path: root/hacks/interference.c

/* interference.c --- colored fields via decaying sinusoidal waves.
 * An entry for the RHAD Labs Screensaver Contest.
 *
 * Author: Hannu Mallat <hmallat@cs.hut.fi>
 *
 * Copyright (C) 1998 Hannu Mallat.
 *
 * Permission to use, copy, modify, distribute, and sell this software and its
 * documentation for any purpose is hereby granted without fee, provided that
 * the above copyright notice appear in all copies and that both that
 * copyright notice and this permission notice appear in supporting
 * documentation.  No representations are made about the suitability of this
 * software for any purpose.  It is provided "as is" without express or 
 * implied warranty.
 *
 * decaying sinusoidal waves, which extend spherically from their
 * respective origins, move around the plane. a sort of interference
 * between them is calculated and the resulting twodimensional wave
 * height map is plotted in a grid, using softly changing colours.
 *
 * not physically (or in any sense) accurate, but fun to look at for 
 * a while. you may tune the speed/resolution/interestingness tradeoff 
 * with X resources, see below.
 *
 * Created      : Wed Apr 22 09:30:30 1998, hmallat
 * Last modified: Wed Apr 22 09:30:30 1998, hmallat
 * Last modified: Sun Aug 31 23:40:14 2003, 
 *              david slimp <rock808@DavidSlimp.com>
 * 		added -hue option to specify base color hue
 * Last modified: Wed May 15 00:04:43 2013,
 *              Dave Odell <dmo2118@gmail.com>
 *              Tuned performance; double-buffering is now off by default.
 *              Made animation speed independent of FPS.
 *              Added cleanup code, fixed a few glitches.
 *              Added gratuitous #ifdefs.
 * Last modified: Fri Feb 21 02:14:29 2014, <dmo2118@gmail.com>
 *              Added support for SMP rendering.
 *              Tweaked math a bit re: performance.
 * Last modified: Tue Dec 30 16:43:33 2014, <dmo2118@gmail.com>
 *              Killed the black margin on the right and bottom.
 *              Reduced the default grid size to 2.
 * Last modified: Sun Oct 9 11:20:48 2016, <dmo2118@gmail.com>
 *              Updated for new xshm.c.
 *              Ditched USE_BIG_XIMAGE.
 */

#include <math.h>
#include <errno.h>

#include "screenhack.h"

#include "thread_util.h"

#ifdef HAVE_INTTYPES_H
# include <inttypes.h>
#else

typedef unsigned int uint32_t;
typedef unsigned short uint16_t;
typedef unsigned char uint8_t;

#endif

/*
Tested on an Intel(R) Pentium(R) 4 CPU 3.00GHz (family 15, model 6, 2 cores),
1 GB PC2-4200, nouveau - Gallium 0.4 on NV44, X.Org version: 1.13.3. A very
modest system by current standards.

Does double-buffering make sense? (gridsize = 2)
USE_XIMAGE is off: Yes (-db: 4.1 FPS, -no-db: 2.9 FPS)
XPutImage in strips: No (-db: 35.9 FPS, -no-db: 38.7 FPS)
XPutImage, whole image: No (-db: 32.3 FPS, -no-db: 33.7 FPS)
MIT-SHM, whole image: Doesn't work anyway: (-no-db: 37.3 FPS)

If gridsize = 1, XPutImage is slow when the XImage is one line at a time.
XPutImage in strips: -db: 21.2 FPS, -no-db: 19.7 FPS
XPutimage, whole image: -db: 23.2 FPS, -no-db: 23.4 FPS
MIT-SHM: 26.0 FPS

So XPutImage in strips is very slightly faster when gridsize >= 2, but
quite a bit worse when gridsize = 1.
*/

/* I thought it would be faster this way, but it turns out not to be... -jwz */
/* It's a lot faster for me, though - D.O. */
#define USE_XIMAGE

/* Numbers are wave_table size, measured in # of unsigned integers.
 * FPS/radius = 50/radius = 800/radius = 1500/Big-O memory usage
 *
 * Use at most one of the following:
 * Both off = regular sqrt() - 13.5 FPS, 50/800/1500. */

/* #define USE_FAST_SQRT_HACKISH */ /* 17.8 FPS/2873/4921/5395/O(lg(radius)) */
#define USE_FAST_SQRT_BIGTABLE2 /* 26.1 FPS/156/2242/5386/O(radius^2) */

