Infinite Progress And Rotation from shai`s

   转载：http://hi.baidu.com/dunwin/blog/item/26abd4ca31c9a6f652664feb.html

 

 

 

 

 

 

When looking at the Makeover demo I posted last week one might assume the rotating wheel animation is a standard animation or gif. This is not the case although I could have gone with that approach I chose to use a different tool to achieve this effect.
I built an infinite progress component which displays a rotating image as an animation, this considerably reduces JAR and heap space usage for an animation that looks quite similar to the one produced by a static animation (which we create from GIF). The reason for this is in the way animations work, animations have no sense of rotation, they store the change to the image (as lines) and when the image rotates the animation sees the entire image as changed and produces a "key frame". Key frames are large both in storage and in memory, to hold a 32x32 pixel animation with 8 keyframes I would need approximately 32x32x8+1024 (1024 for palette). This might not seem big, but with increase in resolution the size rises quite a bit...
Rotation is not always efficient, in fact it can be just as inefficient as a keyframe since it needs to create a new image. However, LWUIT has a special optimization on MIDP (this doesn't not apply to LWUIT on CDC) which uses the platforms built in rotation abilities for square angles (90, 180 & 270) hence removing completely the overhead of the image. This isn't enough since rotating on square angles would produce a jumpy effect, which is why I rotate once to 45 degrees and then rotate 2 images in square angles only thus producing what seems to be 8 images but only paying the cost for 2 images.

Another significant advantage is that unlike animations I can make full use of translucency since the alpha channel isn't removed in these images, this allows for flowing rotation effects.

The rotate method currently makes many assumptions and is mostly useful for square images, however it works rather nicely for all angles which is something that currently plain MIDP doesn't support.

public class InfiniteProgressIndicator extends com.sun.lwuit.Label {
 private Image[] angles;
 private int angle;

 public InfiniteProgressIndicator(Image image) {
     Image fourtyFiveDeg = image.rotate(45);
     angles = new Image[] {image, fourtyFiveDeg, image.rotate(90), fourtyFiveDeg.rotate(90),
         image.rotate(180), fourtyFiveDeg.rotate(180), image.rotate(270), fourtyFiveDeg.rotate(270)};
     getStyle().setBgTransparency(0);
     setIcon(image);
     setAlignment(Component.CENTER);
 }
  
 public void initComponent() {
     getComponentForm().registerAnimated(this);
 }
  
 public boolean animate() {
     angle++;
     setIcon(angles[Math.abs(angle % angles.length)]);
     return true;
 }
}