Creating a GraphicsPath from a semi-transparent bitmap

Question

I want to create a GraphicsPath and a list of Points to form the outline of the non-transparent area of a bitmap. If needed, I can guarantee that each image has only one solid collection of nontransparent pixels. So for example, I should be able to record the points either clockwise or counter-clockwise along the edge of the pixels and perform a full closed loop.

The speed of this algorithm is not important. However, the efficiency of the resulting points is semi-important if I can skip some points to reduce in a smaller and less complex GraphicsPath.

I will list my current code below which works perfectly with most images. However, some images which are more complex end up with paths which seem to connect in the wrong order. I think I know why this occurs, but I can't come up with a solution.

public static Point[] GetOutlinePoints(Bitmap image)
{
    List<Point> outlinePoints = new List<Point>();

    BitmapData bitmapData = image.LockBits(new Rectangle(0, 0, image.Width, image.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

    byte[] originalBytes = new byte[image.Width * image.Height * 4];
    Marshal.Copy(bitmapData.Scan0, originalBytes, 0, originalBytes.Length);

    for (int x = 0; x < bitmapData.Width; x++)
    {
        for (int y = 0; y < bitmapData.Height; y++)
        {
            byte alpha = originalBytes[y * bitmapData.Stride + 4 * x + 3];

            if (alpha != 0)
            {
                Point p = new Point(x, y);

                if (!ContainsPoint(outlinePoints, p))
                    outlinePoints.Add(p);

                break;
            }
        }
    }

    for (int y = 0; y < bitmapData.Height; y++)
    {
        for (int x = bitmapData.Width - 1; x >= 0; x--)
        {
            byte alpha = originalBytes[y * bitmapData.Stride + 4 * x + 3];

            if (alpha != 0)
            {
                Point p = new Point(x, y);

                if (!ContainsPoint(outlinePoints, p))
                    outlinePoints.Add(p);

                break;
            }
        }
    }

    for (int x = bitmapData.Width - 1; x >= 0; x--)
    {
        for (int y = bitmapData.Height - 1; y >= 0; y--)
        {
            byte alpha = originalBytes[y * bitmapData.Stride + 4 * x + 3];

            if (alpha != 0)
            {
                Point p = new Point(x, y);

                if (!ContainsPoint(outlinePoints, p))
                    outlinePoints.Add(p);

                break;
            }
        }
    }

    for (int y = bitmapData.Height - 1; y >= 0; y--)
    {
        for (int x = 0; x < bitmapData.Width; x++)
        {
            byte alpha = originalBytes[y * bitmapData.Stride + 4 * x + 3];

            if (alpha != 0)
            {
                Point p = new Point(x, y);

                if (!ContainsPoint(outlinePoints, p))
                    outlinePoints.Add(p);

                break;
            }
        }
    }

    // Added to close the loop
    outlinePoints.Add(outlinePoints[0]);

    image.UnlockBits(bitmapData);

    return outlinePoints.ToArray();
}

public static bool ContainsPoint(IEnumerable<Point> points, Point value)
{
    foreach (Point p in points)
    {
        if (p == value)
            return true;
    }

    return false;
}

And when I turn the points into a path:

GraphicsPath outlinePath = new GraphicsPath();
outlinePath.AddLines(_outlinePoints);

Here's an example showing what I want. The red outline should be an array of points which can be made into a GraphicsPath in order to perform hit detection, draw an outline pen, and fill it with a brush.

Answer 1

Like both of you desrcipt you just have to find the first non transparent point and afterward move along the none transparent pixels with a transparent neighbor.

Additionaly you'll have to save the point you've already visisted and how often you visited them or you'll end in same cases in an invinity loop. If the point doesn't have a neighbor which already was visited you must go back each point, in revered direction, until a unvisited point is available again.

That's it.

---

See revision history for old versions.