#ifdef HAVE_DOUBLE_BUFFER_EXTENSION
# include "xdbe.h"
#endif /* HAVE_DOUBLE_BUFFER_EXTENSION */

#ifdef USE_XIMAGE
# include "xshm.h"
#endif /* USE_XIMAGE */

static const char *interference_defaults [] = {
  ".background:  black",
  ".foreground:  white",
  "*count:       3",     /* number of waves */
  "*gridsize:    2",     /* pixel size, smaller values for better resolution */
  "*ncolors:     192",   /* number of colours used */
  "*hue:         0",     /* hue to use for base color (0-360) */
  "*speed:       30",    /* speed of wave origins moving around */
  "*delay:       30000", /* or something */
  "*color-shift: 60",    /* h in hsv space, smaller values for smaller
			  * color gradients */
  "*radius:      800",   /* wave extent */
  "*gray:        false", /* color or grayscale */
  "*mono:        false", /* monochrome, not very much fun */

  "*doubleBuffer: False", /* doubleBuffer slows things down for me - D.O. */
#ifdef HAVE_DOUBLE_BUFFER_EXTENSION
  "*useDBE:      True", /* use double buffering extension */
#endif /* HAVE_DOUBLE_BUFFER_EXTENSION */

#ifdef HAVE_XSHM_EXTENSION
  "*useSHM:      True", /* use shared memory extension */
#endif /*  HAVE_XSHM_EXTENSION */
#ifdef HAVE_MOBILE
  "*ignoreRotation: True",
#endif
  THREAD_DEFAULTS
  0
};

static XrmOptionDescRec interference_options [] = {
  { "-count",       ".count",       XrmoptionSepArg, 0 }, 
  { "-ncolors",     ".ncolors",     XrmoptionSepArg, 0 }, 
  { "-gridsize",    ".gridsize",    XrmoptionSepArg, 0 }, 
  { "-hue",         ".hue",         XrmoptionSepArg, 0 },
  { "-speed",       ".speed",       XrmoptionSepArg, 0 },
  { "-delay",       ".delay",       XrmoptionSepArg, 0 },
  { "-color-shift", ".color-shift", XrmoptionSepArg, 0 },
  { "-radius",      ".radius",      XrmoptionSepArg, 0 },
  { "-gray",        ".gray",        XrmoptionNoArg,  "True" },
  { "-mono",        ".mono",        XrmoptionNoArg,  "True" },
  { "-db",	    ".doubleBuffer", XrmoptionNoArg,  "True" },
  { "-no-db",	    ".doubleBuffer", XrmoptionNoArg,  "False" },
#ifdef HAVE_XSHM_EXTENSION
  { "-shm",	".useSHM",	XrmoptionNoArg, "True" },
  { "-no-shm",	".useSHM",	XrmoptionNoArg, "False" },
#endif /*  HAVE_XSHM_EXTENSION */
  THREAD_OPTIONS
  { 0, 0, 0, 0 }
};

struct inter_source {
  int x;
  int y;
  double x_theta;
  double y_theta;
};

struct inter_context {
  /*
   * Display-related entries
   */
  Display* dpy;
  Window   win;

#ifdef HAVE_DOUBLE_BUFFER_EXTENSION
  XdbeBackBuffer back_buf;
#endif /* HAVE_DOUBLE_BUFFER_EXTENSION */
  Pixmap   pix_buf;

  GC       copy_gc;
#ifdef USE_XIMAGE
  XImage  *ximage;

  Bool shm_can_draw;
  XShmSegmentInfo shm_info;
#endif /* USE_XIMAGE */

  /*
   * Resources
   */
  int count;
  int grid_size;
  int colors;
  float hue;
  int speed;
  int delay;
  int shift;

  /*
   * Drawing-related entries
   */
  int w;
  int h;
  unsigned w_div_g, h_div_g;
  Colormap cmap;
  Screen *screen;
  unsigned bits_per_pixel;
  XColor* pal;
#ifndef USE_XIMAGE
  GC* gcs;
#endif
  int radius; /* Not always the same as the X resource. */
  double last_frame;

  struct threadpool threadpool;

