Processing 1.0 - Processing Discourse - image dithering: 8bit -> monochrome

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Index › Programming Questions & Help › Programs › image dithering: 8bit -> monochrome

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image dithering: 8bit -> monochrome (Read 655 times)

extrapixel

image dithering: 8bit -> monochrome
Feb 20^th, 2008, 2:08pm

hi guys,

I have a monochrome (b/w) graphics lcd here and i'd like to generate "nice" b/w images for it. at the moment i'm using a threshold (if brightness>127: pixel=white). This results in quite ugly images, as one might expect.

so i figured i need some "dithering". there's information about it out there, but i really don't know where to start to implement a realtime dither algorithm in java. I found some solutions using JAI, but actually i want to stick with "normal" processing java. I don't care that much about what dithering algorithm to use (http://en.wikipedia.org/wiki/Dither#Dithering_algorithms), all of them are probably better than thresholding.

thanks for thoughts and hints!

http://www.extrapixel.ch

JohnG Ex Member	Re: image dithering: 8bit -> monochrome Reply #1 - Feb 20^th, 2008, 2:38pm Floyd-Steinberg looks promisingly simple, yet sufficiently effective.

antiplastik YaBB Moderator Offline Posts: 699 France	Re: image dithering: 8bit -> monochrome Reply #2 - Feb 20^th, 2008, 2:51pm There's a link describing a simple dithering algorithm, "not as good as Floyd-Steinberg", as it says, but I gave it a try, and it's quite efficient : http://wwwhome.cs.utwente.nl/~schooten/graphics/

extrapixel

Re: image dithering: 8bit -> monochrome
Reply #3 - Feb 21^st, 2008, 2:48am

thanks! antiplastik, your link is great. i tried to implement it... it kind of works. i'm getting 3 "shades of grey" only, but i guess it could be 5. For example i can't get a white-white area. Maybe this is a good oportunity for me to finaly learn how to use bitwise operations and rewrite this thing more elegantly...

http://www.extrapixel.ch/processing/dithering.zip

Quote:

// image dithering
// http://wwwhome.cs.utwente.nl/~schooten/graphics/
PImage ditherImage(PImage img) {
PImage ditheredImg = img.get(0,0,img.width, img.height);

// tile = 2*2 pixels
int[] tilePixelColors = new int[4];
float[] bias = new float[] {
0.25, .75, 0.5, 0.0 };
int pixelPosition;
float valScale = 1.0 / 255.0;

// rows
for (int i=0; i<img.height/2.0; i++) {
// coloumns
for (int j=0; j<img.width/2.0; j++) {
pixelPosition = i*2*img.width + j*2;

tilePixelColors[0] = (int) (bias[0] + valScale*greyValue(img.pixels[pixelPosition]));
tilePixelColors[1] = (int) (bias[1] + valScale*greyValue(img.pixels[pixelPosition+1]));
tilePixelColors[2] = (int) (bias[2] + valScale*greyValue(img.pixels[pixelPosition+img.width]));
tilePixelColors[2] = (int) (bias[3] + valScale*greyValue(img.pixels[pixelPosition+img.width+1]));

ditheredImg.pixels[pixelPosition] = color(255*tilePixelColors[0]);
ditheredImg.pixels[pixelPosition+1] = color(255*tilePixelColors[1]);
ditheredImg.pixels[pixelPosition+img.width] = color(255*tilePixelColors[2]);
ditheredImg.pixels[pixelPosition+img.width+1] = color(255*tilePixelColors[3]);
}
}
return ditheredImg;
}

int greyValue(int _color) {
return (int)((red(_color) + green(_color) + blue(_color)) / 3.0) ;
}

http://www.extrapixel.ch

antiplastik

Re: image dithering: 8bit -> monochrome
Reply #4 - Feb 21^st, 2008, 12:07pm

Yep, I had the same "can't get a white area" problem. I think it is due to the bias values (more exactly the last one which is 0.0). After playing a while, I had a pretty good result with those : {0.25, .75, 0.5, 0.25}.

I'm not sure, but maybe using a larger grid (i.e. 4x4 pixels with different bias values) would result in more shades of grey?

Quote:

Maybe this is a good oportunity for me to finaly learn how to use bitwise operations

/ 256 equals to >> 8
* 256 equals to << 8

Classiclll Full Member Offline Posts: 187	Re: image dithering: 8bit -> monochrome Reply #5 - Feb 21^st, 2008, 1:24pm my sketch is here http://www.geocities.jp/classiclll_newweb/DitherTest/applet/index.html hope it is for your information

extrapixel Full Member Offline Posts: 210 Switzerland	Re: image dithering: 8bit -> monochrome Reply #6 - Feb 21^st, 2008, 2:01pm Classiclll, thank you! your code seems to produce much better results than my attempts. what kind of dithering algorithm does it implement? i'm having having a hard time finding out what your code does exactly
	http://www.extrapixel.ch

Classiclll

Re: image dithering: 8bit -> monochrome
Reply #7 - Feb 21^st, 2008, 4:28pm

hi extrapixel

my idea is

- dither pattern 2*2
0, 2
3, 1
so
P1(i,j) := ( i%2!=j%2 ? 2: 0 ) + j%2

- dither psttern 4*4
00,08,02,10
12,04,14,06
03,11,01,09
15,07,13,05
so
P2(i,j) := 4*P1(i%2,j%2) + P1(i/2,j/2)

- dither pattern 8*8
[-omitted-]
so
P3(i,j) := 4*P2(i%2,j%2) + P2(i/2,j/2)
:= 4*{ 4*P1(i%2%2,j%2%2) + P1(i%2/2,j%2/2) }
+ 4*P1(i/2%2,j/2%2) + P1(i/2/2,j/2/2)

- so generally
- color depth is 2^(2n)
and
- Pn(i,j) := (i%2<>j%2 ? 1 : 0) + j%2 when n=1
:= 4*Pn-1(i%2,j%2) when n>1
+ Pn-1(i/2,j/2)

- dither conversion
- let k(x,y) be color value at (x,y)
- following gives dither bit at (x,y)
( Pn(x, y) <= k(x,y) ? black : white )

extrapixel Full Member Offline Posts: 210 Switzerland	Re: image dithering: 8bit -> monochrome Reply #8 - Feb 21^st, 2008, 10:30pm ok, i'm slowly getting it thank you! here's a video that compares your algorithm (at the start and end of the video) with my quick'n'dirty method (middle part): http://www.extrapixel.ch/processing/compare_dither.mov it's a 128x64 graphic lcd... looks kinda crappy in the video, but it's so cool in real.
	http://www.extrapixel.ch

Classiclll Full Member Offline Posts: 187	Re: image dithering: 8bit -> monochrome Reply #9 - Feb 22^nd, 2008, 5:09am may be "dither matrix" or "ordered dithering" http://www.jhlabs.com/ip/filters/DitherFilter.html my logic is an algorithm creating any sized matrix, i think.
	« Last Edit: Feb 22^nd, 2008, 6:52pm by Classiclll »

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