Blepo Tutorial

This tutorial provides a quick introduction to the use of Blepo. To begin, let us load and display an image in a window:

    #include "blepo.h"
    using namespace blepo;

      ImgGray img;              // an 8-bit-per-pixel grayscale image
      Figure fig;               // a figure
      Load("image.bmp", &img);  // load file into image
      fig.Draw(img);            // draw image in figure
This example illustrates the simplicity of the library. There is a single header file, and the image data are allocated and deallocated automatically. All functions and classes in the library are scoped by the blepo namespace, so you have the choice of either scoping each call with the prefix blepo:: or calling using namespace blepo, as is done here. In a console-based application, be sure to call EventLoop() before returning from main() or else the program will exit before the figure has the chance to display.

In this example, the first line of code simply instantiates a grayscale image. This line incurs essentially no overhead because no memory is allocated for the image data itself (since the size of the image is not given). The second line instantiates a figure, which causes a small window to display on the screen. The Load function loads an image from a file, automatically resizing the img object to hold it, and converting from color to grayscale if necessary. The last line copies the image to the figure, which causes the image to be drawn in the window (and redrawn whenever the window needs to be repainted). When the closing brace is encountered, the image and figure objects fall out of scope, and the image destructor causes all of the memory for the image data to be automatically deallocated. By default, the figure destructor does nothing, so that the window persists on the screen until the user closes the window or the application exits. This behavior makes it easy to rapidly inspect images and data for debugging purposes, but it can be overridden by a parameter to the Figure contructor if desired.

The image class

Because the image class is the backbone of the library, let us look at it in more detail. An image is simply an array of pixels, along with a small header containing the image width and height. When the constructor is called with no parameters (as is done above), the width and height are set to zero, and the internal data pointer is set to NULL. As a result, this is an extremely quick operation. Alternatively, the image dimensions can be passed to the constructor, in which case memory is allocated (on the heap) to hold the image data. Either way, once an image has been created, the Reset function can be called to reallocate the memory at any time, and the destructor automatically frees the memory when the image falls out of scope. (Note that the Load function above calls Reset.)
      ImgGray img;             // an empty image
      img.Reset(320, 240);     // allocates memory for a 320 x 240 image
      img.Reset(160, 120);     // reallocates memory for a 160 x 120 image
      ImgGray img2(320, 240);  // a 320 x 240 image
      img2.Reset(160, 120);    // reallocates memory for a 160 x 120 image
      img2.Reset(160, 120);    // does nothing (no penalty incurred) because data already allocated
    }                          // all memory is freed when images fall out of scope
Thus, the user should never have to worry about explicitly allocating or deallocating memory, because the library takes care of those details automatically. And yet the memory management is done in a transparent way, so that the user remains in complete control. Those familiar with the Standard Template Library (STL) will notice the similarity.

Each image manages its own data. Therefore the assignment operator and copy constructor make an exact copy of the image dimensions and data. Subsequent changes to one image have no effect on the other image.

    ImgGray img2(img);  // copy constructor
    img2 = img;         // assignment operator
As a sidenote, be sure when writing your own functions never to pass an image "by value," which will cause the copy constructor to be called (an expensive operation). For efficiency reasons, images should always be passed "by reference" or "by pointer," as they are in Blepo.

Accessing pixels

Image data are arranged in memory in row-major order in sequential bytes, starting from the top-left. There are no extra bytes for alignment, so a 7 x 3 grayscale image is stored in 21 contiguous bytes. To access an individual pixel in a grayscale image, simply pass the zero-based column (x), followed by the zero-based row (y), to the parenthesis operator:
    unsigned char val;
    val = img(33, 22);  // get pixel at column 34, row 23 -- since the top-left pixel is at (0,0)
    img(33, 22) = val;  // set pixel at column 34, row 23
To change all the pixels in the image, simply iterate through them:
    unsigned char val = 255;  // new value
    for (y=0 ; y<img.Height() ; y++)
    	for (x=0 ; x<img.Width() ; x++)
    		img(x,y) = val);

The parenthesis operator is inlined, so no function overhead is incurred. Moreover, bounds checking is performed using assert statements so that they disappear in Release mode, thus incurring no run-time penalty except during development. Nevertheless, the operator is fairly inefficient, since it uses arithmetic to compute the offset from the first pixel -- i.e., img(x,y) computes dataptr+y*width+x. A more efficient way to access pixels is to use pointers to the data:

    unsigned char* p;
    for (p = img.Begin() ; p != img.End() ; p++)  *p = 15;  // set all pixels to value 15
The Begin method returns a pointer to the first pixel, while End returns a pointer to the byte just after the last pixel. Like Begin, End returns a precomputed value and is inlined, so there is no penalty for calling this method each time through the loop.

