Grouping Pixels in Objective-C - ios

Question

I am making a drawing app, and I have a NSArray with a list of all the pixels the user forget or didn't clean, what i want is make groups of that list pixels using proximity. so i can highlight all the pixel area with a black circle. this is what i got for the moment but is not grouping well
-(NSMutableArray*)orderMissingPixelsByGroups:(NSMutableArray*)missingPixels
{
NSMutableArray *finalArray=[[NSMutableArray alloc]init];
NSMutableArray *auxArray = [[NSMutableArray alloc]init];
for (int i=0; i<[missingPixels count]; i++) {
NSValue *value = [missingPixels objectAtIndex:i];
if (i<[missingPixels count]-1) {
NSValue *nextValue = [missingPixels objectAtIndex:i+1];
CGPoint point = value.CGPointValue;
CGPoint point2 = nextValue.CGPointValue;
if (abs(point.x-point2.x)<5 ) {
[auxArray addObject:[NSValue valueWithCGPoint:point]];
}
else if (abs(point.y-point2.y)<5){
[auxArray addObject:[NSValue valueWithCGPoint:point]];
}
else
{
[auxArray addObject:[NSValue valueWithCGPoint:point]];
[finalArray addObject:auxArray];
auxArray = [[NSMutableArray alloc]init];
}
}
else
{
[auxArray addObject:value];
[finalArray addObject:auxArray];
}
}
return finalArray;
}
If you have advices I will very thankful.

Answer 1

Yes I can imagine this is not the best approach. Try doing something like this. Note I did not test it also there is just too much room for optimizing this approach but I hope it will help you get a bit better understanding on how to create "blobs". Otherwise you can try to search for some fast approach under "blob" keyword...
- (NSMutableArray *)createBlobsFromPixels:(NSMutableArray *)pixels {
NSMutableArray *initialBlobs = [[NSMutableArray alloc] init];
for(NSValue *pixel in pixels)
{
[initialBlobs addObject:#[pixel]];
}
while (YES) {
NSIndexSet *indexSet = [self joinBlobs:initialBlobs minimumRadius:5.0f];
if(indexSet.count < 2) {
break;
}
else {
[self mergeBlobs:initialBlobs atIndexA:indexSet.firstIndex andIndexB:indexSet.lastIndex];
}
}
return initialBlobs;
}
- (NSIndexSet *)joinBlobs:(NSMutableArray *)blobs minimumRadius:(CGFloat)minRadius {
// this method will try to join 2 blobs
// if successfull it will return an index set with 2 blob indices which should be joined
// if no blobs are able to join the method will return nil
if(blobs.count < 2) {
// can not join a single blub
return nil;
}
NSUInteger count = blobs.count;
// iterate through every pair
for(NSUInteger i=0; i<count-1; i++) {
for(NSUInteger j=i+1; j<count; j++) {
NSArray *blobA = blobs[i];
NSArray *blobB = blobs[j];
// check if any of the pixels from blob A are close enough from any pixel in blob B and merge those two blobs
for(NSValue *pixelA in blobA) {
for(NSValue *pixelB in blobB) {
CGPoint pointA = [pixelA CGPointValue];
CGPoint pointB = [pixelB CGPointValue];
if(fabsf(pointA.x-pointB.x)<minRadius && fabsf(pointA.y-pointB.y)<minRadius) {
// can join these 2 blobs
NSMutableIndexSet *indexSet = [[NSMutableIndexSet alloc] init];
[indexSet addIndex:i];
[indexSet addIndex:j];
return indexSet; // returns YES as the blob can been joined
}
}
}
}
}
return nil;
}
- (void)mergeBlobs:(NSMutableArray *)blobs atIndexA:(NSUInteger)indexA andIndexB:(NSUInteger)indexB {
NSArray *blobA = blobs[indexA];
NSArray *blobB = blobs[indexB];
[blobs removeObject:blobA];
[blobs removeObject:blobB];
[blobs addObject:[blobA arrayByAddingObjectsFromArray:blobB]];
}