Prevent white border effect when converting an image to grayscale with ImageMagick

Question

For printing images on a Epson TM-T88v receipt printer, I had to convert a PNG file to a 4-bit grayscale PNG by using ImageMagick.

Using the following command I'm getting pretty good results except of a white border around certain parts of the converted image:

convert test.png -depth 4 -colorspace gray -define png:color-type=0 -define png:bit-depth=4 test-4bit-gray.png

Original input image:

Converted image:

If I use Epson's native JS library to convert PNG images to 4-bit grayscale I get following result which looks way smoother:

Which ImageMagick options could help me to avoid the white border around the heart eyes of the Emoji or make it to look smoother at all?

Looks like an anti-aliasing issue to me. I never use imagemagic, so I'm not sure if/how to adjust those settings, but if you do, start there. On second thought, you could increase your bit depth. That might help. increase it to 8-24, if you can. — Manly, Commented Aug 11, 2016 at 15:01
@Manly, thanks. Need to have exactly a 4-bit depth. The Epson example has also 4-bit. Will look for an anti-aliasing option... — Toni, Commented Aug 11, 2016 at 15:05

Cai · Accepted Answer · 2016-08-11 15:31:46Z

1

What you need is some dithering. There is some information on dithering techniques using ImageMagick here:

Using -ordered-dither for the dither and removing the alpha gets you pretty close:

Converted with:

convert test.png -depth 4 -colorspace gray -alpha remove -ordered-dither o2x2,16 -define png:color-type=0 -define png:bit-depth=4 test-4bit-gray.png

answered Aug 11, 2016 at 15:31

Cai

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