Dynamic masking using the drawing API - Page 5

Movie 3: Pie preloader

So far we've only used the drawing API to draw squares. Certainly there are other things we can draw with it! In this movie we'll create a portion of circle (pie slice) in order to create a pie preloader. It should look something like this in the end:

Here the double arcs on the right represents the percentage of movie loaded (in this case about 45%).

Some background would be useful before presenting the script. Drawing a pie slice turns out to be a pain because it involves curves which are not nearly as simple to work with as lines in the drawing API. Now instead of creating "real" pie slices we will add a series of small triangles with a relative angle between them so as to create a section of a regular polygon. We'll use triangles with a side length greater than the radius of the masked circle and a sharp angle of 3.6 degrees (1/100 of a circle). What we need is a way to transform these angle and radius coordinates to more traditional Cartesian x and y pairs. A diagram will certainly help:

Convince yourself that the following relations hold:

x1 = r*sin(O)
x2 = r*sin(O+dO)
y1 = r*cos(O)
y2 = r*cos(O+dO)

With that in mind we may create our pie preloader. Create a circle with no interior and with a large outline (donut shape). Convert to movie clip (F8), assuring that that you select the center as the registration point (this is very important). Name the instance maskee. Now place it anywhere on the stage.

Select the movie clip and add this action to it (once again, not to the frame but to the clip itself):

onClipEvent(load)
{
  _root.stop();
  dO = 3.6;
  r = 75;
  function addslice(O)
  {
    x1 = r*Math.sin(O*Math.PI/180);
    x2 = r*Math.sin((O+dO)*Math.PI/180);
    y1 = r*Math.cos((O)*Math.PI/180);
    y2 = r*Math.cos((O+dO)*Math.PI/180);
    trace(x1 + ":" + y1);
    with(_root.mask)
    {
      moveTo(0,0);
      beginFill(0x000088);
      lineTo(x1,y1);
      lineTo(x2,y2);
      endFill();
    }
  }
  _root.createEmptyMovieClip("mask",0);
  this.setMask(_root.mask);
  _root.mask._yscale = -100;
  _root.mask._x = this._x;
  _root.mask._y = this._y;
  oldLoaded = 0;
}
onClipEvent(enterFrame)
{
  loaded = Math.ceil(_root.getBytesLoaded()/_root.getBytesTotal()*100);
  for(i = oldLoaded; i < loaded; i++)
  {
    addSlice(dO*i);
  }
  oldLoaded = loaded;
}