auto-crop in ruby-vips

trim.rb

#!/usr/bin/ruby

# An equivalent of ImageMagick's -trim in ruby-vips ... automatically remove
# "boring" image edges.

# We use .project to sum the rows and columns of a 0/255 mask image, the first
# non-zero row or column is the object edge. We make the mask image with an
# amount-differnt-from-background image plus a threshold.

require 'rubygems'
require 'vips'

im = VIPS::Image.new(ARGV[0])

class VIPS::Image
    # split an image to a list of one-band images
    def bandsplit
        (0...bands).map {|i| extract_band(i)}
    end

    # a median filter
    def median(size = 3)
        rank(size, size, (size * size) / 2)
    end

    # get the value of a pixel as an array
    def get_point(x, y)
        extract_area(x, y, 1, 1).bandsplit.map {|i| i.avg}
    end
end

# find the value of the pixel at (0, 0) ... we will search for all pixels 
# significantly different from this
background = im.get_point(0, 0)

# we need to smooth the image, subtract the background from every pixel, take 
# the absolute value of the difference, then threshold

# b = a.lin(c, d) calculates (b = a * c + d), we just want the d, so we must
# set the as to 1
ones = Array.new(background.length, 1)

mask = im.median.lin(ones, background.map {|x| x * -1}).abs.more(10)

# sum mask rows and columns, then search for the first non-zero sum in each
# direction
columns, rows = mask.project

left = columns.profile_h.min
right = columns.x_size - columns.fliphor.profile_h.min
top = rows.profile_v.min
bottom = rows.y_size - rows.flipver.profile_v.min

# and now crop the original image

im = im.extract_area(left, top, right - left, bottom - top)

im.write(ARGV[1])