Changes:

• Point.EMPTY was replaced by Point(-1,-1), or a non transparent pixel in the upper left corner causes an invinityloop
• Check for borderpoint at the image border

---

class BorderFinder {

    int stride = 0;
    int[] visited = null;
    byte[] bytes = null;
    PointData borderdata = null;
    Size size = Size.Empty;
    bool outside = false;
    Point zeropoint = new Point(-1,-1);

    public List<Point[]> Find(Bitmap bmp, bool outside = true) {
        this.outside = outside;
        List<Point> border = new List<Point>();
        BitmapData bmpdata = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

        stride = bmpdata.Stride;
        bytes = new byte[bmp.Width * bmp.Height * 4];
        size = bmp.Size;


        Marshal.Copy(bmpdata.Scan0, bytes, 0, bytes.Length);


        // Get all Borderpoint
        borderdata = getBorderData(bytes);

        bmp.UnlockBits(bmpdata);

        List<List<Point>> regions = new List<List<Point>>();

        //Loop until no more borderpoints are available
        while (borderdata.PointCount > 0) {
            List<Point> region = new List<Point>();

            //if valid is false the region doesn't close
            bool valid = true;

            //Find the first borderpoint from where whe start crawling
            Point startpos = getFirstPoint(borderdata);

            //we need this to know if and how often we already visted the point.
            //we somtime have to visit a point a second time because we have to go backward until a unvisted point is found again
            //for example if we go int a narrow 1px hole
            visited = new int[bmp.Size.Width * bmp.Size.Height];

            region.Add(startpos);

            //Find the next possible point
            Point current = getNextPoint(startpos);

            if (current != zeropoint) {
                visited[current.Y * bmp.Width + current.X]++;
                region.Add(current);
            }

            //May occure with just one transparent pixel without neighbors
            if (current == zeropoint)
                valid = false;

            //Loop until the area closed or colsing the area wasn't poosible
            while (!current.Equals(startpos) && valid) {
                var pos = current;
                //Check if the area was aready visited
                if (visited[current.Y * bmp.Width + current.X] < 2) {
                    current = getNextPoint(pos);
                    visited[pos.Y * bmp.Width + pos.X]++;
                    //If no possible point was found, search in reversed direction
                    if (current == zeropoint)
                        current = getNextPointBackwards(pos);
                } else { //If point was already visited, search in reversed direction
                    current = getNextPointBackwards(pos);
                }

                //No possible point was found. Closing isn't possible
                if (current == zeropoint) {
                    valid = false;
                    break;
                }

                visited[current.Y * bmp.Width + current.X]++;

                region.Add(current);
            }
            //Remove point from source borderdata
            foreach (var p in region) {
                borderdata.SetPoint(p.Y * bmp.Width + p.X, false);
            }
            //Add region if closing was possible
            if (valid)
                regions.Add(region);
        }

        //Checks if Region goes the same way back and trims it in this case
        foreach (var region in regions) {
            int duplicatedpos = -1;

            bool[] duplicatecheck = new bool[size.Width * size.Height];
            int length = region.Count;
            for (int i = 0; i < length; i++) {
                var p = region[i];
                if (duplicatecheck[p.Y * size.Width + p.X]) {
                    duplicatedpos = i - 1;
                    break;
                }
                duplicatecheck[p.Y * size.Width + p.X] = true;
            }

            if (duplicatedpos == -1)
                continue;

            if (duplicatedpos != ((region.Count - 1) / 2))
                continue;

            bool reversed = true;

            for (int i = 0; i < duplicatedpos; i++) {
                if (region[duplicatedpos - i - 1] != region[duplicatedpos + i + 1]) {
                    reversed = false;
                    break;
                }
            }

            if (!reversed)
                continue;

            region.RemoveRange(duplicatedpos + 1, region.Count - duplicatedpos - 1);
        }

        List<List<Point>> tempregions = new List<List<Point>>(regions);
        regions.Clear();

        bool connected = true;
        //Connects region if possible
        while (connected) {
            connected = false;
            foreach (var region in tempregions) {
                int connectionpos = -1;
                int connectionregion = -1;
                Point pointstart = region.First();
                Point pointend = region.Last();
                for (int ir = 0; ir < regions.Count; ir++) {
                    var otherregion = regions[ir];
                    if (region == otherregion)
                        continue;

                    for (int ip = 0; ip < otherregion.Count; ip++) {
                        var p = otherregion[ip];
                        if ((isConnected(pointstart, p) && isConnected(pointend, p)) ||
                            (isConnected(pointstart, p) && isConnected(pointstart, p))) {
                            connectionregion = ir;
                            connectionpos = ip;
                        }

