Legacy ImageMagick Discussions Archive

How would I check if an Image has atleast one corner pixel that is transparent?

gotskill10 wrote:How would I check if an Image has atleast one corner pixel that is transparent?

convert each of the four corner pixels, one at a time to txt: format and look at the information

Make test image as gradient with left half transparent and right half opaque:
convert -size 100x100 gradient: -rotate 90 grad100.png
freds-mac-mini:~ fred$ convert grad100.png -threshold 50% grad100t50.png
freds-mac-mini:~ fred$ convert grad100.png grad100t50.png -compose copy_opacity -composite grad100_at50.png

convert grad100_at50.png[1x1+0+0] txt:-
# ImageMagick pixel enumeration: 1,1,65535,rgba
0,0: ( 0, 0, 0, 0) #000000000000000 none

So the above is transparent:


convert grad100_at50.png[1x1+99+99] txt:-
# ImageMagick pixel enumeration: 1,1,65535,rgba
0,0: (65535,65535,65535,65535) #FFFFFFFFFFFF white

So the above is opaque

Their was also a post recently about extracting the colors of the four corners, this may provide a better check. For example. produces

# ImageMagick pixel enumeration: 2,2,65535,rgba
0,0: (65535,65535,65535,65535) #FFFFFFFFFFFF white
1,0: ( 0, 0, 0, 0) #0000000000000000 none
0,1: (65535,65535,65535,65535) #FFFFFFFFFFFF white
1,1: ( 0, 0, 0, 0) #0000000000000000 none

or two corners are white and two are transparent.

convert grad100_at50.png -roll +1+1 -crop 2x2+0+0 +repage txt:

neat way to get only the 4 corners

And by using -roll you avoid needing to know the size of the image