get fuzzy borders if an input image is of alpha channel

[Rongronggg9]

	Original Image	waifu2x-ncnn-vulkan	waifu2x-caffe
1x		/	/
2x [cunet, denoise-level 3, TTA]	/
2x*2x [cunet, denoise-level 3, TTA]	/

	Original Image (alpha channel deleted)	waifu2x-ncnn-vulkan
1x		/
2x [cunet, denoise-level 3, TTA]	/
2x*2x [cunet, denoise-level 3, TTA]	/

Have tested [cunet, denoise-level -1, no TTA] and got the same fuzzy borders.

[Rongronggg9]

Still reproducible on release 20220419:

-s 2 -n 3 -x:

-s 2 -n 3 -x*2:

[nagadomi]

This is not a request, just information.

waifu2x(web) and waifu2x-caffe use a preprocessing method for padding the borders of the alpha channel.

https://github.com/nagadomi/nunif/blob/eab6952d93e85951ed4e4cff30cd26c09e1dbb63/nunif/utils/render.py#L35
https://github.com/lltcggie/waifu2x-caffe/blob/3812b90f68b33256f040e6c8eafaacebd51490a0/common/stImage.cpp#L167

Also the alpha channel is 2x by the scale2.0 model.
This takes more processing time, but it gives better alpha border results, such as for a sprite image with alpha channel in game-dev.

[chrisjbillington]

To work around this issue, I've been compositing the original image over two differently-coloured solid backgrounds, upscaling those two alpha-free images separately, then doing some pixel maths on the upscaled images to recover the alpha channel and invert the composition to obtain an upscaled image with alpha. E.g. with imagemagick:

# Compose on two solid backgrounds:
convert orig.png -background green1 -flatten green.png
convert orig.png -background magenta -flatten magenta.png

# Upscale:
waifu2x-ncnn-vulkan -s 2 -n 3 -x -i green.png -o green-2x.png
waifu2x-ncnn-vulkan -s 2 -n 3 -x -i magenta.png -o magenta-2x.png

# Extract alpha
magick green-2x.png magenta-2x.png -compose difference -composite -separate \
  -evaluate-sequence max -auto-level -negate alpha-2x.png

# Invert componsition of both 2x images:
magick green-2x.png alpha-2x.png -alpha Off \
  -fx "v==0 ? 0 : u/v - green1/v + green1" alpha-2x.png -compose Copy_Opacity \
  -composite green-2x-decomposed.png

magick magenta-2x.png alpha-2x.png -alpha Off \
  -fx "v==0 ? 0 : u/v - magenta/v + magenta" alpha-2x.png -compose Copy_Opacity \
  -composite magenta-2x-decomposed.png

# Average the two decomposed images together
convert -average green-2x-decomposed.png magenta-2x-decomposed.png result.png

There is the slightest magenta and green tinge around some of the semitransparent edges, but averaging together the two decomposed images instead of just using one of them improves it, and it's not too bad.

Obviously this takes about twice as long since it has to upscale the image twice, but it's what I'm using for now.

orig.png

green.png

magenta.png

green-2x.png

magenta-2x.png

green-2x-decomposed.png

magenta-2x-decomposed.png

result.png