                        if ((isConnected(pointend, p) && isConnected(pointend, p))) {
                            region.Reverse();
                            connectionregion = ir;
                            connectionpos = ip;
                        }
                    }

                }

                if (connectionpos == -1) {
                    regions.Add(region);
                } else {
                    regions[connectionregion].InsertRange(connectionpos, region);
                }

            }

            tempregions = new List<List<Point>>(regions);
            regions.Clear();
        }

        List<Point[]> returnregions = new List<Point[]>();

        foreach (var region in tempregions)
            returnregions.Add(region.ToArray());

        return returnregions;
    }

    private bool isConnected(Point p0, Point p1) {

        if (p0.X == p1.X && p0.Y - 1 == p1.Y)
            return true;

        if (p0.X + 1 == p1.X && p0.Y - 1 == p1.Y)
            return true;

        if (p0.X + 1 == p1.X && p0.Y == p1.Y)
            return true;

        if (p0.X + 1 == p1.X && p0.Y + 1 == p1.Y)
            return true;

        if (p0.X == p1.X && p0.Y + 1 == p1.Y)
            return true;

        if (p0.X - 1 == p1.X && p0.Y + 1 == p1.Y)
            return true;

        if (p0.X - 1 == p1.X && p0.Y == p1.Y)
            return true;

        if (p0.X - 1 == p1.X && p0.Y - 1 == p1.Y)
            return true;

        return false;
    }

    private Point getNextPoint(Point pos) {
        if (pos.Y > 0) {
            int x = pos.X;
            int y = pos.Y - 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
            }
        }

        if (pos.Y > 0 && pos.X < size.Width - 1) {
            int x = pos.X + 1;
            int y = pos.Y - 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
            }
        }

        if (pos.X < size.Width - 1) {
            int x = pos.X + 1;
            int y = pos.Y;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
            }
        }

        if (pos.X < size.Width - 1 && pos.Y < size.Height - 1) {
            int x = pos.X + 1;
            int y = pos.Y + 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
            }
        }

        if (pos.Y < size.Height - 1) {
            int x = pos.X;
            int y = pos.Y + 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
            }
        }


        if (pos.Y < size.Height - 1 && pos.X > 0) {
            int x = pos.X - 1;
            int y = pos.Y + 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
            }
        }

        if (pos.X > 0) {
            int x = pos.X - 1;
            int y = pos.Y;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
            }
        }

        if (pos.X > 0 && pos.Y > 0) {
            int x = pos.X - 1;
            int y = pos.Y - 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
            }
        }


        return zeropoint;
    }

    private Point getNextPointBackwards(Point pos) {
        Point backpoint = zeropoint;

        int trys = 0;

        if (pos.X > 0 && pos.Y > 0) {
            int x = pos.X - 1;
            int y = pos.Y - 1;
            if (ValidPoint(x, y) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
                if (backpoint == zeropoint || trys > visited[y * size.Width + x]) {
                    backpoint = new Point(x, y);
                    trys = visited[y * size.Width + x];
                }
            }
        }

        if (pos.X > 0) {
            int x = pos.X - 1;
            int y = pos.Y;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
                if (backpoint == zeropoint || trys > visited[y * size.Width + x]) {
                    backpoint = new Point(x, y);
                    trys = visited[y * size.Width + x];
                }
            }
        }

        if (pos.Y < size.Height - 1 && pos.X > 0) {
            int x = pos.X - 1;
            int y = pos.Y + 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
                if (backpoint == zeropoint || trys > visited[y * size.Width + x]) {
                    backpoint = new Point(x, y);
                    trys = visited[y * size.Width + x];
                }
            }
        }

        if (pos.Y < size.Height - 1) {
            int x = pos.X;
            int y = pos.Y + 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
                if (backpoint == zeropoint || trys > visited[y * size.Width + x]) {
                    backpoint = new Point(x, y);
                    trys = visited[y * size.Width + x];
                }
            }
        }