To get read-only access to the pixels, use the const keyword. Although this is not, strictly speaking, necessary, it does improve code readability and provides for additional compile-time checking:

    const unsigned char* p = img.Begin();
    unsigned char val = *p;                   // get value of pixel

To get a pointer to a particular pixel, pass the coordinates of the pixel to the Begin method. This operation can be useful when processing a subarray within an image.

    unsigned char* p = img.Begin(x, y);  // returns a pointer to pixel (x, y)

Built-in image operations

For simple operations such as setting all the pixels to a constant value, it is much easier to call a built-in function of the library. In general, built-in functions should be used whenever possible because they are often optimized using SIMD operations, making them faster than doing it yourself.
  ImgGray img, img2;
  Rect r(10, 10, 50, 50);  // a rect structure (left, top, right, bottom)
  Point p(20, 30);         // a point structure (x, y)
  Set(&img, 15);           // set all pixels to value 15
  Set(&img, r, 15);        // set all pixels within a rectangle to value 15
  Set(&img, p, img2);      // copy entire 'img2' to 'img' at (20, 30)
                           //   ('img2' is smaller than 'img')                                    
  Set(&img, p, img2, r);   // copy rect of 'img2' to 'img' at (20, 30)
  Extract(img2, r, &img);  // extract rectangle from 'img2'
In addition, there are built-in functions for performing arithmetic and logical operations (And, Or, Xor, ...), along with functions for comparing images (Equal, LessThan, GreaterThan, IsSameSize, IsIdentical, ...).

Parameter ordering and conventions

If you look carefully at all the preceding code, you will notice that the ordering of function parameters is systematic: Inputs precede outputs, and outputs are passed as pointers. Parameters that are both input and output are also passed as pointers, but at the beginning rather than the end. Thus, img is an output in Load and Extract, whereas it is both input and output in Set:
    Load("image.bmp", &img);  // 'img' is output (img.Reset is called)
    Extract(img2, r, &img);   // 'img2' is input, 'img' is output (img.Reset is called)
    Set(&img, 15);            // 'img' is input/output; img.Reset is NOT called
These simple conventions make it easy to remember how to call a function. They also greatly improve code readability, because they make it possible to distinguish immediately the inputs from the outputs simply by looking at a function is called, without having to know anything about the semantics of the function itself.

Efficient passing

Images are large data structures.  For efficiency reasons, you do not want to pass them around by value.  Instead, you should pass them as pointers or by reference.  Here is an example of a good function prototype:

    void f(const ImgGray& a, ImgGray* b);  // 'a' is input, 'b' is output

In this function, the parameter a is passed as a const reference.  The const keyword indicates that the function has read-only access to the variable a.  The ampersand (&) indicates that it is passed as a reference, which means that the image itself is not copied; rather, inside the function a is simply another name (an alias) for the image that was passed in.  The variable b is passed as a pointer, which also means that the image itself is not copied; rather, b is a pointer that can be dereferenced to access the image that was passed in. 

To call this function, pass it two images:

    ImgGray a, b;
    f(a, &b);
Note that pass by reference and pass as pointer achieve the same behavior with different syntax. With the former the ampersand (meaning 'reference') is in the prototype, while with the latter the ampersand (meaning 'take the address of') must be used each time the function is called.  Which approach to use is largely a matter of style -- the library adopts the convention that inputs are passed using const references while outputs are passed using pointers.

