coregisterImages: Image to Image Co-Registration based on Mutual Information

In RStoolbox: Tools for Remote Sensing Data Analysis

Image to Image Co-Registration based on Mutual Information

Description

Shifts an image to match a reference image. Matching is based on maximum mutual information.

Usage

coregisterImages(
  img,
  ref,
  shift = 3,
  shiftInc = 1,
  nSamples = 1e+05,
  reportStats = FALSE,
  verbose,
  nBins = 100,
  master = deprecated(),
  slave = deprecated(),
  ...
)

Arguments

img	Raster* or SpatRast object. Image to shift to match reference image. img and ref must have equal numbers of bands.
ref	Raster* or SpatRast object. Reference image. img and ref must have equal numbers of bands.
shift	Numeric or matrix. If numeric, then shift is the maximal absolute radius (in pixels of img resolution) which img is shifted (seq(-shift, shift, by=shiftInc)). If shift is a matrix it must have two columns (x shift and y shift), then only these shift values will be tested.
shiftInc	Numeric. Shift increment (in pixels, but not restricted to integer). Ignored if shift is a matrix.
nSamples	Integer. Number of samples to calculate mutual information.
reportStats	Logical. If FALSE it will return only the shifted images. Otherwise it will return the shifted image in a list containing stats such as mutual information per shift and joint histograms.
verbose	Logical. Print status messages. Overrides global RStoolbox.verbose option.
nBins	Integer. Number of bins to calculate joint histogram.
master	DEPRECATED! Argument was renamed. Please use ref from now on.
slave	DEPRECATED! Argument was renamed. Please use img from now on.
...	further arguments passed to writeRaster.

Details

Currently only a simple linear x - y shift is considered and tested. No higher order shifts (e.g. rotation, non-linear transformation) are performed. This means that your imagery should already be properly geometrically corrected.

Mutual information is a similarity metric originating from information theory. Roughly speaking, the higher the mutual information of two data-sets, the higher is their shared information content, i.e. their similarity. When two images are exactly co-registered their mutual information is maximal. By trying different image shifts, we aim to find the best overlap which maximises the mutual information.

Value

reportStats=FALSE returns a Raster* object (x-y shifted image). reportStats=TRUE returns a list containing a data.frame with mutual information per shift ($MI), the shift of maximum MI ($bestShift), the joint histograms per shift in a list ($jointHist) and the shifted image ($coregImg).

Examples

library(raster)
library(ggplot2)
library(reshape2)
data(rlogo)
reference <- rlogo
## Shift reference 2 pixels to the right and 3 up
missreg <- shift(reference,  2,  3)

## Compare shift
p <- ggR(reference, sat = 1, alpha = .5) 
p + ggR(missreg, sat = 1, hue = .5, alpha = 0.5, ggLayer=TRUE) 

## Coregister images (and report statistics)
coreg <- coregisterImages(missreg, ref = reference,
                         nSamples = 500, reportStats = TRUE)

## Plot mutual information per shift
ggplot(coreg$MI) + geom_raster(aes(x,y,fill=mi))

## Plot joint histograms per shift (x/y shift in facet labels)
 
df <- melt(coreg$jointHist)   
df$L1 <- factor(df$L1, levels = names(coreg$jointHist))
df[df$value == 0, "value"] <- NA ## don't display p = 0
ggplot(df) + geom_raster(aes(x = Var2, y = Var1,fill=value)) + facet_wrap(~L1) + 
       scale_fill_gradientn(name = "p", colours =  heat.colors(10), na.value = NA)

## Compare correction
ggR(reference, sat = 1, alpha = .5) +
  ggR(coreg$coregImg, sat = 1, hue = .5, alpha = 0.5, ggLayer=TRUE) 

RStoolbox documentation built on March 18, 2022, 5:37 p.m.