Walkthrough & Usage

Line-by-line code walk-through

tip

See the source code (mergeSmallRegionsInPlace.h and mergeSmallRegionsInPlace.cpp) for the full source listing. Here we explain the important parts.

• struct Pixel { int x,y; }; — small POD for BFS queue.

• inline int idx(int x, int y, int width) — maps (x,y) to a linear index into labels (not into pixel bytes; bytes index uses *4). This makes it simpler to index 2D data in a 1D array.

• sameColor(...) — compares 4 bytes (in RGBA form) at two pixel coordinates for exact equality.

  It uses exact equality

  Compression or anti-aliased edges will produce many colors that are visually similar but not equal.

• struct Region — collects size, minX, maxX, minY, maxY, provides convenience width(), height(), and isBigEnough(...).

• Flood-fill labeling loop — For every unlabeled pixel, perform a BFS:

  List

  1. Push initial pixel
  2. Mark its labels[...] = nextLabel
  3. Add pixel to region: r.add(x,y)
  4. While queue non-empty
     • Pop and consider 4 neighbours
       • If their label is -1 and sameColor(...) holds
         • label them and push to queue
  5. Now done with n^th region:
     • Add region to list of regions: regions.push_back(r)
     • Increment nextLabel

  Flowchart

• Merge phase: iterate every pixel; if its region is too small (isBigEnough(...) == false), check the 4 immediate neighbours; if any neighbour is in a different label nl and regions[nl].isBigEnough(...) is true, copy the neighbour's color bytes into the small pixel and set its label to nl.

  note

  The merge phase uses the labels array to pick neighbour region IDs and the regions metadata to determine which regions are "big".

Example usage

Load an image with stb_image, run merge, write out with stb_image_write

#include "mergeSmallRegionsInPlace.h"
#include <stb_image.h>
#include <stb_image_write.h>

int main() {
  int w,h,comp;
  unsigned char *img = stbi_load("segmented.png", &w, &h, &comp, 4);
  if (!img) return 1;

  // Remove tiny islands smaller than 50px and require bbox at least 3x3
  mergeSmallRegionsInPlace(img, w, h, 50, 3, 3);

  stbi_write_png("cleaned.png", w, h, 4, img, w*4);
  stbi_image_free(img);
  return 0;
}

tip

Tweak the thresholds to match your use-case.