test-png-res.R

library(ggplot2)

p <- ggplot(mtcars, aes(cyl, mpg)) + geom_point()
res <- 300
fn <- "test.png"

# Default png()
png(fn, res = res)
print(p)
dev.off()
system(paste0('identify -format "%x x %y" ', fn))

# quartz
png(fn, type = "quartz", res = res)
print(p)
dev.off()
system(paste0('identify -format "%x x %y" ', fn))

# cairo
png(fn, type = "cairo", res = res)
print(p)
dev.off()
system(paste0('identify -format "%x x %y" ', fn))

# cairo-png
png(fn, type = "cairo-png", res = res)
print(p)
dev.off()
system(paste0('identify -format "%x x %y" ', fn))

# CairoPNG
Cairo::CairoPNG(fn, res = res)
print(p)
dev.off()
system(paste0('identify -format "%x x %y" ', fn))