  /*
   * lookup tables
   */
  unsigned* wave_height;
    
  /*
   * Interference sources
   */
  struct inter_source* source;
};

struct inter_thread
{
  const struct inter_context *context;
  unsigned thread_id;

#ifdef USE_XIMAGE
  uint32_t* row;
#endif

  unsigned* result_row;
};

#ifdef HAVE_DOUBLE_BUFFER_EXTENSION
# define TARGET(c) ((c)->back_buf ? (c)->back_buf : \
                    (c)->pix_buf ? (c)->pix_buf : (c)->win)
#else  /* HAVE_DOUBLE_BUFFER_EXTENSION */
# define TARGET(c) ((c)->pix_buf ? (c)->pix_buf : (c)->win)
#endif /* !HAVE_DOUBLE_BUFFER_EXTENSION */

#ifdef USE_FAST_SQRT_HACKISH
/* Based loosely on code from Wikipedia:
 * https://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Approximations_that_depend_on_IEEE_representation
 */

/* FAST_SQRT_EXTRA_BITS = 3: Smallest useful value
 * = 5/6: A little bit of banding, wave_height table is on par with regular
 *        sqrt() code.
 * = 7: No apparent difference with original @ radius = 800.
 * = 8: One more just to be comfortable.
 */

# define FAST_SQRT_EXTRA_BITS 8

union int_float
{
  uint32_t i;
  float f;
};

static unsigned fast_log2(unsigned x)
{
  union int_float u;
  if(!x)
    return x;
  u.f = x;
  return ((u.i - 0x3f800000) >> (23 - FAST_SQRT_EXTRA_BITS)) + 1;
}

static float fast_inv_log2(unsigned x)
{
  union int_float u;
  if(!x)
    return 0.0f;
  u.i = ((x - 1) << (23 - FAST_SQRT_EXTRA_BITS)) + 0x3f800000;
  return u.f;
}

#endif

#ifdef USE_FAST_SQRT_BIGTABLE2

/* I eyeballed these figures. They could be improved. - D.O. */

# define FAST_SQRT_DISCARD_BITS1 4
/* = 5: Dot in center is almost invisible at radius = 800. */
/* = 4: Dot in center looks OK at radius = 50. */

/* 156/2740/9029 */
/* # define FAST_SQRT_DISCARD_BITS2 8 */
/* # define FAST_SQRT_CUTOFF 64 * 64 */

/* 156/2242/5386 */
# define FAST_SQRT_DISCARD_BITS2 9
# define FAST_SQRT_CUTOFF 128 * 128

/*
 * This is a little faster:
 * 44.5 FPS, 19/5000/17578
 *
 * # define FAST_SQRT_DISCARD_BITS1 7
 * # define FAST_SQRT_DISCARD_BITS2 7
 * # define FAST_SQRT_CUTOFF 0
 *
 * For radius = 800, FAST_SQRT_DISCARD_BITS2 =
 * = 9/10: Approximately the original table size, some banding near origins.
 * = 7: wave_height is 20 KB, and just fits inside a 32K L1 cache.
 * = 6: Nearly indistinguishable from original
 */

/*
  FAST_TABLE(x) is equivalent to, but slightly faster than:
  x < FAST_SQRT_CUTOFF ?
    (x >> FAST_SQRT_DISCARD_BITS1) :
    ((x - FAST_SQRT_CUTOFF) >> FAST_SQRT_DISCARD_BITS2) +
      (FAST_SQRT_CUTOFF >> FAST_SQRT_DISCARD_BITS1);
*/

#define FAST_TABLE(x) \
  ((x) < FAST_SQRT_CUTOFF ? \
    ((x) >> FAST_SQRT_DISCARD_BITS1) : \
    (((x) + \
      ((FAST_SQRT_CUTOFF << (FAST_SQRT_DISCARD_BITS2 - \
        FAST_SQRT_DISCARD_BITS1)) - FAST_SQRT_CUTOFF)) >> \
      FAST_SQRT_DISCARD_BITS2))

static double fast_inv_table(unsigned x)
{
  return x < (FAST_SQRT_CUTOFF >> FAST_SQRT_DISCARD_BITS1) ?
    (x << FAST_SQRT_DISCARD_BITS1) :
    ((x - (FAST_SQRT_CUTOFF >> FAST_SQRT_DISCARD_BITS1)) <<
      FAST_SQRT_DISCARD_BITS2) + FAST_SQRT_CUTOFF;
}

