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The above image is a Windows 95 default background. I wanted to know how they achieved this effect and was wondering if it is possible in Photoshop or if I need a dot-matrix printer or maybe a graphic-design program from the 90's to replicate it.

Open a photograph in Photoshop

Do Image > Adjusments > Desaturate

Then Image > Mode > Indexed color, and use the settings shown below.

Note: Afterwards, you can switch back to other modes such as RGB for further editing, by using Image > Mode > RGB

It's not dot-matrix printing simulation. It's stipple effect - a form of halftoning. It's well possible that some older software could use dot matrix printers this way to make greyshades with halftoning. As far as I know, generally the results of greyscale printing with dot matrix printers resembled much more the computer+hands image that user Danielillo has attached to his explanation. White was got by no hit, light grey = one hit, dark grey = two hits at the same place.

Photoshop is already discussed well, so I insert other possiblities:

The finest results I have seen were produced by using Astute Graphics plugin Stipplism for Illustrator. It costs some money, but free raster image editors have usable methods, too.

In GIMP you can convert a desaturated RGB image to Indexed mode (Image > Mode > Indexed) with certain settings and get quite good results. The key is to use Floyd-Steinberg Dithering method. Start with 1 bit black and white coloring. You can also experiment with a few more colors to get more possible shades of grey with the same number of dots.

This is an example of the results in GIMP:

This is the original and the conversion dialog:

The resulted image is indexed, but you can convert it back to RGB or normal grayscale, if needed and you do not lose the quality. But the file size grows.

In Krita you can use BW > Dithering effect of the included G'MIC filter pack and get quite same final result See a screenshot:

You should note that the simulated greyscale isn't complete nor fully linear, so you probably must experiment with curves tool to get well transformable greyscale. In G'MIC package's BW > Dither you have gamma, contrast and brightness sliders which partially do the same.

NOTE: If you have too high resolution in the original image, the result is easily so dense that it fights with the display pixel grid producing weird interference patterns. Printing also needs some care for the same reason.

_{Image from unsplash.com}

From Photoshop:

• Menu Image → Mode → Grayscale


• Menu Image → Mode → Bitmap → Method Use: Halftone Screen

• Shape: Square

It is a black and white dithering effect you can have in many graphics software. For illustration I used the G'MIC filters which can be used for free online, or as a plugin to Gimp.

^{Source:Pixabay}

Filter > Black & White > Dithering

For more options there also is a filter Pattern > Halftone which is available in the plugin only.

This technique is called dithering. It's not primarily used as an effect, but as a method of choosing the pixel values used when doing colour reduction on an image (e.g. conversion to an indexed palette).

There are many dithering algorithms available. Some produce an obvious regular pattern, while some - like this one - produce a more scattered effect. The above page on dithering provides many examples.

I remember that image - it's a black-and-white copy of the setup.bmp image that came with Windows 95 (it was originally black and blue, but otherwise exactly the same).

It's hard to know the exact algorithm that was used on it, but it looks quite a bit like Floyd-Steinberg dithering:

This algorithm can be chosen in GIMP when converting an image to use Indexed mode.