[magick-users] Photoshop "save for web" gif size

[Illtud Daniel]

Hi,

I've tried to google, but I can't answer this specific question.
I suspect the answer is in the filtering, but I can't find one
that works for me. If anybody knows the answer I'd appreciate a
point in the right direction.

I have a 4-bit (16 color) greyscale tiff that's 3755390 bytes.
Photoshop's 'save for web' conversion to gif saves it as a
16-color non-interlaced LZW gif of size 226014 bytes with the
original dimensions.

There are indeed 16 colors, but it's done some kind of
sharpening/dithering, which I would have expected to decrease the
compressibility. The Histogram and the mean have changed slightly.

I can't get Imagemagick to create a gif smaller than 392366
bytes, significantly bigger. I've tried -strip, -filter
Lanczos, -unsharp, -sampling-factor, - everything I could find
but I can't get smaller. What am I missing? The only thing I can
think that Photoshop is doing is prefiltering intelligently to
increase the redundancy so that it LZW compresses better. Is
there any way of doing that in Imagemagick?

I can post the 3 images somewhere if that would help.

-- 
Illtud Daniel                                 illtud.daniel at llgc.org.uk
Uwch Ddadansoddwr Systemau                       Senior Systems Analyst
Llyfrgell Genedlaethol Cymru                  National Library of Wales
Yn siarad drosof fy hun, nid LlGC   -  Speaking personally, not for NLW