I know that [Floyd–Steinberg dithering](http://en.wikipedia.org/wiki/Floyd%E2%80%93Steinberg_dithering) algorithm can't be implemented with pixel shader, because that algorithm is strictly sequential. But maybe there exist some higly parallel dithering algorithm which by it's visual output is similar to Floyd-Steinberg algorithm ?

So the question is - What are dithering algorithms which are suitable to implement on pixel shader (preferably GLSL) and with output quality (very) similar to Floyd-Steinberg dithering ?

BTW. Multi-pass algorithms are allowed until there are not more than 2 passes and CPU overhead between those passes is small.

Any ideas ?

**EDIT:**  
I need dithering from 24-bit color to 21-bit color.   
 (That is - i need to convert from 8 bits/channel to 7 bits/channel.)

**EDIT 2**
Maybe I've not explained problem very well. So i'll try to expand a bit on exact problem.
Problem is this - consider we have this picture:  

![alt text](https://i.stack.imgur.com/sFXx5.png)

And we have above picture, but processed with dithering algorithm:  

![alt text](https://i.stack.imgur.com/DNDpP.png)

Now this is procedure which will test your dithering is good for me or not:

1. Load these pictures in Photoshop as one picture with 2 layers.

2. Choose Layers blending mode to "Difference".

3. Perform "Merge Visible" operation on layers, to get just one layer.

4. Perform operation => Image/Adjustments/Equalize

After that you must get such image:  

![alt text](https://i.stack.imgur.com/AjAfT.png)

As you see - middle pixels which was in monotone red color was not dithered at all.
Also dithering of left and right image zones is a bit different. Try to reconstruct dithering algorithm with such behavior.