This function can compute a gray matrix from the RGB in an
image section.Such image section can be compressed in either
portable network graphics format (png) or tagged image file
format (tif).
NULL or integer. If NULL the image resolution
in points per inch is extracted from attributes
in image. If this attribute is not
embedded then users should provide it
rgb
vector with three fractions, all of
them adding to one, to combine RGB channels into gray
matrix.
p.row
proportion of rows of gray matrix to be
processed.
Value
a gray matrix containing the image reflectances.
Author(s)
Wilson Lara, Carlos Sierra, Felipe Bravo
Examples
## (not run) Read two image sections in package measuRing:
image1 <- system.file("P105_a.tif", package="measuRing")
image2 <- system.file("P105_a.png", package="measuRing")
## (not run) compute a gray matrix:
gray <- imageTogray(image1)
## (not run) - the ppi is embedded in the image:
attributes(gray)
## (not run) but, the ppi is not embedded in image2:
## - imageTogray will return an error:
## uncoment and run
## gray2 <- imageTogray(image2)
## attributes(gray2)
## - the ppi should be provided (i.e. ppi = 1200):
gray3 <- imageTogray(image2,ppi = 1200)
attributes(gray3)
##(not run) a plot of the gray matrix
xrange <- range(0:ncol(gray)) + c(-1,1)
yrange <- range(0:nrow(gray)) + c(-1,1)
{plot(xrange,yrange,xlim=xrange,ylim=yrange,xlab='',
ylab='',type='n',asp=0)
rasterImage(gray,xrange[1],yrange[1],xrange[2],yrange[2])}