#endif

static void destroy_image(Display* dpy, struct inter_context* c)
{
#ifdef USE_XIMAGE
  if(c->ximage) {
    destroy_xshm_image(dpy, c->ximage, &c->shm_info);
  }
#endif

  if(c->threadpool.count)
  {
    threadpool_destroy(&c->threadpool);
    c->threadpool.count = 0;
  }
}

static void inter_free(Display* dpy, struct inter_context* c)
{
#ifndef USE_XIMAGE
  unsigned i;
#endif

  if(c->pix_buf)
    XFreePixmap(dpy, c->pix_buf);

  if(c->copy_gc)
    XFreeGC(dpy, c->copy_gc);

  destroy_image(dpy, c);

  if(c->colors <= 2)
    free(c->pal);
  else if(c->pal)
    free_colors(c->screen, c->cmap, c->pal, c->colors);

#ifndef USE_XIMAGE
  for(i = 0; i != c->colors; ++i)
    XFreeGC(dpy, c->gcs[i]);
  free(c->gcs);
#endif

  free(c->wave_height);
  free(c->source);
}

static void abort_on_error(int error)
{
  fprintf(stderr, "interference: %s\n", strerror(error));
  exit(1);
}

static void abort_no_mem(void)
{
  abort_on_error(ENOMEM);
}

static void check_no_mem(Display* dpy, struct inter_context* c, void* ptr)
{
  if(!ptr) {
    inter_free(dpy, c);
    abort_no_mem();
  }
}

static int inter_thread_create(
  void* self_raw,
  struct threadpool* pool,
  unsigned id)
{
  struct inter_thread* self = (struct inter_thread*)self_raw;
  const struct inter_context* c = GET_PARENT_OBJ(struct inter_context, threadpool, pool);

  self->context = c;
  self->thread_id = id;

  self->result_row = malloc(c->w_div_g * sizeof(unsigned));
  if(!self->result_row)
    return ENOMEM;

#ifdef USE_XIMAGE
  self->row = malloc(c->w_div_g * sizeof(uint32_t));
  if(!self->row) {
    free(self->result_row);
    return ENOMEM;
  }
#endif

  return 0;
}

static void inter_thread_destroy(void* self_raw)
{
  struct inter_thread* self = (struct inter_thread*)self_raw;
#ifdef USE_XIMAGE
  free(self->row);
#endif
  free(self->result_row);
}

/*
A higher performance design would have input and output queues, so that when
worker threads finish with one frame, they can pull the next work order from
the queue and get started on it immediately, rather than going straight to
sleep. The current "single-buffered" design still provides reasonable
performance at low frame rates; high frame rates are noticeably less efficient.
*/

static void inter_thread_run(void* self_raw)
{
  struct inter_thread* self = (struct inter_thread*)self_raw;
  const struct inter_context* c = self->context;

  int i, j, k;
  unsigned result;
  int dist1;
  int g = c->grid_size;

  int dx, dy, g2 = 2 * g * g;
  int px, py, px2g;

  int dist0, ddist;

#ifdef USE_XIMAGE
  unsigned img_y = g * self->thread_id;
  void *scanline = c->ximage->data + c->ximage->bytes_per_line * g * self->thread_id;
#endif

  for(j = self->thread_id; j < c->h_div_g; j += c->threadpool.count) {
    px = g/2;
    py = j*g + px;

    memset(self->result_row, 0, c->w_div_g * sizeof(unsigned));

    for(k = 0; k < c->count; k++) {

      dx = px - c->source[k].x;
      dy = py - c->source[k].y;

      dist0 = dx*dx + dy*dy;
      ddist = -2 * g * c->source[k].x;

      /* px2g = g*(px*2 + g); */
      px2g = g2;

      for(i = 0; i < c->w_div_g; i++) {
        /*
         * Discarded possibilities for improving performance here:
         * 1. Using octagon-based distance estimation
         *    (Which causes giant octagons to appear.)
         * 2. Square root approximation by reinterpret-casting IEEE floats to
         *    integers.
         *    (Which causes angles to appear when two waves interfere.)
         */

