third_party/fastfeat/nonmax.c

// clang-format off
#include <stdlib.h>
#include "fast.h"


#define Compare(X, Y) ((X)>=(Y))

xy* nonmax_suppression(const xy* corners, const int* scores, int num_corners, int* ret_num_nonmax)
{
	int num_nonmax=0;
	int last_row;
	int* row_start;
	int i, j;
	xy* ret_nonmax;
	const int sz = (int)num_corners;

	/*Point above points (roughly) to the pixel above the one of interest, if there
    is a feature there.*/
	int point_above = 0;
	int point_below = 0;


	if(num_corners < 1)
	{
		*ret_num_nonmax = 0;
		return 0;
	}

	ret_nonmax = (xy*)malloc(num_corners * sizeof(xy));

	/* Find where each row begins
	   (the corners are output in raster scan order). A beginning of -1 signifies
	   that there are no corners on that row. */
	last_row = corners[num_corners-1].y;
	row_start = (int*)malloc((last_row+1)*sizeof(int));

	for(i=0; i < last_row+1; i++)
		row_start[i] = -1;

	{
		int prev_row = -1;
		for(i=0; i< num_corners; i++)
			if(corners[i].y != prev_row)
			{
				row_start[corners[i].y] = i;
				prev_row = corners[i].y;
			}
	}



	for(i=0; i < sz; i++)
	{
		int score = scores[i];
		xy pos = corners[i];

		/*Check left */
		if(i > 0)
			if(corners[i-1].x == pos.x-1 && corners[i-1].y == pos.y && Compare(scores[i-1], score))
				continue;

		/*Check right*/
		if(i < (sz - 1))
			if(corners[i+1].x == pos.x+1 && corners[i+1].y == pos.y && Compare(scores[i+1], score))
				continue;

		/*Check above (if there is a valid row above)*/
		if(pos.y != 0 && row_start[pos.y - 1] != -1)
		{
			/*Make sure that current point_above is one
			  row above.*/
			if(corners[point_above].y < pos.y - 1)
				point_above = row_start[pos.y-1];

			/*Make point_above point to the first of the pixels above the current point,
			  if it exists.*/
			for(; corners[point_above].y < pos.y && corners[point_above].x < pos.x - 1; point_above++)
			{}


			for(j=point_above; corners[j].y < pos.y && corners[j].x <= pos.x + 1; j++)
			{
				int x = corners[j].x;
				if( (x == pos.x - 1 || x ==pos.x || x == pos.x+1) && Compare(scores[j], score))
					goto cont;
			}

		}

		/*Check below (if there is anything below)*/
		if(pos.y != last_row && row_start[pos.y + 1] != -1 && point_below < sz) /*Nothing below*/
		{
			if(corners[point_below].y < pos.y + 1)
				point_below = row_start[pos.y+1];

			/* Make point below point to one of the pixels belowthe current point, if it
			   exists.*/
			for(; point_below < sz && corners[point_below].y == pos.y+1 && corners[point_below].x < pos.x - 1; point_below++)
			{}

			for(j=point_below; j < sz && corners[j].y == pos.y+1 && corners[j].x <= pos.x + 1; j++)
			{
				int x = corners[j].x;
				if( (x == pos.x - 1 || x ==pos.x || x == pos.x+1) && Compare(scores[j],score))
					goto cont;
			}
		}

		ret_nonmax[num_nonmax++] = corners[i];
		cont:
			;
	}

	free(row_start);
	*ret_num_nonmax = num_nonmax;
	return ret_nonmax;
}

// clang-format on