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Posted at 2012-09-14 14:16 | RSS feed (Full text feed) | Blog Index
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Friday Q&A 2012-09-14: Implementing a Flood Fill
by Mike Ash  

Following up from the previous article about manipulating image data, Marc Palluat de Besset suggested a followup discussing the implementation of a flood fill. Today, I'm going to walk through the theory and practice of implementing flood fill on an NSBitmapImageRep.

A flood fill takes a starting point and a color. It then searches for contiguous points from that starting point which match the starting point's color. It sets all of those points to the fill color. In short, if you start a flood fill on a patch of color, it will change the color of the whole patch, but no other parts of the image, including parts that have the same color but aren't connected.

A refinement to this is to have a threshold instead of simply looking for an exact color match. Any color that's close enough to the starting point's color counts as being part of the fill region.

To implement this, we'll keep two lists of pixels. One list will be a list of candidate pixels to examine. This list will start off containing the starting point. The other list will be a list of pixels that have already been examined. We'll then enter a loop. Each time through the loop, we'll pull a pixel off the candidate list and check it out. If the candidate's color is close enough to the starting pixel's color, we'll fill the candidate by setting its color to the fill color. We'll then add the candidate to the list of pixels that have been examined, and add all four of its neighbors to the list of candidates, except for those neighbors which have already been examined.

One question is how to implement the lists of pixels. The most obvious way to do it in a Cocoa application is to use NSMutableSets containing NSValue objects containing NSPoint values. This works, but it's really slow. Too slow to be usable. Running a flood fill over a reasonably sized section of a 640x480 image takes a minute or two, which is just awful. We need something faster.

A pixel coordinate is just two integers, an x and a y. However, as we saw earlier, for a given image, we can convert the coordinate into a single index by simply calculating x + y * width. So these lists really just need to be sets of indexes. As it happens, Cocoa has a class made just for storing sets of indexes: NSIndexSet and its mutable cousin NSMutableIndexSet.

I don't think that NSIndexSet was built with this application in mind, but it works perfectly for it, giving good performance with a straightforward API. With NSMutableIndexSet to store the pixel lists, and some simple calculations for the rest of the flood fill, we're on our way.

Here's the pixel structure from last time, which corresponds nicely to the RGBA image representation you get from a properly configured NSBitmapImageRep:

    struct Pixel { uint8_t r, g, b, a; };

To use the threshold, we need to compute the difference between two pixels. To do that, we need to compute the difference between two pixel components, which is just a simple subtraction:

    static inline int ComponentDiff(uint8_t a, uint8_t b)
        return MAX(a, b) - MIN(a, b);

To compute the difference of two pixels, we'll just add together the differences of the red, green, and blue components:

    static inline int PixelDiff(struct Pixel p1, struct Pixel p2)
        return ComponentDiff(p1.r, p2.r)
             + ComponentDiff(p1.g, p2.g)
             + ComponentDiff(p1.b, p2.b);

Now on to the flood fill function itself. It takes image data, its width and height, the starting pixel coordinate, the fill value, and the threshold:

    void FloodFill(struct Pixel *image, int width, int height, int startx, int starty, struct Pixel fillValue, int threshold)

This function uses a few macros to convert pixel coordinates to indexes and back again:

    #define PIXEL_TO_INDEX(x, y) ((x) + (y) * width)
    #define INDEX_TO_X(index) ((index) % width)
    #define INDEX_TO_Y(index) ((index) / width)

There's also a macro to make it easier to access an individual pixel. Because this expands to a simple array access, it works for both reading and writing:

    #define PIXEL(x, y) image[PIXEL_TO_INDEX(x, y)]

We set up the two index sets that hold the lists of candidate pixels and pixels that already have been examined:

        NSMutableIndexSet *pixelsToExamine = [NSMutableIndexSet indexSet];
        NSMutableIndexSet *pixelsSeen = [NSMutableIndexSet indexSet];

Then add the starting pixel to the list of pixels to examine, and also store the starting pixel's color to use in the threshold computation:

        [pixelsToExamine addIndex: PIXEL_TO_INDEX(startx, starty)];
        struct Pixel startPixel = PIXEL(startx, starty);

Now it's time to enter the loop. We keep going as long as there are candidate pixels to examine:

        while([pixelsToExamine count] > 0)

It grabs the first index in the list, removes it, and adds it to pixelsSeen:

            int index = [pixelsToExamine firstIndex];
            [pixelsToExamine removeIndex: index];
            [pixelsSeen addIndex: index];

Note that the choice of firstIndex is completely arbitrary. This could just as well be lastIndex or any other way of getting some index in the set.

