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3.1.3 Animation

Widget classes (mglWindow, mglGLUT) support a delayed drawing, when all plotting functions are called once at the beginning of writing to memory lists. Further program displays the saved lists faster. Resulting redrawing will be faster but it requires sufficient memory. Several lists (frames) can be displayed one after another (by pressing ‘,’, ‘.’) or run as cinema. To switch these feature on one needs to modify function sample:

int sample(mglGraph *gr)
{
  gr->NewFrame();             // the first frame
  gr->Rotate(60,40);
  gr->Box();
  gr->EndFrame();             // end of the first frame
  gr->NewFrame();             // the second frame
  gr->Box();
  gr->Axis("xy");
  gr->EndFrame();             // end of the second frame
  return gr->GetNumFrame();   // returns the frame number
}

First, the function creates a frame by calling NewFrame() for rotated axes and draws the bounding box. The function EndFrame() must be called after the frame drawing! The second frame contains the bounding box and axes Axis("xy") in the initial (unrotated) coordinates. Function sample returns the number of created frames GetNumFrame().

Note, that animation can be also done as visualization of running calculations (see Draw and calculate).

Pictures with animation can be saved in file(s) as well. You can: export in animated GIF, or save each frame in separate file (usually JPEG) and convert these files into the movie (for example, by help of ImageMagic). Let me show both methods.

The simplest methods is making animated GIF. There are 3 steps: (1) open GIF file by StartGIF() function; (2) create the frames by calling NewFrame() before and EndFrame() after plotting; (3) close GIF by CloseGIF() function. So the simplest code for “running” sinusoid will look like this:

#include <mgl2/mgl.h>
int main(int ,char **)
{
  mglGraph gr;
  mglData dat(100);
  char str[32];
  gr.StartGIF("sample.gif");
  for(int i=0;i<40;i++)
  {
    gr.NewFrame();     // start frame
    gr.Box();          // some plotting
    for(int j=0;j<dat.nx;j++)
      dat.a[j]=sin(M_PI*j/dat.nx+M_PI*0.05*i);
    gr.Plot(dat,"b");
    gr.EndFrame();     // end frame
  }
  gr.CloseGIF();
  return 0;
}

The second way is saving each frame in separate file (usually JPEG) and later make the movie from them. MathGL have special function for saving frames – it is WriteFrame(). This function save each frame with automatic name ‘frame0001.jpg, frame0002.jpg’ and so on. Here prefix ‘frame’ is defined by PlotId variable of mglGraph class. So the similar code will look like this:

#include <mgl2/mgl.h>
int main(int ,char **)
{
  mglGraph gr;
  mglData dat(100);
  char str[32];
  for(int i=0;i<40;i++)
  {
    gr.NewFrame();     // start frame
    gr.Box();          // some plotting
    for(int j=0;j<dat.nx;j++)
      dat.a[j]=sin(M_PI*j/dat.nx+M_PI*0.05*i);
    gr.Plot(dat,"b");
    gr.EndFrame();     // end frame
    gr.WriteFrame();   // save frame
  }
  return 0;
}

Created files can be converted to movie by help of a lot of programs. For example, you can use ImageMagic (command ‘convert frame*.jpg movie.mpg’), MPEG library, GIMP and so on.

Finally, you can use mglconv tool for doing the same with MGL scripts (see Utilities).





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