/*      int_float u;
        u.f = dx*dx + dy*dy;
        u.i = (1 << 29) + (u.i >> 1) - (1 << 22);
        dist = u.f; */

#if defined USE_FAST_SQRT_BIGTABLE2
        dist1 = FAST_TABLE(dist0);
#elif defined USE_FAST_SQRT_HACKISH
        dist1 = fast_log2(dist0);
#else
        dist1 = sqrt(dist0);
#endif

        if(dist1 < c->radius)
          self->result_row[i] += c->wave_height[dist1];

        dist0 += px2g + ddist;
        px2g += g2;
      }
    }

    for(i = 0; i < c->w_div_g; i++) {

      result = self->result_row[i];

      /* It's slightly faster to do a subtraction or two before calculating the
       * modulus. - D.O. */
      if(result >= c->colors)
      {
        result -= c->colors;
        if(result >= c->colors)
          result %= (unsigned)c->colors;
      }

#ifdef USE_XIMAGE
      self->row[i] = c->pal[result].pixel;
#else
      XFillRectangle(c->dpy, TARGET(c), c->gcs[result], g*i, g*j, g, g);
#endif /* USE_XIMAGE */
    }

#ifdef USE_XIMAGE
    /* Fill in these `gridsize' horizontal bits in the scanline */
    if(c->ximage->bits_per_pixel == 32)
    {
      uint32_t *ptr = (uint32_t *)scanline;
      for(i = 0; i < c->w_div_g; i++) {
        for(k = 0; k < g; k++)
          ptr[g*i+k] = self->row[i];
      }
    }
    else if(c->ximage->bits_per_pixel == 24)
    {
      uint8_t *ptr = (uint8_t *)scanline;
      for(i = 0; i < c->w_div_g; i++) {
        for(k = 0; k < g; k++) {
          uint32_t pixel = self->row[i];
          /* Might not work on big-endian. */
          ptr[0] = pixel;
          ptr[1] = (pixel & 0x0000ff00) >> 8;
          ptr[2] = (pixel & 0x00ff0000) >> 16;
          ptr += 3;
        }
      }
    }
    else if(c->ximage->bits_per_pixel == 16)
    {
      uint16_t *ptr = (uint16_t *)scanline;
      for(i = 0; i < c->w_div_g; i++) {
        for(k = 0; k < g; k++)
          ptr[g*i+k] = self->row[i];
      }
    }
    else if(c->ximage->bits_per_pixel == 8)
    {
      uint8_t *ptr = (uint8_t *)scanline;
      for(i = 0; i < c->w_div_g; i++) {
        for(k = 0; k < g; k++)
          ptr[g*i+k] = self->row[i];
      }
    }
    else
    {
      for(i = 0; i < c->w_div_g; i++) {
        for(k = 0; k < g; k++)
          /* XPutPixel is thread safe as long as the XImage didn't have its
           * bits_per_pixel changed. */
          XPutPixel(c->ximage, (g*i)+k, img_y, self->row[i]);
      }
    }

    /* Only the first scanline of the image has been filled in; clone that
       scanline to the rest of the `gridsize' lines in the ximage */
    for(k = 0; k < (g-1); k++)
      memcpy(c->ximage->data + (c->ximage->bytes_per_line * (img_y + k + 1)),
             c->ximage->data + (c->ximage->bytes_per_line * img_y),
             c->ximage->bytes_per_line);

    scanline = (char *)scanline +
                 c->ximage->bytes_per_line * g * c->threadpool.count;
    img_y += g * c->threadpool.count;

#endif /* USE_XIMAGE */
  }
}

/* On allocation error, c->ximage == NULL. */
static void create_image(
  Display* dpy, 
  struct inter_context* c, 
  const XWindowAttributes* xgwa)
{
#ifdef USE_XIMAGE

  /* Set the width so that each thread can work on a different line. */
  unsigned align = thread_memory_alignment(dpy) * 8 - 1;
  unsigned wbits, w, h;
#endif /* USE_XIMAGE */

  c->w = xgwa->width;
  c->h = xgwa->height;
  c->w_div_g = (c->w + c->grid_size - 1) / c->grid_size;
  c->h_div_g = (c->h + c->grid_size - 1) / c->grid_size;

