Tutorial

Namespace

Emgu

All libraries provided by Emgu™ use the namespace Emgu.

Emgu.CV

The Emgu.CV namespace implement wrapper functions for OpenCV

Emgu.CV.CvInvoke class

This class is written to provided a way to directly invoke opencv function within .NET languages. Each method in this class corresponds to the same function in opencv. For example, a call to

CvInvoke.cvCreateImage(new MCvSize(400, 300), CvEnum.IPL_DEPTH.IPL_DEPTH_8U, 1); 

is equivalent to calling the function in opencv

cvCreateImage(cvSize(400, 300), IPL_DEPTH_8U, 1); 

Both of which create a 400x300 single-channel image of 8-bit depth.

Emgu.CV.CvEnum namespace

This namespace provides direct mapping to opencv enumerations. For example, CvEnum.IPL_DEPTH.IPL_DEPTH_8U is equivalent to the value in opencv IPL_DEPTH_8U . Both of which equals 8.

Emgu.CV.Mxxx Structure

This type of structure is a direct mapping to opencv structures. For example

• MIplImage is equivalent to IplImage structure in opencv
• MCvMat is equivalent to CvMat structure
• Mxxxx is equivalent to xxxx structure

Working with images

Creating Image

Although it is possible to create image by calling CvInvoke.cvCreateImage, we suggest using the generic class Image< Color, Depth> for image creation. There are serveral advantage of using the Managed Image<Color, Depth> class, among those are

• Memory is automatically released when the garbage collector dispose the Image< Color, Depth> Object
• Image< Color, Depth> class contains advanced method that is not available on OpenCV, for example, generic operation

To create an 480x320 image with Bgr color and 8-bit unsigned value, in C# you can call

Image<Bgr, Byte> img1 = new Image<Bgr, Byte>(480, 320);

Note that the image initialized this way contains random pixel values, if you wants to specify the background value of the image, let's say in Blue, in C# you write

Image<Bgr, Byte> img1 = new Image<Bgr, Byte>(480, 320, new Bgr(255, 0, 0));

Creating image from file is also simple, in C# just call

Image<Bgr, Byte> img1 = new Image<Bgr, Byte>("MyImage.jpg");

assuming the image file is call "MyImage.jpg"

Image Color

Image Color is specified using the first generic parameter Color Available Color Types are:

• Gray
• Bgr (Blue Green Red)
• Hsv (Hue Satuation value)

Image Depth

Image Depth is specified using the second generic parameter Depth Available Color Depths are:

• Byte
• Single (float)

Methods

Naming Convention

• Method XYZ in Image< Color, Depth> class corresponse to the OpenCV function cvXYZ. For example, Image< Color, Depth>.Not() function corresponse to cvNot function with the resulting image being returned.
• Method _XYZ is usually the same as Method XYZ except that the operation is performed inplace rather than returning a value. For example, Image< Color, Depth>._Not() function performs the bitwise inversion inplace.

Generic Operation

One of the advantage of using Emgu CV is the ability to perform generic operations.

It's best if I demostrate this using with example. Suppose we have an gray scale image of bytes

Image<Gray, Byte> img1 = new Image<Gray, Byte>(400, 300, new Gray(30));

To invert all the pixels in this image we can call the function using plain old CvInvoke

Image<Gray, Byte> img2 = img1.Not();

As an alternative, we can also use the generic method Convert available from the Image< Color, Depth> class

Image<Gray, Byte> img3 = img1.Convert<Byte>( delegate(Byte b) { return (Byte) (255-b); } );

The resulting image img2 and img3 contains the same value for each pixel.

At first glance it wouldn't seems to be a big gain when using generic operations. In fact, since opencv already has an implementation of the Not function and performance-wise it is better than the generic version of the equailent Convert function call. However, there comes to cases when generic functions provides the flexibility with minor performance cost.

Let's say you have an Image<Gray, Byte> img1 with pixels set. and you wants to create a single channel float point image of the same size, when each pixel of the new image, corresponse to the old image, can be describe in the following delegate

delegate(Byte b) { return (Single) Math.cos( b * b / 255.0); } 

This operation can be completed as follows in Emgu CV

Image<Gray, Single> img4 = img1.Convert<Single>( delegate(Byte b) { return (Single) Math.cos( b * b / 255.0); }  );

Which is simple and meaningfull. This operation in OpenCV is hard to perform since equivalent function such as Math.cos is not available.

Drawing

The Draw( ) method in Image< Color, Depth> can be used to draw different types of objects, including fonts, lines, circles, rectangles, boxes, ellipses as well as contours. Use the documentation and intellisense as a guideline to discover the many functionality of the Draw function.

Color and Depth Conversion

Converting an Image< Color, Depth> between different colors and depths are simple. For example, if you have Image<Bgr, Byte> img1 and you wants to convert it to a Grayscale image of Single, all you need to do is

Image<Gray, Single> img2 = img1.Convert<Gray, Single>();

There is no need to worry about the color convertion code as it is handled by the Emgu CV library.

XML serialization

One of the future of Emgu CV is that Image< Color, Depth> can be XML serializated. You might ask why we need to serialization an Image. The answer is simple, we wants to use it in a web service!

Since the Image< Color, Depth> class implements ISerializable, when you work in WCF (Windows Communication Fundation), you are free to use Image< Color, Depth> type as parameters or return value of a web service.

This will be ideal, for example, if you are building a cluster of computers to recongnize different groups of object and have a center computer to coordinate the tasks. I will also be useful if your wants to implement remote monitoring software that constainly query image from a remote server, which use the Capture class in Emgu CV to capture images from camera.

Examples

Hello, World

We will start by the Hello World sample, written in C#

String win1 = "Test Window"; //The name of the window

CvInvoke.cvNamedWindow(win1); //Create the window using the specific name


using (Image<Bgr, Byte> img = new Image<Bgr, byte>(400, 200, new Bgr(255, 0, 0))) //Create an image of 400x200 of Blue color

using (Font f = new Font(CvEnum.FONT.CV_FONT_HERSHEY_COMPLEX, 1.0, 1.0)) //Create the font

{

img.Draw("Hello, world", f, new Point2D<int>(10, 80), new Bgr(0, 255, 0)); //Draw "Hello, world." on the image using the specific font


CvInvoke.cvShowImage(win1, img.Ptr); //Show the image

CvInvoke.cvWaitKey(0); //Wait for the key pressing event

CvInvoke.cvDestroyWindow(win1); //Destory the window

}

The above code will create an image of 400x200 with blue background color and the String "Hello, world" in green on the forground. The image will be displayed a window named "Test Window".