Search code examples
imagemagick

Can I use imagemagick to detect border color?

I have to deal with a lot of images and I'm looking for a way to detect the border color. For example, take this image:

enter image description here

is it somehow possible to determine that the image has a white background? A workaround can be to extract a border of 1px and check if the color of that extracted border is equally white. But I don't think the shave-feature of ImageMagick supports this, right?

Solution

  • There are lots of techniques...

    • you could get the top-left (or other) corner pixel value, or
    • all 4 corners and take the median, or
    • take a 1px wide slice off each side and select the one with smallest variance.

    Let's make a test image 200x100:

    convert xc:red xc:lime +append \( xc:blue xc:white +append \) -append -scale 200x100\! image.png

    enter image description here

    Here are some ways of getting the corner pixels:

    # Top left pixel
magick image.png -format "%[hex:u.p{0,0}]\n" info:
FF0000

# Top right pixel
magick image.png -crop "1x1+%[fx:w-1]+0" -format "%[hex:u.p{0,0}]\n" info:
00FF00

# Bottom left pixel
magick image.png -crop "1x1+0+%[fx:h-1]+0" -format "%[hex:u.p{0,0}]\n" info:
0000FF

# Bottom right pixel
magick image.png -crop "1x1+%[fx:w-1]+%[fx:h-1]" -format "%[hex:u.p{0,0}]\n" info:
FFFFFF

    Now let's look at the top, bottom, left and right edges for which we could use a better sample image:

    convert -size 256x256 gradient:red-yellow image.png

    enter image description here

    # Top row mean and standard deviation
magick image.png +repage -crop x1\!+0+0 -format "%[fx:mean],%[fx:standard_deviation]\n" info:
0.333333,0

    Standard deviation is zero, so all pixels are the same in this row.

    # Bottom row mean and standard deviation
magick image.png +repage -crop "x1\!+0+%[fx:h-1]" -format "%[fx:mean],%[fx:standard_deviation]\n" info:
0.666667,0

    Standard deviation is zero, so all pixels are the same in this row, but brighter (0.6666 vs 0.3333) because both the red and green pixels are on - making yellow.

    # Left edge mean and standard deviation
magick image.png +repage -crop "1\!x+0+0" -format "%[fx:mean],%[fx:standard_deviation]\n" info:
0.5,0.0967909

    There is some deviation down the edge because the colours are changing.

    # Right edge mean and standard deviation
magick image.png +repage -crop "1\!x+%[fx:w-1]+0" -format "%[fx:mean],%[fx:standard_deviation]\n" info:
0.5,0.0967909