crop image, convert to black and white and sharpen it

Question

I want to take x, convert it into a black and white image. Then i want to convert all pixels that are below 100 to 0 and convert all pixels that are above 100 to 255. I want to get a sharper image where background will be white and objects within image will be black. And then store new x (black and white sharp image) on my hard drive. How could i do it?

library(raster)
r1 <- brick(system.file("external/rlogo.grd", package="raster"))
x <- crop(r1, extent(0,50,0,50))
plotRGB(x)

Based upon earlier question, i wrote below lines to convert r1 to black and white. But plotRGB function fails :(. Plot function works but it doesnt return me a black and white image

flat <- sum(x * c(0.2989, 0.5870, 0.1140))
plotRGB(flat)
flat
plot(flat)

========update1==========

in case anyone interested,

image colors can be swapped easily by using "1-twoclasses" in place of "twoclasses" in image and plot functions below

plotRGB does not fail. It is meant for a three layer Raster, not for a single layer — Robert Hijmans, May 23 '15 at 00:46

Robert Hijmans · Accepted Answer · 2015-05-23T04:13:00.067

2

To actually see grays you need to provide these colors.

library(raster)
r1 <- brick(system.file("external/rlogo.grd", package="raster"))
x <- crop(r1, extent(0,50,0,50))
flat <- sum(x * c(0.2989, 0.5870, 0.1140))
plot(flat, col=gray(seq(0,1,0.1)))

You can use the reclassify function to get values that are either 0 or 255 but in this case it would seem more sensible to use 1 and 2

twoclasses <- cut(flat, c(0,100,255))
plot(twoclasses, col=gray(c(0,1)))

To write to disk, you can do something like:

png(width=ncol(twoclasses)*10, height=nrow(twoclasses)*10)
par(mai=c(0,0,0,0))
image(twoclasses, col=gray(c(0,1)))
dev.off()

edited May 23 '15 at 04:13

answered May 23 '15 at 00:44

Robert Hijmans

40,301
4
55
63

I want to save the resulting black and white image as a jpg or png. How can i save the cropped image exactly at the same size. (size remains 50X50 pixels and I dont get legend and x and y axis and axis names/numbers - i get them after i run plot(twoclasses, col=gray(c(0,1))) ). – user2543622 May 23 '15 at 01:05
I have added a png writing example – Robert Hijmans May 23 '15 at 04:13
thanks, any reason for changing width and height by 10x? – user2543622 May 23 '15 at 04:26
No. Just showing how you can adjust the size if the image would be too small. – Robert Hijmans May 23 '15 at 05:13
thanks, any idea how i could smooth the image if i zoom it 10x (multiply height and width by 10 and then saved that image)? – user2543622 May 23 '15 at 16:03
1

plot(, interpolate=TRUE) – Robert Hijmans May 23 '15 at 16:48
is there easy way to swap image colors? I want background to be black and actual image parts to be white – user2543622 May 27 '15 at 20:56

crop image, convert to black and white and sharpen it

1 Answers1