I've seen a few threads on dithering lately and I've done some work in the past on this aspect of drawing, both with hand drawn art and using programming to see what works best. The work I've done is rather experimental but I think it might be of interest to some people here, as well as being useful in your own artwork.

A year or so ago I wanted to see how to get the best results via programming to colour reduce a 24bit truecolour image to a fixed palette 256 colour picture.
I deliberately chose a fixed palette as adaptive palette reduction in paint programs is already very good, but fixed colour palettes I've seen are normally pretty poor. They are often variations on a 332 bit RGB colourspace which covers most of the palette range but is pretty poor in every other way (only 8 shades for a start).

As artists will know, HSV colourspace is closer to how we actually see and is also a better way of approaching dithering (and probably computer art in general).

I proceeded along the following assumptions:

+ Our eyes are most sensitive to detecting differences in luminance (this can be seen in black and white photography and how easy it is to tell shades apart)
+ Our eyes are less sensitive in detecting difference in hue (one example of this is that JPEG's reduce the hue channel data without the average person noticing, another is putting say a blue and a green next to each other of the same brightness in noticing the edges)
+ Our eyes are least sensitive in detecting differences in saturation (this can be seen by the difficulties in seeing differences between a grey and a desaturated version of the same brightness when next to each other)

There is nothing new about dithering in hue and saturation dimensions of course. On the Amiga the Bitmap Brothers often differed various shades of two hues together to make a third hue in their games. However I think there is more potential when using a limited palette for this kind of dithering that is often used, as luminance dithering is easier to do and much more common and well known.

So we now have the order of importance for selecting colours for each HSV dimension in the fixed palette. Because we are least sensitive to saturation we will have one range of grey shades and then fully saturated shades of the hues. This effectively only gives us a one bit scale for saturation and even another range of half-saturated colours would be very useful when dithering, but would take up too many colours in our 256 colour fixed palette to be done.

We now add 8 hues (I also tried 7 with an increased number of shades per hue, but 8 provides a bit more hue coverage). The 8 hues are pretty close to the colours of a rainbow, but each hue's luminance value is adjusted so it is the same brightness to all other hues for that shade. What I mean by that is that a yellow is not blended with a full red, but more of a pink colour (as pure red can't reach the brightness of a full yellow). By doing this we can now dither across the hue dimension without also dithering across the luminance dimension (which is what makes the dithering so noticable when the two colours are quite different in brightness)

So with 8 different hues and 1 grey scale 'hue' we can have 28 shades x 9 hues for 252 palette entries. We use the final 4 entries for a pure black and white and a very dark grey and slightly dim white, to make a total shading range of 32 for each colour:



Now we need a dither pattern to blend between the hues, saturation and luminance dimensions so I used a 64 shade 8 x 8 ordered dither pattern.

I tested the program I made with a variety of images and you can find a selection of some of these (original and colour reduced version) below. With only 256 colours you are always trying to find the best balance between gaps in hue coverage, saturation coverage and shading, especially in a fixed colour palette. The results tend to be higher in dithering across the images compared to other fixed colour palettes, but the dithering is much less noticable overall. In fact even when magnified the images work better overall, and the dithering is less noticable than with other methods when seen at distance.


Landscape test photo
(low detail clouds with testing of grey clouds against bright blue sky, high texture detail in rocks, grass and building with varying hues from orange to green)

Original: http://www.retroidea.com/Fixed256/output6_24.png
Fixed palette (222 colours): http://www.retroidea.com/Fixed256/output6.png


Human test photo
(Fashion style picture of Keira Knightly, showing low contrast and detail areas such as skin areas and background as well as other areas of high detail and contrast such as facial features, and hair)

Original: http://www.retroidea.com/Fixed256/output4_24.png
Fixed palette (128 colours): http://www.retroidea.com/Fixed256/output4.png


Computer art pixel painting
(Old truecolour demo picture to test against this more artifical image)

Original: http://www.retroidea.com/Fixed256/output2_24.png
Fixed palette (245 colours): http://www.retroidea.com/Fixed256/output2.png


Photo scene
(Difficult test of saturation blending with subtle hues and a range of details and contrast)

Original: http://www.retroidea.com/Fixed256/output1_24.png
Fixed palette (157 colours): http://www.retroidea.com/Fixed256/output1.png


These images aren't perfect and there are errors in hue, saturation and some obvious dithering areas. But I hope it shows the potential of dithering in Hues and Saturation and creating the illusion of more colours.

I hope it helps!
Richard

Is it me or has board activity decreased quite a lot from a few years ago?

Hi all,

As some of you may know I am working on my own paint program and am seeking advice and ideas from the people who have the most experience of art, artists!
It is likely that I will have a lot of questions about different aspects of the program and people's experience with other paint programs they have used. Would it be appropriate to start different threads on this section of Pixelation to ask these kinds of question? I don't want to spam up the board, but then again this isn't the most active section and putting everything in one thread would probably quickly get disorganised.

What do you advise please?

Thanks,
Richard

Hi there,

I'm currently trying (for the 3rd time!) to make and complete a paint program for my own use (and anyone else who wants to use it too). I'm not 100% happy with any paint program I have tried.

This will be aimed at 3 aspects of art:
1. Low colour (palette based) sprites/tiles/art
2. True colour manipulation (nothing too advanced)
3. Good alpha mask/channel editing and support.

These are in order of importance as well. The program won't try to compete with photoshop, but will fill a niche for the 3 aspects I described above.

So are there any tools, ideas or features you have that I can make a note of? I'm looking for unique ideas that will be practical and useful and preferably aren't offered in any other program (besides the basic tools common to most paint programs). No guarantees I will use or implement any suggestions of course, but they may well prove useful!

Since I don't consider myself an artist, I created a little prototype to make art through the power of chaos and evolution. I started a thread on this here:
http://www.retroremakes.com/forum2/showthread.php?t=10804

It's great for small 2 colour symmetrical sprites. And my avatar is also completely created with this tool.

Don't kill me!