6 levels of grey scale

[Luke St. Everything]

I'm a casual user of Affinity and there's a lot I don't understand (or need to).  But I'm trying to convert a colour image to grey scale but without numerous levels of grey.  Ideally I would like the image to covert to 6 shades of grey that are clearly outlined from each other.  Is this possible and if so can anyone help me to achieve it.  Many thanks.

[Optische Ausrichtung]

1. Add a layer "Soft proof Adjustment". Choose a Generic Grey Profile as Proof Profile.
2. Add a layer "Posterize Adjustment". Choose 6 Posterize Levels.
3. Export as Greyscale Image

 

[GarryP]

I think a good answer would depend on which six levels of grey and which colours/tints relate to which of those levels.

In the meantime, you could add a Posterise Adjustment (with 6 levels) and a Black & White Adjustment to the image/layer – see attached image.

[Luke St. Everything]

Thank you so much for taking the time to help me.  I'll certainly try this.

[Luke St. Everything]

26 minutes ago, Optische Ausrichtung said:

1. Add a layer "Soft proof Adjustment". Choose a Generic Grey Profile as Proof Profile.
2. Add a layer "Posterize Adjustment". Choose 6 Posterize Levels.
3. Export as Greyscale Image

 

Thank you so much for taking the time to help me.  I'll certainly try this.

[David in Яuislip]

Try a gradient map, a little fussy to create but you get to choose where the tone splits are rather than rely on posterise which I've never had much luck with. The gradient map is in the psd as they can't be exported, at least I don't know how to

GradMap.psd

[walt.farrell]

7 minutes ago, David in Яuislip said:

The gradient map is in the psd as they can't be exported, at least I don't know how to.

I'm curious why you're sharing it as a .psd rather than an Affinity format such as .afphoto.

But for an exported format, "TIFF with Affinity Layers" or PSD may be the only choices.

[David in Яuislip]

2 minutes ago, walt.farrell said:

I'm curious why you're sharing it as a .psd rather than an Affinity format such as .afphoto.

Gotta think of the forum server space

8,533 GradMap.afphoto
4,618 GradMap.psd

[walt.farrell]

1 hour ago, David in Яuislip said:

Gotta think of the forum server space

8,533 GradMap.afphoto
4,618 GradMap.psd

A little smaller for me (using 2.5 beta), but interesting. Thanks.

[lacerto]

18 hours ago, Luke St. Everything said:

Ideally I would like the image to covert to 6 shades of grey that are clearly outlined from each other.

If you want to have exactly 6 levels then you need to combine the Posterize adjustment with Gradient and Black & White adjustments (the last one being useful when working with a color image). The exact number of levels will then be determined by the Posterize adjustment while the Gradient adjustment determines the color scale. With Black & White adjustment you can determine the brightness level of primaries and how they interact with the defined gradient:

You can use the Histogram to check the number of levels in the image, or create a document palette (note that only max 64 tones/colors will be created). Often it is also necessary to add a solid layer at the bottom of the design (I suppose in situations an image has partial transparencies). If exact number of levels is not required, then the shown methods will produce appearance of decreased levels and are adequate.

[Luke St. Everything]

A big thank you to everyone who helped me.  I've now achieved exactly what I wanted thanks to you.  I really do appreciate your help.

[GarryP]

You’re welcome.

Just out of interest, and for anyone else wanting to do the same sort of thing, which approach did you find most useful?

[Luke St. Everything]

There was one a found too technical for me but I did experiment with a few.  I settled on your response and to some extent I was influenced by the pic. you included that demonstrated exactly what I wanted to achieve.  I did raise the contrast a little before proceeding but I don't know if it was relevant to the outcome.  The point is that I got what I was looking for.  Thank you again for the really useful help.

[walt.farrell]

3 hours ago, lacerto said:

If you want to have exactly 6 levels then you need to combine the Posterize adjustment with Gradient and Black & White adjustments

Why doesn't the Gradient Map with just 6 levels (suggested above by David) do it?

[lacerto]

17 hours ago, walt.farrell said:

Why doesn't the Gradient Map with just 6 levels (suggested above by David) do it?

My experience is that there will always be intermediate tones besides those specified in the Gradient Map adjustment. The stray tones can especially be seen when using color images. In context of black and white images this does not normally show, so if exact number of levels is not important (and it often is not, as images are normally not viewed at pixel level), the shown methods would be fine (though setting up a kind of accurate stepwise gradient is rather tedious). In low-resolution images, sporadic intermediate tones, even grayscale, are of course more easily discernible.

 

[David in Яuislip]

My previous gradient map was sloppy as the stop points between the regions didn't coincide so I fixed that

Now this below shows both of them over a sort of spectrum gradient. Exporting the exact gradient I got 8 colors rather than 6 so at great risk to my sanity I scanned across the bands and the figures show the results. The bands with 2 numbers have 1px wide strips spoiling the effect, only forensic scientists and nutters will notice, normal people are immune. As lacerto notes, a posterise layer is needed and it is all very tedious

Colours counted thus:
magick Greys-6exact.png -format '%k' info:
'8'
magick Greys-6exactPosterised.png -format '%k' info:
'6'

Happy friday!

[Luke St. Everything]

Crikey, what have I started.  All really interesting and helpful so thank you all.