#ifdef USE_XIMAGE
  w = c->w_div_g * c->grid_size;
  h = c->h_div_g * c->grid_size;

  /* The width of a scan line, in *bits*. */
  wbits = (w * c->bits_per_pixel + align) & ~align;

  /* This uses a lot more RAM than the single line approach. Users without
   * enough RAM to fit even a single framebuffer should consider an upgrade for
   * their 386. - D.O.
   */

  c->ximage = create_xshm_image(dpy, xgwa->visual, xgwa->depth,
                                ZPixmap, &c->shm_info,
                                wbits / c->bits_per_pixel, h);

  c->shm_can_draw = True;

  check_no_mem(dpy, c, c->ximage);
#endif /* USE_XIMAGE */

  {
    static const struct threadpool_class cls =
    {
      sizeof(struct inter_thread),
      inter_thread_create,
      inter_thread_destroy
    };

    int error = threadpool_create(
      &c->threadpool,
      &cls,
      dpy,
#ifdef USE_XIMAGE
      hardware_concurrency(dpy)
#else
      1
      /* At least two issues with threads without USE_XIMAGE:
       * 1. Most of Xlib isn't thread safe without XInitThreads.
       * 2. X(Un)LockDisplay would need to be called for each line, which is
       *    terrible.
       */
#endif
      );

    if(error) {
      c->threadpool.count = 0; /* See the note in thread_util.h. */
      inter_free(dpy, c);
      abort_on_error(error);
    }
  }
}

static void create_pix_buf(Display* dpy, Window win, struct inter_context *c,
                           const XWindowAttributes* xgwa)
{
#ifdef HAVE_DOUBLE_BUFFER_EXTENSION
  if(c->back_buf)
    return;
#endif /* HAVE_DOUBLE_BUFFER_EXTENSION */
  c->pix_buf = XCreatePixmap(dpy, win, xgwa->width, xgwa->height, xgwa->depth);
}

static double float_time(void)
{
  struct timeval result;
  gettimeofday(
    &result
#ifdef GETTIMEOFDAY_TWO_ARGS
    , NULL
#endif
    );

  return result.tv_usec * 1.0e-6 + result.tv_sec;
}

static void inter_init(Display* dpy, Window win, struct inter_context* c) 
{
  XWindowAttributes xgwa;
  double H[3], S[3], V[3];
  int i;
  int mono;
  int gray;
  int radius;
  double scale = 1;
  XGCValues val;
  Bool dbuf = get_boolean_resource (dpy, "doubleBuffer", "Boolean");

#ifndef USE_XIMAGE
  unsigned long valmask = 0;
#endif

# ifdef HAVE_JWXYZ	/* Don't second-guess Quartz's double-buffering */
  dbuf = False;
# endif

  memset (c, 0, sizeof(*c));

  c->dpy = dpy;
  c->win = win;


  c->delay = get_integer_resource(dpy, "delay", "Integer");

  XGetWindowAttributes(c->dpy, c->win, &xgwa);
  c->cmap = xgwa.colormap;
  c->screen = xgwa.screen;
  c->bits_per_pixel = visual_pixmap_depth(xgwa.screen, xgwa.visual);
  check_no_mem(dpy, c, (void *)(ptrdiff_t)c->bits_per_pixel);

  val.function = GXcopy;
  c->copy_gc = XCreateGC(c->dpy, TARGET(c), GCFunction, &val);

  c->count = get_integer_resource(dpy, "count", "Integer");
  if(c->count < 1)
    c->count = 1;
  c->grid_size = get_integer_resource(dpy, "gridsize", "Integer");
  if(c->grid_size < 1)
    c->grid_size = 1;
  mono = get_boolean_resource(dpy, "mono", "Boolean");
  if(!mono) {
    c->colors = get_integer_resource(dpy, "ncolors", "Integer");
    if(c->colors < 2)
      c->colors = 2;
  }
  c->hue = get_integer_resource(dpy, "hue", "Float");
  while (c->hue < 0 || c->hue >= 360)
    c->hue = frand(360.0);
  c->speed = get_integer_resource(dpy, "speed", "Integer");
  c->shift = get_float_resource(dpy, "color-shift", "Float");
  while(c->shift >= 360.0)
    c->shift -= 360.0;
  while(c->shift <= -360.0)
    c->shift += 360.0;
  radius = get_integer_resource(dpy, "radius", "Integer");;
  if(radius < 1)
    radius = 1;

