ordered dither checks and remapping possible?

[mattyrb]

Hi,
I've been playing with dithering and remapping. I have images I want to reduce to a selected 16 colours from a colourmap of 512. I've got all that working nicely. However I'd like to use -ordered-dither checks rather than Riemersma or FloydSteinberg. Is this possible?

Best wishes,
Matt

[snibgo]

Riemersma and FloydSteinberg are error-diffusion methods. "-ordered-dither" is a dither with no error diffusion. So I don't understand the question.

[mattyrb]

Okay I'm trying to remap an image to a colourmap of 512 then order dither it using checks to 16 colours?

[snibgo]

Sorry, I still don't understand. "-ordered-dither" makes each pixel either "on" or "off", so it can only make 8 colours, which are: black, white, red, green, blue, cyan, magenta and cyan.

It can't make 16 colours, or colours of your choosing. See http://www.imagemagick.org/Usage/quanti ... red-dither

Can you show an example of what you want?

[mattyrb]

I literally want to reduce my images to 16 colours but use a dither pattern that's consistent

[snibgo]

Consistent in what sense?

If you want to reduce an image to 16 colours that you choose, then "-remap" will do that. But each pixel is assigned the closest of the 16 colours, either with error-diffusion dithering, or no with dithering. IM has no simple method for ordered-dithering with 16 chosen colours.

[mattyrb]

When I say consistent the dither pattern needs to be consistent obviously error-diffusion is too random

[fmw42]

When I say consistent the dither pattern needs to be consistent obviously error-diffusion is too random

I suspect he means the same unique pattern over the whole image, i.e. an ordered-dither

[mattyrb]

When I say consistent the dither pattern needs to be consistent obviously error-diffusion is too random

I suspect he means the same unique pattern over the whole image, i.e. an ordered-dither

Yes! Sorry. I'm really new to image processing!

[fmw42]

Perhaps this is what you want. See https://imagemagick.org/Usage/quantize/#ordered-dither

[fmw42]

Also see the section https://imagemagick.org/Usage/quantize/#od_posterize for comparison of different methods.

[mattyrb]

Order-dither is definitely what I'm after but I can't fathom how to restrict it to 16 colours? It looks like you can only restrict to RGB steps?

-ordered-dither checks,8,8,8 is what I have at present but I then have to run a quantize to reduce the colouring but that can't be right surely!

[fmw42]

I am not a dithering expert. But you could reduce the colors to 16 using +dither -colors 16, then do you ordered dither

Code: Select all

convert inputimage +dither -colors 16 -ordered-dither checks,8,8,8 outputimage

The +dither before -colors turns off dithering for reducing colors.

Alternately, you could create a 16 color colormap image

convert xc:color1 xc:color2 .... xc:color16 +append colormap.gif


and then do

Code: Select all

convert inputimage +dither -remap colormap.gif -ordered-dither checks,8,8,8 outputimage

[fmw42]

Please always provide your IM version and platform/OS, since syntax may vary.

[mattyrb]

Thanks I'll post back in the morning