Detect object from image based on object color by C#

To detect a object based on its color , there is an easy algorithm for that .u have to choose a filtering method . Steps normally are

• Take the image
• Apply ur filtering
• Apply greyscalling
• subtract background and get ur objects
• find all the objects position
• mark the objects

First u have to choose a filtering method , there are many filtering method provided for c#. mainly i prefer aforge filters , for this purpose they have few filter they are

• ColorFiltering
• ChannelFiltering
• HSLFiltering
• YCbCrFiltering
• EuclideanColorFiltering

my favorite is EuclideanColorFiltering it is easy and simple. for other filtering u can know more about them here. U have to download Aforge dll for apply these in ur code .

So see EuclideanColorFiltering work first see

we are going to apply a color filter it is very simple code to use euclideanfiltering

// create filter
EuclideanColorFiltering filter = new EuclideanColorFiltering( );
// set center colol and radius
filter.CenterColor = Color.FromArgb( 215, 30, 30 );
filter.Radius = 100;
// apply the filter
filter.ApplyInPlace( image );

now see the effect

well to understand how it work look closely at the code

filter.CenterColor = Color.FromArgb( 215, 30, 30 );
filter.Radius = 100;

first line select the select color value. you all know color has a value 0 to 255.
by filter.CenterColor = Color.FromArgb( 215, 30, 30 ); i specified my center color will be a red effected color because here value of red is 215, green and blue is 30. and filter.Radius = 100 means that all color value near than 100 in my specified color.
now my filter filters pixels, which color is inside/outside of RGB sphere with specified center and radius – it keeps pixels with colors inside/outside of the specified sphere and fills the rest with specified color.

Now for detect objects i use bitmap data and use lockbits method to understand clearly this method see here . then we make it greyscale algorithom hten unlock it.

BitmapData objectsData = image.LockBits(new Rectangle(0, 0, image.Width, image.Height),ImageLockMode.ReadOnly, image.PixelFormat);
            // grayscaling
            UnmanagedImage grayImage = grayscaleFilter.Apply(new UnmanagedImage(objectsData));
            // unlock image
            image.UnlockBits(objectsData);

now the object we use blobcounter for that. it is a very strong class that aforge provided.

  blobCounter.MinWidth = 5;
            blobCounter.MinHeight = 5;
            blobCounter.FilterBlobs = true;
            blobCounter.ProcessImage(grayImage);
            Rectangle[] rects = blobCounter.GetObjectRectangles();
            foreach(Rectangle recs in rects)
            if (rects.Length > 0)
            {
                foreach (Rectangle objectRect in rects)
                {

                    Graphics g = Graphics.FromImage(image);

                    using (Pen pen = new Pen(Color.FromArgb(160, 255, 160), 5))
                    {
                        g.DrawRectangle(pen, objectRect);
                    }

                    g.Dispose();
                }

            }

blobCounter.MinWidth and blobCounter.MinHeight define the smallest size of the object in pixel.
and blobCounter.GetObjectRectangles() return all the objects rectangle position. and using graphics class i draw rectangle over the images.

now if u want to take only the biggest object there is a method for that

   blobCounter.MinWidth = 5;
            blobCounter.MinHeight = 5;
            blobCounter.FilterBlobs = true;
            blobCounter.ObjectsOrder = ObjectsOrder.Size;
 blobCounter.ProcessImage(grayImage);
            Rectangle[] rects = blobCounter.GetObjectRectangles();
            foreach(Rectangle recs in rects)
            if (rects.Length > 0)
            {
                   Rectangle objectRect = rects[0];
                    Graphics g = Graphics.FromImage(image);
                    using (Pen pen = new Pen(Color.FromArgb(160, 255, 160), 5))
                    {
                        g.DrawRectangle(pen, objectRect);
                    }
                    g.Dispose();
                           }

now image is like that

but if u want to extract u can use following code

Bitmap bmp = new Bitmap(rects[0].Width, rects[0].Height);
Graphics g = Graphics.FromImage(bmp);
g.DrawImage(c, 0, 0, rects[0], GraphicsUnit.Pixel);

so u will get ur object like that

now if u want to draw the rectangles in main image use that image reference in graphics. u will find u result. Hope this can be helpful

see update at this post