Atkinson Dither

# Atkinson Dither

A filter that takes a region in a monolith buffer and applies dither with two arbitrary colors. Implemented as a Janet plugin that works with Monolith.

Many thanks to this blog postfor providing clear instructions on how to implement this.

## Top

<<dither.c>>=
``````#include <math.h>
#define MONOLITH_ANSI
#include "janet/janet.h"
#include "patchwerk.h"
#include "runt.h"
#include "monolith.h"
<<grayscale>>
<<algorithm>>
<<janet_function>>
<<funclist>>
<<plugin_entry>>``````

## Boilerplate Janet stuff

To get this algorithm wrapped inside of Janet, you need some boilerplate. Quite dull actually. Why are you still reading this?

<<funclist>>=
``````#ifndef BUILD_DITHER_PLUGIN
#define NAME(s) "monolith/gfx-"s
#else
#define NAME(s) s
#endif

static const JanetReg cfuns[] = {
{NAME("dither"),
j_dither,
"Dithers a rectangular region between two colors."},
{NULL, NULL, NULL}
};``````
<<plugin_entry>>=
``````void monolith_gfx_dither_loader(JanetTable *env,
const char *name)
{
janet_cfuns(env, name, cfuns);
}
#ifdef BUILD_DITHER_PLUGIN
JANET_MODULE_ENTRY(JanetTable *env)
{
}
#endif``````

### Janet Function

<<janet_function>>=
``````static monolith_pixel rgbval(int *rgb)
{
return monolith_pixel_make(rgb, rgb, rgb, 255);
}
static Janet j_dither(int32_t argc, Janet *argv)
{
int x, y;
int w, h;
int fg;
int bg;
int i;
monolith_d *m;
monolith_framebuffer *fb;

janet_fixarity(argc, 10);
m = monolith_data_get();
x = janet_getinteger(argv, 0);
y = janet_getinteger(argv, 1);
w = janet_getinteger(argv, 2);
h = janet_getinteger(argv, 3);

for (i = 0; i < 3; i++) {
fg[i] = janet_getinteger(argv, 4 + i);
bg[i] = janet_getinteger(argv, 7 + i);
}

fb = monolith_fb_get(m);

atkinson_dither(fb, x, y, w, h, rgbval(fg), rgbval(bg));
return janet_wrap_nil();
}``````

## Grayscale Conversion

Before dither can be applied, the RGB colorspace needs to be converted to grayscale. This `pixel2grayscale` function will take in an RGB pixel and return an integer between 0 and 225.

A standard linear conversion is used (no gamma correction). Not great, but it's fast and good enough.

<<grayscale>>=
``````static int pixel2grayscale(monolith_pixel *p)
{
double oned255;
double r, g, b;
double gray;

oned255 = 1.0 / 255;

r = p->r * oned255;
g = p->g * oned255;
b = p->b * oned255;

gray = 0.2126 * r + 0.715 * g + 0.0722 * b;

return floor(gray * 255);
}``````

## The Dither Algorithm

The actual dither algorithm itself. There's some great places to learn about dither, including the blog post I linked at the top of this document. These are good places to get a thorough overview of the topic, not here.

Dithering is a matter of taking each pixel and approximating it to the nearest color. If a pixel is kinda light, make it white. If it's kinda dark, make it black. For our filter, these two colors are referred to as light and dark.

Two dimensional dither works by taking the error (how much it snapped to the color), and distributing it to the surrounding pixels, in a process known as error diffusion. How this error is divied up and distrubuted determines the diffusion algorithm.

Atkinson dithering works by taking the error, dividing it by 8, and then distributing that value amongst 6 surrounding pixels: two pixels down, two pixels forward, one pixel down to the left, and one pixel down to the right. It helps to draw a picture (the blog post I used had an excellent one).

The full error does not get distributed. Apparently, this sort of technique is known to provide a "reduced color bleed", reducing speckling artifacts, but causing contiguous light/dark regions to be more pronounced.

And that's it in a nutshell. To implement the error distribution, three row buffers were used: the first row is used to store errors for the current row, the second for the next row, the third for the row after that. At the end of every row, the row buffers rotate up in position, causing the first row to be the new third row. While in in the first row, the buffer gets zero'd out as it goes left-to-right, causing it to be fully re-initialized by the time the rotation happens.

<<algorithm>>=
``````static void atkinson_dither(monolith_framebuffer *f,
int xpos, int ypos,
int w, int h,
monolith_pixel light,
monolith_pixel dark)
{
int x, y;
int *mem;
int *rows;
int rowpos;
int *err;

mem = malloc(3 * w * sizeof(int));
rowpos = 0;

rows = &mem;
rows = &mem[w];
rows = &mem[w*2];

err = rows[rowpos];
err = rows[(rowpos + 1) % 3];
err = rows[(rowpos + 2) % 3];

for (x = 0; x < 3 * w; x++) mem[x] = 0;

for (y = 0; y < h; y++) {
for (x = 0; x < w; x++) {
monolith_pixel p;
monolith_pixel *out;
int gray;
int s;
int e;
e = 0;

s = 0;
monolith_gfx_pixel_get(f, x + xpos, y + ypos, &p);
gray = pixel2grayscale(&p) + err[x];

/* clear error */
err[x] = 0;

s = (gray > 127);

if (s) {
out = &light;
e = gray - 255;
} else {
out = &dark;
e = gray;
}

/* divide error by 8 */
e >>= 3;

if ((x + 2) < w) {
err[x + 2] += e;

/* also do x + 1 */
err[x + 1] += e;
err[x + 1] += e;
} else if ((x + 1) < w) {
err[x + 1] += e;
err[x + 1] += e;
}

/* x + 1 distribution */
if (x > 0) {
err[x - 1] += e;
}

err[x] += e;
err[x] += e;

monolith_gfx_pixel_set(f, x + xpos, y + ypos, *out);

/* shift rows circularly */
rowpos = (rowpos + 1) % 3;
err = rows[rowpos];
err = rows[(rowpos + 1) % 3];
err = rows[(rowpos + 2) % 3];
}
}

free(mem);
}``````