Volume Number: 3

Issue Number: 7

Column Tag: Pascal Procedures

Animated BitMaps

By Scott Boyd, The MacHax™ Group, Bryan, Texas

Offscreen BitMagic

Remember the first time you picked up an object in Déja Vú or Uninvited?

Absolutely dazzling! The object lifted off the page. You dragged IT around, not some

gray outline. Ever wondered how animation like that is done? Obviously some of you

have, because we’ve seen similar behavior in other programs. Just about all of the

paint programs have it. Maybe you can think of others that do it, too.

Alan Kay loves to point out that, about ten years ago, Alto computers at XEROX

Palo Alto Research Center (PARC) performed at about the same level as today’s

Macintosh Plus. We now have the power to pay attention to feel and effect. Alan Kay

showed a film of a current XEROX PARC project called the Alternate Reality Kit. It

served as my inspiration for the effect that serves as the focus of this article. I

realized that the Mac is plenty powerful to drag whole objects, and it might even be

powerful enough to add shadows. The sample programs below show that my hunch was

right. Try them out and see if you don’t feel a sense of excitement as the pictures leap

off the screen and move around.

In addition to the samples, this article discusses some of the how-to’s of

offscreen bitmap magic. It also provides you with code that you can plug directly into

your programs. Now that we have a machine that can handle it, let’s push it. Let’s see

what this can do for our interfaces. Figure 0 shows our example program, which

allows us to drag various bitmaps without flicker.

Where do we start?

Try to imagine how you might solve the problem of dragging a picture around the

screen if you could do it any way you wanted. I envision a graphics system where I