  if (xgwa.width > 2560) scale = 3.5;  /* Retina displays */
  radius *= scale;

  create_image(dpy, c, &xgwa);

  if(!mono) {
    c->pal = calloc(c->colors, sizeof(XColor));
    check_no_mem(dpy, c, c->pal);

    gray = get_boolean_resource(dpy, "gray", "Boolean");
    if(!gray) {
      H[0] = c->hue;
      H[1] = H[0] + c->shift < 360.0 ? H[0]+c->shift : H[0] + c->shift-360.0; 
      H[2] = H[1] + c->shift < 360.0 ? H[1]+c->shift : H[1] + c->shift-360.0; 
      S[0] = S[1] = S[2] = 1.0;
      V[0] = V[1] = V[2] = 1.0;
    } else {
      H[0] = H[1] = H[2] = 0.0;
      S[0] = S[1] = S[2] = 0.0;
      V[0] = 1.0; V[1] = 0.5; V[2] = 0.0;
    }

    make_color_loop(c->screen, xgwa.visual, c->cmap, 
		    H[0], S[0], V[0], 
		    H[1], S[1], V[1], 
		    H[2], S[2], V[2], 
		    c->pal, &(c->colors), 
		    True, False);
    if(c->colors < 2) { /* color allocation failure */
      mono = 1;
      free(c->pal);
    }
  }

  if(mono) { /* DON'T else this with the previous if! */
    c->colors = 2;
    c->pal = calloc(2, sizeof(XColor));
    check_no_mem(dpy, c, c->pal);
    c->pal[0].pixel = BlackPixel(c->dpy, DefaultScreen(c->dpy));
    c->pal[1].pixel = WhitePixel(c->dpy, DefaultScreen(c->dpy));
  }

#ifdef USE_XIMAGE
  dbuf = False;
  /* Double-buffering doesn't work with MIT-SHM: XShmPutImage must draw to the
   * window. Otherwise, XShmCompletion events will have the XAnyEvent::window
   * field set to the back buffer, and XScreenSaver will ignore the event.
   */
#endif

  if (dbuf)
    {
#ifdef HAVE_DOUBLE_BUFFER_EXTENSION
      c->back_buf = xdbe_get_backbuffer (c->dpy, c->win, XdbeUndefined);
#endif /* HAVE_DOUBLE_BUFFER_EXTENSION */

      create_pix_buf(dpy, win, c, &xgwa);
    }

#ifndef USE_XIMAGE
  valmask = GCForeground;
  c->gcs = calloc(c->colors, sizeof(GC));
  check_no_mem(dpy, c, c->gcs);
  for(i = 0; i < c->colors; i++) {
    val.foreground = c->pal[i].pixel;    
    c->gcs[i] = XCreateGC(c->dpy, TARGET(c), valmask, &val);
  }
#endif

#if defined USE_FAST_SQRT_HACKISH
  c->radius = fast_log2(radius * radius);
#elif defined USE_FAST_SQRT_BIGTABLE2
  c->radius = radius * radius;
  c->radius = FAST_TABLE(c->radius);
#else
  c->radius = radius;
#endif

  if (c->radius < 1) c->radius = 1;
  c->wave_height = calloc(c->radius, sizeof(unsigned));
  check_no_mem(dpy, c, c->wave_height);

  for(i = 0; i < c->radius; i++) {
    float max, fi;
#if defined USE_FAST_SQRT_HACKISH
    fi = sqrt(fast_inv_log2(i));
#elif defined USE_FAST_SQRT_BIGTABLE2
    fi = sqrt(fast_inv_table(i));
#else
    fi = i;
#endif
    max = 
      ((float)c->colors) * 
      ((float)radius - fi) /
      ((float)radius);
    c->wave_height[i] = 
      (unsigned)
      ((max + max*cos(fi/(50.0 * scale))) / 2.0);
  }

  c->source = calloc(c->count, sizeof(struct inter_source));
  check_no_mem(dpy, c, c->source);

  for(i = 0; i < c->count; i++) {
    c->source[i].x_theta = frand(2.0)*3.14159;
    c->source[i].y_theta = frand(2.0)*3.14159;
  }

  c->last_frame = float_time();
}

#define source_x(c, i) \
  (c->w/2 + ((int)(cos(c->source[i].x_theta)*((float)c->w/2.0))))
#define source_y(c, i) \
  (c->h/2 + ((int)(cos(c->source[i].y_theta)*((float)c->h/2.0))))

