diffviewer fails to show differences in some PNG files

[wch]

With these two files, there are a few pixels that are different, but the visual diff does not color them in red.
However, magick::image_compare() does show the differences.

oldfile <- "file1.png"
newfile <- "file2.png"

library(magick)
old <- magick::image_read(oldfile)
new <- magick::image_read(newfile)

# Shows difference
image_compare(old, new)

# Does not show difference
library(diffviewer)
diffviewer::visual_diff(oldfile, newfile)

This is the difference image generated by image_compare(). I've added arrows pointing to the pixels that are different.

Here's the info about the pixels that are different:

old_data <- old[[1]]
new_data <- new[[1]]

idx <- old_data != new_data
which(idx, arr.ind = TRUE)
#>       dim1 dim2 dim3
#>  [1,]    1  512  185
#>  [2,]    2  512  185
#>  [3,]    3  512  185
#>  [4,]    1  512  196
#>  [5,]    2  512  196
#>  [6,]    3  512  196
#>  [7,]    1 1022  670
#>  [8,]    2 1022  670
#>  [9,]    3 1022  670
#> [10,]    1 1022  686
#> [11,]    2 1022  686
#> [12,]    3 1022  686
#> [13,]    1 1022 1136
#> [14,]    2 1022 1136
#> [15,]    3 1022 1136

old_data[idx]
#>  [1] 0f 82 fe 0e 81 fe e6 e6 e6 d7 d7 d7 e6 e6 e6

new_data[idx]
#>  [1] 00 7b ff 00 7b ff e7 e7 e7 d8 d8 d8 e7 e7 e7

cc @nstrayer

[wch]

I've fixed this in rstudio/shinytest#391, and the same fix should work here.

[hadley]

Would you mind doing a PR?