        if (pos.X < size.Width - 1 && pos.Y < size.Height - 1) {
            int x = pos.X + 1;
            int y = pos.Y + 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
                if (backpoint == zeropoint || trys > visited[y * size.Width + x]) {
                    backpoint = new Point(x, y);
                    trys = visited[y * size.Width + x];
                }
            }
        }


        if (pos.X < size.Width - 1) {
            int x = pos.X + 1;
            int y = pos.Y;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
                if (backpoint == zeropoint || trys > visited[y * size.Width + x]) {
                    backpoint = new Point(x, y);
                    trys = visited[y * size.Width + x];
                }
            }
        }

        if (pos.Y > 0 && pos.X < size.Width - 1) {
            int x = pos.X + 1;
            int y = pos.Y - 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
                if (backpoint == zeropoint || trys > visited[y * size.Width + x]) {
                    backpoint = new Point(x, y);
                    trys = visited[y * size.Width + x];
                }
            }
        }


        if (pos.Y > 0) {
            int x = pos.X;
            int y = pos.Y - 1;
            if ((ValidPoint(x, y)) && HasNeighbor(x, y)) {
                if (visited[y * size.Width + x] == 0) {
                    return new Point(x, y);
                }
                if (backpoint == zeropoint || trys > visited[y * size.Width + x]) {
                    backpoint = new Point(x, y);
                    trys = visited[y * size.Width + x];
                }
            }
        }

        return backpoint;
    }

    private bool ValidPoint(int x, int y) {
        return (borderdata[y * size.Width + x]);
    }

    private bool HasNeighbor(int x, int y) {
        if (y > 0) {
            if (!borderdata[(y - 1) * size.Width + x]) {
                return true;
            }
        } else if (ValidPoint(x, y)) {
            return true;
        }

        if (x < size.Width - 1) {
            if (!borderdata[y * size.Width + (x + 1)]) {
                return true;
            }
        } else if (ValidPoint(x, y)) {
            return true;
        }

        if (y < size.Height - 1) {
            if (!borderdata[(y + 1) * size.Width + x]) {
                return true;
            }
        } else if (ValidPoint(x, y)) {
            return true;
        }

        if (x > 0) {
            if (!borderdata[y * size.Width + (x - 1)]) {
                return true;
            }
        } else if (ValidPoint(x, y)) {
            return true;
        }

        return false;
    }

    private Point getFirstPoint(PointData data) {
        Point startpos = zeropoint;
        for (int y = 0; y < size.Height; y++) {
            for (int x = 0; x < size.Width; x++) {
                if (data[y * size.Width + x]) {
                    startpos = new Point(x, y);
                    return startpos;
                }
            }
        }
        return startpos;
    }

    private PointData getBorderData(byte[] bytes) {

        PointData isborderpoint = new PointData(size.Height * size.Width);
        bool prevtrans = false;
        bool currenttrans = false;
        for (int y = 0; y < size.Height; y++) {
            prevtrans = false;
            for (int x = 0; x <= size.Width; x++) {
                if (x == size.Width) {
                    if (!prevtrans) {
                        isborderpoint.SetPoint(y * size.Width + x - 1, true);
                    }
                    continue;
                }
                currenttrans = bytes[y * stride + x * 4 + 3] == 0;
                if (x == 0 && !currenttrans)
                    isborderpoint.SetPoint(y * size.Width + x, true);
                if (prevtrans && !currenttrans)
                    isborderpoint.SetPoint(y * size.Width + x - 1, true);
                if (!prevtrans && currenttrans && x != 0)
                    isborderpoint.SetPoint(y * size.Width + x, true);
                prevtrans = currenttrans;
            }
        }
        for (int x = 0; x < size.Width; x++) {
            prevtrans = false;
            for (int y = 0; y <= size.Height; y++) {
                if (y == size.Height) {
                    if (!prevtrans) {
                        isborderpoint.SetPoint((y - 1) * size.Width + x, true);
                    }
                    continue;
                }
                currenttrans = bytes[y * stride + x * 4 + 3] == 0;
                if(y == 0 && !currenttrans)
                    isborderpoint.SetPoint(y * size.Width + x, true);
                if (prevtrans && !currenttrans)
                    isborderpoint.SetPoint((y - 1) * size.Width + x, true);
                if (!prevtrans && currenttrans && y != 0)
                    isborderpoint.SetPoint(y * size.Width + x, true);
                prevtrans = currenttrans;
            }
        }
        return isborderpoint;
    }
}

