dither.cc

// PipelineC and LiteX integration demo: basic graphics with dither
// Copyright (c) 2022 Victor Suarez Rovere <suarezvictor@gmail.com>

#include "pipelinec_compat.h"

extern int FRAME_WIDTH;
extern int FRAME_HEIGHT;
struct pixel_t { uint8_t a, b, g, r; };

#define DITHER

inline uint16_t gray(uint8_t v) { return v ^ (v<<1); }

inline uint9_t dither(uint8_t x, uint8_t y, uint9_t v)
{
 uint8_t pattern = gray((x&7)^gray(y&7)) & 0xF;
 return (pattern + v) & 0x1F0;
}

pixel_t render_pixel(uint16_t i, uint16_t j)
{
  int16_t x = i << 1; x = x - (FRAME_WIDTH + 1);
  int16_t y = j << 1; y = (FRAME_HEIGHT + 1) - y;

  uint9_t r = 0;
  uint9_t g = 0;
  uint9_t b = 0;

  if(x*x*4 + y*y*3 < FRAME_HEIGHT*FRAME_HEIGHT*4*4/(3*3))
   g = (i-j) & 0xFF;
  else
   b = (i+j) & 0xFF;

#ifdef DITHER
  r = dither(i, j, r);
  g = dither(i, j, g);
  b = dither(i, j, b);
#endif

  pixel_t pix = {0, 0, 0, 0};

  if(r >= 256) pix.r = 255; else pix.r = r;
  if(g >= 256) pix.g = 255; else pix.g = g;
  if(b >= 256) pix.b = 255; else pix.b = b;

  return pix;
}