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dejitter.c
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dejitter.c
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/*
* tslib/plugins/dejitter.c
*
* Copyright (C) 2001 Russell King.
*
* This file is placed under the LGPL. Please see the file
* COPYING for more details.
*
*
* Problem: some touchscreens read the X/Y values from ADC with a
* great level of noise in their lowest bits. This produces "jitter"
* in touchscreen output, e.g. even if we hold the stylus still,
* we get a great deal of X/Y coordinate pairs that are close enough
* but not equal. Also if we try to draw a straight line in a painter
* program, we'll get a line full of spikes.
*
* Solution: we apply a smoothing filter on the last several values
* thus excluding spikes from output. If we detect a substantial change
* in coordinates, we reset the backlog of pen positions, thus avoiding
* smoothing coordinates that are not supposed to be smoothed. This
* supposes all noise has been filtered by the lower-level filter,
* e.g. by the "variance" module.
*/
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <stdio.h>
#include "config.h"
#include "tslib.h"
#include "tslib-filter.h"
/**
* This filter works as follows: we keep track of latest N samples,
* and average them with certain weights. The oldest samples have the
* least weight and the most recent samples have the most weight.
* This helps remove the jitter and at the same time doesn't influence
* responsivity because for each input sample we generate one output
* sample; pen movement becomes just somehow more smooth.
*/
#define NR_SAMPHISTLEN 4
/* To keep things simple (avoiding division) we ensure that
* SUM(weight) = power-of-two. Also we must know how to approximate
* measurements when we have less than NR_SAMPHISTLEN samples.
*/
static const unsigned char weight [NR_SAMPHISTLEN - 1][NR_SAMPHISTLEN + 1] =
{
/* The last element is pow2(SUM(0..3)) */
{ 5, 3, 0, 0, 3 }, /* When we have 2 samples ... */
{ 8, 5, 3, 0, 4 }, /* When we have 3 samples ... */
{ 6, 4, 3, 3, 4 }, /* When we have 4 samples ... */
};
struct ts_hist {
int x;
int y;
unsigned int p;
};
struct tslib_dejitter {
struct tslib_module_info module;
int delta;
int x;
int y;
int down;
int nr;
int head;
struct ts_hist hist[NR_SAMPHISTLEN];
};
static int sqr (int x)
{
return x * x;
}
static void average (struct tslib_dejitter *djt, struct ts_sample *samp)
{
const unsigned char *w;
int sn = djt->head;
int i, x = 0, y = 0;
unsigned int p = 0;
w = weight [djt->nr - 2];
for (i = 0; i < djt->nr; i++) {
x += djt->hist [sn].x * w [i];
y += djt->hist [sn].y * w [i];
p += djt->hist [sn].p * w [i];
sn = (sn - 1) & (NR_SAMPHISTLEN - 1);
}
samp->x = x >> w [NR_SAMPHISTLEN];
samp->y = y >> w [NR_SAMPHISTLEN];
samp->pressure = p >> w [NR_SAMPHISTLEN];
#ifdef DEBUG
fprintf(stderr,"DEJITTER----------------> %d %d %d\n",
samp->x, samp->y, samp->pressure);
#endif
}
static int dejitter_read(struct tslib_module_info *info, struct ts_sample *samp, int nr)
{
struct tslib_dejitter *djt = (struct tslib_dejitter *)info;
struct ts_sample *s;
int count = 0, ret;
ret = info->next->ops->read(info->next, samp, nr);
for (s = samp; ret > 0; s++, ret--) {
if (s->pressure == 0) {
/*
* Pen was released. Reset the state and
* forget all history events.
*/
djt->nr = 0;
samp [count++] = *s;
continue;
}
/* If the pen moves too fast, reset the backlog. */
if (djt->nr) {
int prev = (djt->head - 1) & (NR_SAMPHISTLEN - 1);
if (sqr (s->x - djt->hist [prev].x) +
sqr (s->y - djt->hist [prev].y) > djt->delta) {
#ifdef DEBUG
fprintf (stderr, "DEJITTER: pen movement exceeds threshold\n");
#endif
djt->nr = 0;
}
}
djt->hist[djt->head].x = s->x;
djt->hist[djt->head].y = s->y;
djt->hist[djt->head].p = s->pressure;
if (djt->nr < NR_SAMPHISTLEN)
djt->nr++;
/* We'll pass through the very first sample since
* we can't average it (no history yet).
*/
if (djt->nr == 1)
samp [count] = *s;
else {
average (djt, samp + count);
samp [count].tv = s->tv;
}
count++;
djt->head = (djt->head + 1) & (NR_SAMPHISTLEN - 1);
}
return count;
}
static int dejitter_fini(struct tslib_module_info *info)
{
free(info);
return 0;
}
static const struct tslib_ops dejitter_ops =
{
.read = dejitter_read,
.fini = dejitter_fini,
};
static int dejitter_limit(struct tslib_module_info *inf, char *str, void *data)
{
struct tslib_dejitter *djt = (struct tslib_dejitter *)inf;
unsigned long v;
int err = errno;
v = strtoul(str, NULL, 0);
if (v == ULONG_MAX && errno == ERANGE)
return -1;
errno = err;
switch ((int)data) {
case 1:
djt->delta = v;
break;
default:
return -1;
}
return 0;
}
static const struct tslib_vars dejitter_vars[] =
{
{ "delta", (void *)1, dejitter_limit },
};
#define NR_VARS (sizeof(dejitter_vars) / sizeof(dejitter_vars[0]))
TSAPI struct tslib_module_info *dejitter_mod_init(struct tsdev *dev, const char *params)
{
struct tslib_dejitter *djt;
djt = malloc(sizeof(struct tslib_dejitter));
if (djt == NULL)
return NULL;
memset(djt, 0, sizeof(struct tslib_dejitter));
djt->module.ops = &dejitter_ops;
djt->delta = 100;
djt->head = 0;
if (tslib_parse_vars(&djt->module, dejitter_vars, NR_VARS, params)) {
free(djt);
return NULL;
}
djt->delta = sqr (djt->delta);
return &djt->module;
}
#ifndef TSLIB_STATIC_DEJITTER_MODULE
TSLIB_MODULE_INIT(dejitter_mod_init);
#endif