Defaults to rowMeans(img.series).

...

Additional arguments passed to ccf (i.e., lag.max).

Details

The algorithm first synthesizes the video frames into a matrix of time series signals. In other words, a time-series is generated for each pixel that records the value of that pixel in each frame. These original pixel time-series are then simply stacked and then analyzed in terms of their frequency components. Additionally, each pixel time-series is cross correlated with the base.vec and then shifted according to the lag associated with the maximum correlation value. These shifted time-series are then stacked and analyzed in terms of their frequency components.

Note the function gives two basic outputs. The first is a simple stack (without applying any shift to the pixel time-series), while the second applies a shift then stack. Both outputs may prove useful and should be investigated.

Value

List of length four whose elements correspond to both original and shifted stacks. Note the nomenclature is that lower case objects correspond to time-series data while upper case corresponds to frequency domain data.

org

Matrix of stacked time-series from original pixel values. The first column corresponds to the time axis while second column is the stacked values.

ORG

Matrix of stacked frequency components from original pixel values. The first column corresponds to the frequency axis while second column is the stacked frequency amplitudes.

shifted

Matrix of stacked time-series from shifted pixel values. The first column corresponds to the time axis while second column is the stacked values.

SHIFTED

Matrix of stacked frequency components from shifted pixel values. The first column corresponds to the frequency axis while second column is the stacked frequency amplitudes.

Author(s)

Alex J.C. Witsil

See Also

gen.eg.img.list

Examples

##############################
### SYNTHETIC VIDEO INPUTS ###
##############################

## x and y dimension of the frame
dim.x = 64
dim.y = 64

## sample rate in frames per second
fps = 30

## how many seconds does the video last
n.secs = 3 

## what is the frequency at which the boxed region oscillates
sig.f = 2 

## what is the peak amplitude of the signal
sig.peak = 0.5

## size of the boxed region
box.width = 20
box.height = 20


################################
### GENERATE SYNTHETIC VIDEO ###
################################

## use the inputs to generate an image list (i.e. video)
img.list <- gen.eg.img.list(dim.x, dim.y, fps, n.secs, sig.f, sig.peak, box.width, box.height)


#################################
### EXTRACT SIGNAL FROM VIDEO ###
#################################

sig.x <- sig.extract(img.list,fps)

################
### PLOTTING ###
################

## set up a plot
split.screen(c(1,2))
screen(1)
plot(sig.x$org,col='blue',type='l',main='Time Domain')
lines(sig.x$shifted,col='red')

screen(2)
plot(sig.x$ORG,col='blue',type='l',xlim=c(0,5),main='Frequency Domain')
lines(sig.x$SHIFTED,col='red')

## close the screens
close.screen(all.screens=TRUE)