This index is converted to pixel coordinates:

            int x = INDEX_TO_X(index);
            int y = INDEX_TO_Y(index);

Next comes the threshold check. The difference between the pixel at x, y and the starting pixel is computed. If it's under the threshold, we proceed with the fill:

            int diff = PixelDiff(startPixel, PIXEL(x, y));
            if(diff <= threshold)

The first thing is to actually perform the fill operation on this pixel by setting its value to the fill value:

                PIXEL(x, y) = fillValue;

The code then loops over the four neighbors of this pixel: above, below, left, and right. I used an array of x coordinates and another array of y coordinates, then just loop over those arrays:

                int nextXs[4] = { x + 1, x - 1, x, x };
                int nextYs[4] = { y, y, y + 1, y - 1 };
                for(int i = 0; i < 4; i++)
                    int nextX = nextXs[i];
                    int nextY = nextYs[i];

The pixel we're looking at may lie at the edge of the image. If it does, the candidate pixels can lie off the edge. Such pixels must be detected and rejected to avoid accessing bad memory:

                    if(nextX >= 0 && nextY >= 0 && nextX < width && nextY < height)

If the candidate is within bounds, we convert the coordinate back to an index so it can be used with the pixel lists:

                        int next = PIXEL_TO_INDEX(nextX, nextY);

The next index is then added to pixelsToExamine, but only if it hasn't already been seen:

                        if(![pixelsSeen containsIndex: next])
                            [pixelsToExamine addIndex: next];

And that's it! The loop exits on its own once there are no more pixels to examine. Since the image is being mutated in place, there's nothing to return. Once the loop finishes, the flood fill has been completed. If you passed in an NSBitmapImageRep's bitmap data, the NSBitmapImageRep will now contain the flood-filled image.

This code plays nicely with the ImageRepFromImage function from the last article, allowing flood fills on any image that NSImage can load. The threshold computation is not particularly sophisticated, but it gets the job done here, and can easily be replaced with something more complex.

That's it for today! Come back next time for another wacky adventure. Friday Q&A is driven by reader submissions, so as always, if you have an idea that you'd like to see covered here, please send it in.

Did you enjoy this article? I'm selling whole books full of them! Volumes II and III are now out! They're available as ePub, PDF, print, and on iBooks and Kindle. Click here for more information.


Great post as usual, Mike. This is obviously a simple introduction to flood filling, but if your readers require a faster implementation they should try using a scanline flood fill, as demonstrated here: http://will.thimbleby.net/scanline-flood-fill/
This is, by the way, an example of breadth-first search: http://en.wikipedia.org/wiki/Breadth-first_search
Great read! I have enjoyed you topics on images. Can you do a page or two on sounds next?
Derek: I'd love to do an article or two on audio. Do you have anything more specific in mind?
Maybe just an introduction like you did with images. I like how you covered how an uncompressed image is stored. I only have a basic idea of how audio works.
Sure, starting with the basics sounds good to me. I think I momentarily forgot that level would be useful to cover. Do you have a last name I can attach to the idea, or shall I just label it "Derek"?
ooh, audio! I've tried to write code that uses fourier transforms (for example to detect that a certain frequency of sound is being played above a threshold volume), but was never able to understand what the values actually meant. since you kick ass at describing things so I understand, an introduction to fourier transforms would be awesome :)
I'm not sure I understand fourier transforms myself well enough to write them up, but I love the idea, and I think I might give it a shot.

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