问题
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
回答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[2] + 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.
来源：https://stackoverflow.com/questions/50202733/how-to-perform-perspective-distort-transformation-in-vips