class PointData {
    bool[] points = null;
    int validpoints = 0;
    public PointData(int length) {
        points = new bool[length];
    }

    public int PointCount {
        get {
            return validpoints;
        }
    }

    public void SetPoint(int pos, bool state) {
        if (points[pos] != state) {
            if (state)
                validpoints++;
            else
                validpoints--;
        }
        points[pos] = state;
    }
    public bool this[int pos] {
        get {
            return points[pos];
        }
    }


}

Answer 2

I've modified GetOutlinePoints by adding a helper variable than verifies the position at which new points should be added.

The idea of the outline detection algorithm in your code is something like looking at the image while standing each of its edges and writing down all non-transparent pixels that are visible. It's ok, however, you always added pixels to the end of the collection, which caused issues. I've added a variable that remembers position of the last pixel visible from the previous edge and the current one and uses it to determine index where the new pixel should be added. I suppose it should work as long as the outline is continuous, but I suppose it is in your case.

I've tested it on several images and it seems to work correctly:

public static Point[] GetOutlinePoints(Bitmap image)
    {
        List<Point> outlinePoints = new List<Point>();

        BitmapData bitmapData = image.LockBits(new Rectangle(0, 0, image.Width, image.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

        byte[] originalBytes = new byte[image.Width * image.Height * 4];
        Marshal.Copy(bitmapData.Scan0, originalBytes, 0, originalBytes.Length);
        //find non-transparent pixels visible from the top of the image
        for (int x = 0; x < bitmapData.Width; x++)
        {
            for (int y = 0; y < bitmapData.Height; y++)
            {
                byte alpha = originalBytes[y * bitmapData.Stride + 4 * x + 3];

                if (alpha != 0)
                {
                    Point p = new Point(x, y);

                    if (!ContainsPoint(outlinePoints, p))
                        outlinePoints.Add(p);

                    break;
                }
            }
        }

        //helper variable for storing position of the last pixel visible from both sides 
        //or last inserted pixel
        int? lastInsertedPosition = null;
        //find non-transparent pixels visible from the right side of the image
        for (int y = 0; y < bitmapData.Height; y++)
        {
            for (int x = bitmapData.Width - 1; x >= 0; x--)
            {
                byte alpha = originalBytes[y * bitmapData.Stride + 4 * x + 3];

                if (alpha != 0)
                {
                    Point p = new Point(x, y);

                    if (!ContainsPoint(outlinePoints, p))
                    {
                        if (lastInsertedPosition.HasValue)
                        {
                            outlinePoints.Insert(lastInsertedPosition.Value + 1, p);
                            lastInsertedPosition += 1;
                        }
                        else
                        {
                            outlinePoints.Add(p);
                        }
                    }
                    else
                    {
                        //save last common pixel from visible from more than one sides
                        lastInsertedPosition = outlinePoints.IndexOf(p);
                    }

                    break;
                }
            }
        }

        lastInsertedPosition = null;
        //find non-transparent pixels visible from the bottom of the image
        for (int x = bitmapData.Width - 1; x >= 0; x--)
        {
            for (int y = bitmapData.Height - 1; y >= 0; y--)
            {
                byte alpha = originalBytes[y * bitmapData.Stride + 4 * x + 3];

                if (alpha != 0)
                {
                    Point p = new Point(x, y);

                    if (!ContainsPoint(outlinePoints, p))
                    {
                        if (lastInsertedPosition.HasValue)
                        {
                            outlinePoints.Insert(lastInsertedPosition.Value + 1, p);
                            lastInsertedPosition += 1;
                        }
                        else
                        {
                            outlinePoints.Add(p);
                        }
                    }
                    else
                    {
                        //save last common pixel from visible from more than one sides
                        lastInsertedPosition = outlinePoints.IndexOf(p);
                    }

