Dithering
Right, we have covered color and resolution now so we can finally start doing something with them. We can exchange color for resolution or resolution for color. If we up the resolution and reduce the number of colors then this is called dithering and is a very common process. In fact when you print out an image then it will be done for you as part of the printer driver even if you dont ask for it. Dithering can be applied to all images but generally paint apps do not perform it on color index type. For the other type, the dithering is done per channel so we might as well stick to one channel and use grayscale images to show the effects.
There are many types of dithering but the most common is halftoning which is what is also known as an ordered dither. Each pixel of a large number of gray shades are converted to less shades (usually black and white only) by substituting them for a small grid of new pixels where the average of the new grid is the same shade as the original pixel. What is vital to understand is that if you dither an image then you must up the resolution of it by an amount that the dither grid size if you want to preserve the image quality. The dither grid size should depend on the amount of shades being converted. So if we were to go from full grayscale to black and white that is a 256 to 2 color drop. So using a 16x16 grid should be the minimum to allow for the full averaging. I doubt any paint app would use that and its more likely to use a 3x3 or 4x4 grid instead. Therefore, expect some loss and at least multiply the resolution by 4 in each direction.
A second but slightly less common form is the error difusion. When converting the PC keeps track of what color the dither would average to and compares with next pixel and applies the dither pixel a color which gets the dither color average as close to the sample image pixel. If that was in black and white only then if the dither average is too bright then it puts a black dot down, if too dark then a white one. Over space, the error wibbles up and down but averages out to track the sample image. If the sample image is made up of thin lines then this technique does not work so well as it confuses it but if the resolutuion is multiplied by 4 in each direction, generally, error diffusion is better than halftone.
An image shown next is the color one dithered into the 16 color windows palette. Not that bad considering the bizarre set of colors contained in that palette.
Now that you can appreciate what is happening behind the scenes you will also appreciate printer DPI settings. A standard color desktop printer might have a DPI rating of 1440 but it only has 4 or maybe 6 ink colors. A color photo has to be dithered in CMYK down to 4 inks before being sprayed. The 1440 rating is the number of dots per inch per component. So it is not how much DPI you can print a full color photo at. A modern color ink jet printer uses error diffusion (sometimes called giclee for some bizarre and cost increasing marketing hype reason) so that you should expect that to print a full color pixel you would need to divide this number by something between 4 and 16 to get the real 'true color' DPI rating of the final print. So a 1440 DPI printer can print photos at about 200 pixels per inch which is still quite good as can be seen from out previous discussion about resolution. Also, as a tip, dont try to pre-dither images before sending to the printer - let the printer do it.
We stated earlier that we can go back the other way too. We can trade resolution for more colors. First, up the image number of colors to full color or grayscale and then average sections of the image out. For example assume that we can see that a halftone image was processed with a 4x4 grid. Then average those grids out. We can do the same with error diffused dither by using a blur. By looking at dithered images from farther than the ability to resolve the dot pattern, your eye is just doing the averaging for you.
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