How to perform perspective distort transformation in VIPS?

Question

Is it possible to do the following ImageMagick perspective distort command using VIPS? If so, what would the command be (using ruby-vips)?

$ convert my_file.png -matte -virtual-pixel transparent +distort Perspective '0,0,0,60 1500,0,300,0 0,2100,0,2310 1500,2100,300,2100' -crop 300x2310+0+0

Answer 1

There isn't a built-in thing for perspective distort, but you can make one using mapim:

http://jcupitt.github.io/libvips/API/current/libvips-resample.html#vips-mapim

#!/usr/bin/ruby
  
require 'vips'

image = Vips::Image.new_from_file ARGV[0]

# perspective distortion: each pixel (x', y') in the output image is
# interpolated from pixel (x, y) in the input using:
#
#   x' = (A x + B y + C) / (G x + H y + 1)
#   y' = (D x + E y + F) / (G x + H y + 1)
#
# where the constants A .. H are from the transform matrix T
#
# T = [A, B, .. H]

T = [1.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0003, 0.0001]

# make an index image where pixels have the value of their (x, y) coordinates
i = Vips::Image.xyz image.width, image.height

x = (i[0] * T[0] + i[1] * T[1] + T[2]) / (i[0] * T[6] + i[1] * T[7] + 1)
y = (i[0] * T[3] + i[1] * T[4] + T[5]) / (i[0] * T[6] + i[1] * T[7] + 1)

# join up x and y as a map image
m = x.bandjoin y

# and use it to transform our original image
image = image.mapim m 

image.write_to_file ARGV[1]

You'd also need something to calculate the transform from a set of tie-points, of course.

edit: fixed typo