Avoiding jagged edges with mapim?

[GavinJoyce]

Hi,

I'm following this example to achieve perspective distortion with mapim.

It's working great, except for one small issue with jagged edges which I haven't yet figured out how to avoid. Perhaps someone knows how to avoid the jagged edges?

output.png:

input.png:

require "vips"

image = Vips::Image.new_from_file "input.png"

t = [1.73945282, -0.41595611, -116.35426798, 5.97741712e-01, 2.47280054e+00, -6.75608244e+02, -1.59808198e-06, 4.97586518e-06]
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)

m = x.bandjoin y
distorted_image = image.mapim m

distorted_image.write_to_file "output.png"

[GavinJoyce]

Seems similar to libvips/php-vips#58 ("affine should have a background parameter, and not have jaggy edges")

[jcupitt]

Hi @GavinJoyce,

You could put a one-pixel black border around the source. Eg.:

#!/usr/bin/ruby

require "vips"

image = Vips::Image.new_from_file ARGV[0]
image = image.embed 1, 1, image.width + 2, image.height + 2

t = [1.73945282, -0.41595611, -116.35426798, 5.97741712e-01, 2.47280054e+00, -6.75608244e+02, -1.59808198e-06, 4.97586518e-06]
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)

m = x.bandjoin y
distorted_image = image.mapim m

distorted_image.write_to_file ARGV[1]

To make:

[jcupitt]

... but you're right, improving mapim's edge handling would be better.

[GavinJoyce]

Thanks for the workaround @jcupitt, that works a treat

[jcupitt]

I made an enhancement issue to fix this properly. Thanks for the suggestion.

[jcupitt]

... this is fixed in master and will be in 8.13.