                    break;
                }
            }
        }

        lastInsertedPosition = null;
        //find non-transparent pixels visible from the left side of the image
        for (int y = bitmapData.Height - 1; y >= 0; y--)
        {
            for (int x = 0; x < bitmapData.Width; x++)
            {
                byte alpha = originalBytes[y * bitmapData.Stride + 4 * x + 3];

                if (alpha != 0)
                {
                    Point p = new Point(x, y);

                    if (!ContainsPoint(outlinePoints, p))
                    {
                        if (lastInsertedPosition.HasValue)
                        {
                            outlinePoints.Insert(lastInsertedPosition.Value + 1, p);
                            lastInsertedPosition += 1;
                        }
                        else
                        {
                            outlinePoints.Add(p);
                        }
                    }
                    else
                    {
                        //save last common pixel from visible from more than one sides
                        lastInsertedPosition = outlinePoints.IndexOf(p);
                    }

                    break;
                }
            }
        }

        // Added to close the loop
        outlinePoints.Add(outlinePoints[0]);

        image.UnlockBits(bitmapData);

        return outlinePoints.ToArray();
    }

Update: This algorithm won't work correctly for images that have outline parts not "visible" from any of the edges. See comments for suggested solutions. I'll try to post a code snippet later.

Update II

I've prepared another algorithm as described in my comments. It just crawls around the object and saves the outline.

First, it finds the first pixel of the outline using a method from the previous algorithm. Then, it looks throught the neighbouring pixels in clockwise order, find the first one that is transparent and then continues browsing, but looking for a non-transparent one. The first non-transparent pixel found is the next one in the outline. The loop continues until it goes around the whole object and gets back to the starting pixel.

public static Point[] GetOutlinePointsNEW(Bitmap image)
    {
        List<Point> outlinePoints = new List<Point>();

        BitmapData bitmapData = image.LockBits(new Rectangle(0, 0, image.Width, image.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

        Point currentP = new Point(0, 0);
        Point firstP = new Point(0, 0);

        byte[] originalBytes = new byte[image.Width * image.Height * 4];
        Marshal.Copy(bitmapData.Scan0, originalBytes, 0, originalBytes.Length);
        //find non-transparent pixels visible from the top of the image
        for (int x = 0; x < bitmapData.Width && outlinePoints.Count == 0; x++)
        {
            for (int y = 0; y < bitmapData.Height && outlinePoints.Count == 0; y++)
            {
                byte alpha = originalBytes[y * bitmapData.Stride + 4 * x + 3];

                if (alpha != 0)
                {
                    Point p = new Point(x, y);

                    outlinePoints.Add(p);
                    currentP = p;
                    firstP = p;

                    break;
                }
            }
        }

        Point[] neighbourPoints = new Point[] { new Point(-1, -1), new Point(0, -1), new Point(1, -1), 
                                                new Point(1, 0), new Point(1, 1), new Point(0, 1), 
                                                new Point(-1, 1), new Point(-1, 0) };

        //crawl around the object and look for the next pixel of the outline
        do
        {
            bool transparentNeighbourFound = false;
            bool nextPixelFound = false;
            int i;
            //searching is done in clockwise order
            for (i = 0; (i < neighbourPoints.Length * 2) && !nextPixelFound; ++i)
            {
                int neighbourPosition = i % neighbourPoints.Length;

                int x = currentP.X + neighbourPoints[neighbourPosition].X;
                int y = currentP.Y + neighbourPoints[neighbourPosition].Y;

                byte alpha = originalBytes[y * bitmapData.Stride + 4 * x + 3];

                //a transparent pixel has to be found first
                if (!transparentNeighbourFound)
                {
                    if (alpha == 0)
                    {
                        transparentNeighbourFound = true;
                    }
                }
                else //after a transparent pixel is found, a next non-transparent one is the next pixel of the outline
                {
                    if (alpha != 0)
                    {
                        Point p = new Point(x, y);

                        currentP = p;
                        outlinePoints.Add(p);
                        nextPixelFound = true;
                    }
                }
            }
        } while (currentP != firstP);

        image.UnlockBits(bitmapData);

        return outlinePoints.ToArray();
    }

One thing to remember is that it does work IF object does NOT ends at the edge of the image (there has to be a transparent space between the object and each of the edges).

