Posterize


Posterize - a short history ...

In the 1970s high contrast black and white images printed onto coloured base papers were very popular, and in vogue with work by Andy Warhol.

Similarly, images generated by separation methods to give high contrast cyan, magenta and yellow became easier to make as colour processing became easier.  In the days of analogue cameras and film, these posters were achieved by making film copies at differing densities on litho and equidensity films, then copying them on to colour film or developing the layers individually in chromogenic developers and binding the layers together mechanically.

Paint Shop Pro makes an attempt at posterization.  Effects>Artistic Effects>Posterize and then a choice of levels.  A minimum of two and a maximum of 16 are available.  The software seems to set the number of bits for each colour (RGB) and so at 16 the colours are barely degraded, and at 2 block tones are approached.  But the choice of tones is unavailable to you.

This is the original unadjusted colour image


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And this is the posterize menu


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The posterized image from a value of 3.  Not very exciting is it?


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Step One



The first step in making your posterized image is to make a greyscale black and white image from the colour original.  Adjust>Hue and Saturation> Hue/Saturation/Lightness and set the saturation slider to -100.  You may need to change the contrast and brightness.

(You can also posterize a greyscale black and white image – if that is what you are using then start at stage 3.)

Posters are best made from pictures containing large blocks of similar tone.  Fussy pictures containing fine detail posterize very badly.  See the tutorial on converting images to black and white if you are unsure of how to achieve this - right here.

Step Two and Three


Second stage.  Image>.Greyscale turns all of the colour information into shades of grey.

Third stage.  Image>Increase Color Depth>RGB 8 bits makes it possible to re-colour the image later.  At this stage you should make an analysis of the different shades of grey in your image.  In the bottom right hand corner of the Paint Shop Pro frame the grey shades (or colour values if we were dealing with a colour image) are shown as you move your mouse or tablet pen over the image area.

Image 4 shows the values area of the screen ...


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Step Four



Fourth stage.  We now need to generate the different layers which we are going to colour.  Image>Split Channel To CMYK makes four layers which will be eventually coloured cyan, magenta, yellow, and black just like printing inks.  We can choose which areas we can colourise in which way.

Step Five



The solution to many issues in PaintShopPro is through curves, and this is no exception.  If you are unfamiliar with how curves work you should take a look at the tutorials on Curves both here and here.

We need to add an adjustment layer to each of the colour separations we have made.  When we eventually re-assemble them, the areas which show black in each of the separations will become coloured, and the areas which are white will remain uncoloured for that layer.

So returning to our analysis in stage 3 you should be able to work out which grey area you want which colour.

When areas overlap you will combine the secondary colours to make primary ones by subtraction.


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In the example I’ve chosen I decided that I didn’t want a contribution to “greyness” and so I’ve removed its contribution totally, but you can experiment with this for yourself.

So we move onto using the curves.

Go to window to select the black separation.  Then select Layers>New Adjustment Layer>Curves.  As you can see, I’ve moved the curve up to the very top of the graph, making any value of grey become 255 or pure white and this will not contribute to the final image.  You might want a contribution of grey in your image.

Image5 shows screen for the black image.


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Repeat this procedure for the magenta image.

Image 6 shows the screen for the magenta image.


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I wanted the coach to have a red colour and so there must be a magenta contribution.  Pulling the curve to that shape isn’t easy in Paint ShopPro.  The easiest way is to put 4 points onto the curve, then hover the mouse over each point in turn to highlight it and pull it into position using the arrow keys.

As you do this the values will appear on the graph itself.  Again return to the curves tutorial where this is explained in more detail.  The values for this curve (from left to right) are ...


Point 1 Point 2 Point 3 Point 4 Point 5 Point 6
0 255 73 255 74 0 154 0 155 255 255 255



because the values for the grey in the coach are around 80, and we want a few other areas to change colour.


Image7 shows the screen for the yellow image

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Two coloured areas are created; one is around 80 to add yellow to the magenta for the bus turning it red; the other lies at higher values adding to the cyan turning it to green.  The values (from left to right) are ...

Point 1 Point 2 Point 3 Point 4 Point 5 Point 6
0 255 39 255 40 0 89 0 90 255 199 255


Point 7 Point 8 Point 9
200 0 254 0 255 255



Image 8 shows the screen for the cyan image.


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The values (from left to right) are ...

Point 1 Point 2 Point 3 Point 4 Point 5 Point 6
0 255 139 255 144 0 219 0 220 255 255 255


Step Six



And so onto the sixth stage in which the magic happens.  Image>Combine Channel>Combine From CMYK will bring you your poster.  Of course, using layers and curves will allow you to return to the separations you’ve made and allow you to choose a larger or smaller range of grey tones which is to be converted.

Here is the final image.  I think it is more personal than the one that Paint Shop Pro chose for me.

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Background

Posterization of an image entails conversion of a continuous gradation of tone to several regions of fewer tones, with abrupt changes from one tone to another. This was originally done with photographic processes to create posters.

It can now be done photographically or with digital image processing, and may be deliberate or may be an unintended artifact of color quantization.
(From Wikipedia)








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