/*
 * This is somewhat suboptimal. Calculating the distance per-pixel is going to
 * be a lot slower than using now-ubiquitous SIMD CPU instructions to do four
 * or eight pixels at a time.
 */

static unsigned long do_inter(struct inter_context* c)
{
  int i;

  double now;
  float elapsed;

#ifdef USE_XIMAGE
  /* Wait a little while for the XServer to become ready if necessary. */
  if(!c->shm_can_draw)
    return 2000;
#endif

  now = float_time();
  elapsed = (now - c->last_frame) * 10.0;

  c->last_frame = now;

  for(i = 0; i < c->count; i++) {
    c->source[i].x_theta += (elapsed*c->speed/1000.0);
    if(c->source[i].x_theta > 2.0*3.14159)
      c->source[i].x_theta -= 2.0*3.14159;
    c->source[i].y_theta += (elapsed*c->speed/1000.0);
    if(c->source[i].y_theta > 2.0*3.14159)
      c->source[i].y_theta -= 2.0*3.14159;
    c->source[i].x = source_x(c, i);
    c->source[i].y = source_y(c, i);
  }

  threadpool_run(&c->threadpool, inter_thread_run);
  threadpool_wait(&c->threadpool);

#ifdef USE_XIMAGE
  put_xshm_image(c->dpy, c->win, c->copy_gc, c->ximage, 0, 0, 0, 0,
                 c->ximage->width, c->ximage->height, &c->shm_info);
  /* c->shm_can_draw = False; */
#endif

#ifdef HAVE_DOUBLE_BUFFER_EXTENSION
  if (c->back_buf)
    {
      XdbeSwapInfo info[1];
      info[0].swap_window = c->win;
      info[0].swap_action = XdbeUndefined;
      XdbeSwapBuffers(c->dpy, info, 1);
    }
  else
#endif /* HAVE_DOUBLE_BUFFER_EXTENSION */
    if (c->pix_buf)
      {
        XCopyArea (c->dpy, c->pix_buf, c->win, c->copy_gc,
                   0, 0, c->w, c->h, 0, 0);
      }

  return c->delay;
}

static void *
interference_init (Display *dpy, Window win)
{
  struct inter_context *c = (struct inter_context *) calloc (1, sizeof(*c));
  if(!c)
    abort_no_mem();
  inter_init(dpy, win, c);
  return c;
}

static unsigned long
interference_draw (Display *dpy, Window win, void *closure)
{
  struct inter_context *c = (struct inter_context *) closure;
  return do_inter(c);
}

static void
interference_reshape (Display *dpy, Window window, void *closure, 
                 unsigned int w, unsigned int h)
{
  struct inter_context *c = (struct inter_context *) closure;
  XWindowAttributes xgwa;
  Bool dbuf = (!!c->pix_buf
# ifdef HAVE_DOUBLE_BUFFER_EXTENSION
               || c->back_buf
# endif
               );

#ifdef USE_XIMAGE
  destroy_image(dpy, c);
  c->ximage = 0;
#endif

  if(c->pix_buf)
    XFreePixmap(dpy, c->pix_buf);
  c->pix_buf = None;

  XGetWindowAttributes(dpy, window, &xgwa);
  xgwa.width = w;
  xgwa.height = h;
  create_image(dpy, c, &xgwa);
  if(dbuf)
    create_pix_buf(dpy, window, c, &xgwa);
}

static Bool
interference_event (Display *dpy, Window window, void *closure, XEvent *event)
{
#if HAVE_XSHM_EXTENSION
  struct inter_context *c = (struct inter_context *) closure;

  if(event->type == XShmGetEventBase(dpy) + ShmCompletion)
  {
    c->shm_can_draw = True;
    return True;
  }
#endif
  return False;
}

static void
interference_free (Display *dpy, Window window, void *closure)
{
  struct inter_context *c = (struct inter_context *) closure;
  inter_free(dpy, c);
}

XSCREENSAVER_MODULE ("Interference", interference)