Point reduction filter for InklingReader

point-reduction.c

#include "point-reduction.h"
#include "../datatypes/coordinate.h"
#include "../datatypes/element.h"

static int
opt_in_between (dt_coordinate* first, dt_coordinate* second, dt_coordinate* third)
{
  int crossproduct =
    (third->y - first->y) * (second->x - first->x) -
    (third->x - first->x) * (second->y - first->y);

  if (crossproduct < 100 && crossproduct > -100) return 1;

  int dotproduct =
    (third->x - first->x) * (second->x - first->x) +
    (third->y - first->y) * (second->y - first->y);

  if (dotproduct < 0) return 1;

  int squaredlength =
    (second->x - first->x) * (second->x - first->x) +
    (second->y - first->y) * (second->y - first->y);

  if (dotproduct > squaredlength) return 1;

  return 0;
}

int
opt_point_reduction_apply (GSList* data)
{
  GSList* head = data;

  dt_coordinate* first = NULL;
  dt_coordinate* second = NULL;
  dt_coordinate* third = NULL;
  
  while (data != NULL)
    {
      dt_element* e = (dt_element *)data->data;
      if (e->type == TYPE_COORDINATE)
	{
	  if (third == NULL)
	    {
	      if (first == NULL) first = (dt_coordinate *)data->data; 
	      else if (second == NULL) second = (dt_coordinate *)data->data; 
	      else if (third == NULL) third = (dt_coordinate *)data->data; 
	    }
	  else
	    {
	      if (opt_in_between (first, second, third))
		head = g_slist_remove (head, second);

	      first = second;
	      second = third;
	      third = NULL;
	    }
	}

      /* Reset the optimization when it's the beginning or end of a stroke. */
      if (e->type == TYPE_STROKE)
	{
	  first = NULL;
	  second = NULL;
	  third = NULL;
	}

      data = data->next;
    }

  return 1;
}

Sorry, can't really say what's best here. I just can "forward" a discussion that followed @n8willis's lightning talk at LGM about the Inklingreader. About point reduction and drawing optimization, Chris Lilley (if I remember well) suggested to look for works or algos done by the mapping community. They have really thought about optimization and efficiency with these kind of problems. So instead of trying to reinvent the wheel, maybe there is some research to do there. But sorry, I can't be much more help here.