This can be easily done if you just make the image one line larger at each side before passing it to the GetOutlinePointsNEW method.

Answer 3

The scenario by the poster was one I just encountered. After applying the GetOutlinePointsNEW method above, I encountered an index issue when the non-transparent pixel is at the edge of the image causing the crawl to be outside the bounds of the image. Below is a managed code update that treats the out of bounds pixels as non-transparent when performing the crawl.

    static public Point[] crawlerPoints = new Point[] { new Point(-1, -1), new Point(0, -1), new Point(1, -1),
                                            new Point(1, 0), new Point(1, 1), new Point(0, 1),
                                            new Point(-1, 1), new Point(-1, 0) };

    private BitmapData _bitmapData;
    private byte[] _originalBytes;
    private Bitmap _bitmap;  //this is loaded from an image passed in during processing

    public Point[] GetOutlinePoints()
    {
        List<Point> outlinePoints = new List<Point>();
        _originalBytes = new byte[_bitmap.Width * _bitmap.Height * 4];
        _bitmapData = _bitmap.LockBits(new Rectangle(0, 0, _bitmap.Width, _bitmap.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
        Marshal.Copy(_bitmapData.Scan0, _originalBytes, 0, _originalBytes.Length);
        GetFirstNonTransparentPoint(outlinePoints);
        if (outlinePoints.Count > 0) { GetNonTransparentPoints(outlinePoints); }
        _bitmap.UnlockBits(_bitmapData);

        return outlinePoints.ToArray();
    }
    private void GetFirstNonTransparentPoint(List<Point> outlinePoints)
    {
        Point firstPoint = new Point(0, 0);

        for (int x = 0; x < _bitmapData.Width; x++)
        {
            for (int y = 0; y < _bitmapData.Height; y++)
            {
                if (!IsPointTransparent(x, y))
                {
                    firstPoint = new Point(x, y);
                    outlinePoints.Add(firstPoint);
                    break;
                }
            }
            if (outlinePoints.Count > 0) { break; }
        }            
    }
    private void GetNonTransparentPoints(List<Point> outlinePoints)
    {
        Point currentPoint = outlinePoints[0];
        do   //Crawl counter clock-wise around the current point
        {
            bool firstTransparentNeighbourFound = false;
            bool nextPixelFound = false;
            for (int i = 0; (i < ApplicationVariables.crawlerPoints.Length * 2) && !nextPixelFound; ++i)
            {
                int crawlPosition = i % ApplicationVariables.crawlerPoints.Length;
                if (!firstTransparentNeighbourFound) { firstTransparentNeighbourFound = IsCrawlPointTransparent(crawlPosition, currentPoint); }
                else
                {
                    if (!IsCrawlPointTransparent(crawlPosition, currentPoint))
                    {
                        outlinePoints.Add(new Point(currentPoint.X + ApplicationVariables.crawlerPoints[crawlPosition].X, currentPoint.Y + ApplicationVariables.crawlerPoints[crawlPosition].Y));
                        currentPoint = outlinePoints[outlinePoints.Count - 1];
                        nextPixelFound = true;
                    }
                }
            }
        } while (currentPoint != outlinePoints[0]);
    }

    private bool IsCrawlPointTransparent(int crawlPosition, Point currentPoint)
    {
        int x = currentPoint.X + ApplicationVariables.crawlerPoints[crawlPosition].X;
        int y = currentPoint.Y + ApplicationVariables.crawlerPoints[crawlPosition].Y;

        if (IsCrawlInBounds(x, y)) { return IsPointTransparent(x, y); }
        return true;
    }
    private bool IsCrawlInBounds(int x, int y)
    {
        return ((x >= 0 & x < _bitmapData.Width) && (y >= 0 & y < _bitmapData.Height));
    }
    private bool IsPointTransparent(int x, int y)
    {
        return _originalBytes[(y * _bitmapData.Stride) + (4 * x) + 3] == 0;
    }