Other image types

In addition to ImgGray, there are several other image classes, the only difference between them being the pixel type. Each class has exactly the same interface:
    ImgX img;                // an image
    ImgX::Pixel val;         // a pixel
    ImgX::Iterator p;        // an iterator (pointer)
    ImgX::ConstIterator q;   // a const iterator (pointer)
    p = img.Begin();         // get an iterator to the first pixel
    val = *p;                // get the value
    val = *q;                // get the value
    *p = val;                // set the value
    *q = val;                // error!  cannot set using const iterator
where ImgX can be replaced by any of the image classes. For most of the classes, this interface is achieved using typedef. For example, ImgGray::Pixel is simply typedefed to unsigned char, an ImgInt pixel is an int, an ImgFloat pixel is a float, and an ImgBgr pixel is a Bgr struct that contains three unsigned chars:
    struct Bgr { unsigned char b, g, r; };
For any of these classes you are free to use either the basic type or the uniform interface, whichever you prefer. The only exception to this is that you must use ImgBinary::Iterator and ImgBinary::ConstIterator, because these are special internal classes that overcome the problem that in C++ bool* is not a pointer to a bit. To convert between images of different types, simply call Convert:
    ImgGray img_gray;
    ImgBgr img_bgr;
    Convert(img_gray, &img_bgr);

To access pixels in one of these other image classes, simply replace unsigned char with the pixel type.  For example, pixels in a BGR image are accessed in the following manner:

    Bgr val;
    val = img(33, 22);  // get pixel at column 34, row 23 -- since the top-left pixel is at (0,0)
    val.b = 255;
    val.g = 0;
    val.r = 0;
    img(33, 22) = val;  // set pixel at column 34, row 23 to blue

and so on.


Once you have an image, it is straightforward to call any of the image processing or computer vision algorithms. Here are some examples:
    ImgGray img, gradx, grady, gradmag, img_smooth, filled;
    unsigned char new_color = 255;
    Point seed(20, 30);
    std::vector<Rect> rectlist;        // array of rectangles
    Prewitt(img, &gradx, &grady);      // compute Prewitt edges
    Sobel(img, &gradx, &grady);        // compute Sobel edges
    PrewittMag(img, &gradmag);         // compute magnitude of Prewitt edges
    Gradient(img, &gradx, &grady);     // compute gradient using Gaussian derivative
    SmoothGauss5x5(img, &img_smooth);  // smooth image by convolving with 5x5 Gaussian
    FloodFill4(img, new_color, 
      seed.x, seed.y, &filled);        // floodfill (4-connected)
    FaceDetector det;
    det.DetectFaces(img, &rectlist);   // detect faces
Some of these functions are optimized to use SIMD.(MMX/SSE/SSE2) operations, while others are unoptimized. Some of these functions are written from scratch for Blepo, while others are wrappers around code obtained from other libraries.

Linear algebra

The matrix class MatDbl contains a 2D array of doubles, stored contiguously in row-major order. The interface is the same as that of the image classes:
    MatDbl mat;           // an empty matrix
    MatDbl mat2(15, 10);  // a matrix with 15 columns, 10 rows
    mat.Reset(15, 10);    // reallocate for 15 columns, 10 rows
    double val;
    val = mat(3, 5);      // get value at column 3, row 5
    mat(3, 5) = val;      // set value at column 3, row 5
Note that the interface is not consistent with standard linear algebra notation in two ways:
  1. Indices are zero-based, not one-based
  2. Elements are indexed by (column, row), rather than (row, column)
We trust that the first inconsistency will meet with little resistance among C/C++ programmers, since zero-based indexing is natural to these languages. Although the second inconsistency is admittedly unnatural, we believe that internal inconsistency within the library is more important than partial consistency with external notation, in order to avoid problems such as Matlab's ginput function, whose ordering is the reverse of most other Matlab functions, leading to confusion. For those who prefer the more traditional (row, column) indexing, an alternate "ij" interface is available:
    MatDbl mat;                 // an empty matrix
    MatDbl mat2(10, 15, "ij");  // a matrix with 10 rows, 15 columns
    mat.ijReset(10, 15);        // reallocate for 10 rows, 15 columns
    double val;
    val = mat.ij(5, 3);         // get value at row 5, column 3
    mat.ij(5, 3) = val;         // set value at row 5, column 3

Basic matrix operations, such as add, multiply, transpose, creating identity matrices, etc., can be called as follows:

    MatDbl a, b, c;
    MatDbl d(4);             // create vertical vector (4 rows)
    double eps = 0.01;
    bool s;
    Eye(4, &a);              // create identity matrix (4 columns, 4 rows)
    Rand(6, 3);              // create random matrix (6 columns, 3 rows)
    Add(a, b, &c);           // compute c = a + b
    Multiply(a, b, &c);      // compute c = a * b (matrix multiply) ????????????????
    s = Similar(a, b, eps);  // returns whether matrix elements are the same, within 'eps'
    Transpose(a, &c);        // computer c = a^T
    Diag(d, &c);             // create diagonal matrix from vector
    Diag(c, &d);             // create vector by extracting diagonal from matrix
Linear algebra routines are provided via wrappers around the GNU Scientific Library (GSL) functions:
    MatDbl a, u, s, v, l, p, q, r, eval, evec, x, inv;
    Svd(a, &u, &s, &v);           // singular value decomposition (SVD) (a = u * Diag(s) * v)
    Lu(a, &l, &u, &p);            // LU decomposition (p * a = l * u)
    Qr(a, &q, &r);                // QR factorization (a = q * r)
    EigenSymm(a, &eval, &evec);   // eigenvalues and eigenvectors of symmetric matrix
    double det = Determinant(a);  // determinant
    SolveLinear(a, b, &x);        // solve (possibly overdetermined) A x = b
    Inverse(a, &inv);             // inverse

When efficiency is not important (e.g., when quickly prototyping a new idea), it may be preferable to use more compact notation, which is provided using overloaded operators. For example:

    MatDbl a, b, c, d, e;
    e = a * b + Transpose(c) * Inverse(d);  // compute d = a * b + c^T * d^{-1}


We have already seen how to display an image in a window using the Draw method of the Figure class. Another useful method of the class is GrabMouseClick:
    ImgGray img;
    Figure fig;
    Point pt = fig.GrabMouseClick();    // wait until user clicks the mouse in the figure
    printf("point = %d, %d\n", pt.x, pt.y);
GrabMouseClick draws cross-hairs in the figure as the mouse is moved; when the user clicks inside the figure, then processing resumes with the next line of code (in this case printing the coordinates). In this manner, you can easily get mouse input without having to write your own callback.

Before displaying an image, you can overlay lines, points, rectangles, ellipses, etc. using the following functions:

    ImgBgr img;
    Bgr color(0, 255, 0);                       // green
    Point pt1(10, 10), pt2(20, 20);             // two points
    int radius = 15;
    DrawLine(pt1, pt2, &img, color);            // draw line
    DrawRect(rect, &img, color);                // draw rectangle
    DrawCircle(pt1, radius, &img, color);       // draw circle
Note that these functions actually change the image itself.

To display an image directly onto an existing window, simply call the Draw function. In this example, MyWindow derives from CWnd:

    void MyWindow::Func() 
      CClientDC dc(this);
      Draw(img, dc, 0, 0);   // draw image in upper-left corner of window


Several classes exist to aid capturing images and video from cameras. The following code captures from a USB webcam via Microsoft's DirectShow framework:
    ImgBgr img;
    bool new_img;
    CaptureDirectShow cap;                  // create capture object
    cap.BuildGraph(320, 240);               // build DirectShow graph
    cap.Start();                            // start live capture
    new_img = cap.GetImage(&img);           // get latest image
    if (new_img)
      // use 'img'
Similar classes exist to capture from other devices.

Error handling

Errors in Blepo are roughly divided into two categories. Errors which are the result of programmer mistakes (e.g., trying to add two images of different sizes) are often checked only with an assert statement. As a result, researchers are encouraged to use Debug mode whenever possible.

If an error occurs that is system-related (e.g., trying to load from a file that does not exist, or trying to capture from a camera that is not plugged in), an Exception is thrown. Unless you are happy with the application crashing because of an unhandled exception, you should handle these errors by wrapping your code with a try / catch block:

      // call blepo code
    catch (const Exception& e)
      e.Display();    // display exception to user in a popup window 
The Display method informs the user of the type of error, along with the line and file number where the error occurred. To see which functions may throw an exception, look at the header files.

For further information

Hopefully this tutorial has provided enough information to get you started using Blepo. For further information, consult the reference manual or the actual source code.