Color/grayscale to monochrome conversion

[Chiter Chater]

Hello,

I'm in the process of scanning some old projects.

The company whose scanning services I used has a professional large format scanner with proprietary color-to-monochrome conversion algorithm built in.

Here are a few samples, with info on each starting from leftmost one:



- Color scan
- B&W scan using the proprietary conversion algorithm of a professional large format scanner
- B&W conversion of a grayscale scan using Threshold set to arbitrary value
- Grayscale to monochrome conversion using Photoshop's Pattern Dither or Diffusion Dither method
- Grayscale to monochrome conversion using Photoshop's Pattern Dither or Diffusion Dither method

I prefer the look of monochrome scans with commercial scanner's algorithm used.

How to obtain a similar result using Photoshop tools on a grayscale image?
Is there a way to modify Photoshop's Pattern Dither and Diffusion Dither methods?

The scan second from left and the one second from right appear to be of similar quality, but the right one has more noise as can be seen when both are zoomed in:



Thanks!

 

[Tom Mann]

Pattern, diffusion, stochastic, and other dithering techniques were originally developed for the printing of contone (ie, continuous tone) images on devices that do not intrinsically have good continuous tone abilities. For example, because inkjet printers can only put out droplets of a small set of specific colors, the software drivers for inkjet printers always include some sort of dithering technique in which the brightness and color of a section of an image determines how close the droplets are to each other, and/or the size of the droplets laid down in that area, etc.

Once non-technical people discovered these dithering effects, they also started to be used for artsy effects that roughly approximate stippling done with conventional pen and pencil.

Originally, these techniques were never intended to be used for general color-to-grayscale conversions, and certainly not for technical and other line drawings such as found in many blueprints, architectural drawings, etc., but because they are computationally easy to implement, the essentially binary files that dithered scanners use can be easily compressed, etc. they eventually found their way into this field, as well.

You already discovered for yourself the reason why you don't like some dithering methods: The results can look noisy.

To be honest, I'm not sure exactly what your preferences are, but an easy way to go between the two example cases you provided is to adjust the black and white endpoint sliders in a "levels" adjustment layer before you convert to a bitmap and do the dithering. I've attached a couple of examples of different white and black levels below.

HTH,

Tom M

 

[Chiter Chater]

Thanks Tom,

I was able to get decent results by changing dithering percentage via Save for Web(B&W Gif) dialog.

Trying different Levels settings, like you suggested, helped even further.

Do you know of any programs or Photoshop plugins that let one control and apply different dithering algorithms?

I suspect being able to control one of many dithering algorithms is all I need to get the result I want.

And yes, I'm using dithering to convert to B&W and then compress...

Thanks!

 

[Tom Mann]

Although it's not quite "dithering", you might like Alpha Plugin's "Engraver":
http://www.alphaplugins.com/EngraverIIIManual/engraverIIImanual.php

Here is a before and after shot. In the interest of full disclosure, I did some very simple tweaks (eg, levels, invert, etc. mostly) to both the original and then after applying "Engraver" to get the effect you see.

I don't own this one, but you might like it as well. It does much more realistic stippling:
http://www.pixeology.com/ArtisticHalftone/features.html

Tom M