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[IMAGE Graphics_col._final_rev1.GIF]

The Macintosh has always provided animation capabilities. From the early Macintosh 128K to current CPUs, animation has consistently played a large part in the development of software. And though CPU models continue to change, the theories and concepts behind animation have stayed basically the same. Simply stated, animation is the process of stepping through a sequence of images, each slightly different from the previous.

The thought of animation on the Macintosh usually brings to mind games and multimedia, when in fact the actual use of animation is more prevalent than most people imagine. I'll describe some common uses and methods of performing animation and get you started on writing your own animation sequences.

One of the most fundamental methods of animation is using the srcXor transfer mode. The basic idea is that once you've drawn something in this mode, you can erase it simply by drawing it again, restoring the bits underneath to their previous state. Primitive though it may be, this method is common to many applications. Probably the most obvious example of it can be found in the Finder. Familiar to even the novice Macintosh user is the dotted rectangle that often appears during desktop navigation. The movement of the dotted rectangle, which appears when the user selects multiple icons or drags windows across the desktop, is a simple form of animation. The dotted rectangle is also used to create the zooming effect when desktop folders are opened and closed.

To use this method, you set the current transfer mode to srcXor before drawing the object you plan to animate. In the desktop example, the Finder switches to srcXor mode and then draws the dotted rectangle with a simple FrameRect call, with the PenPat set to 50% gray. The movement of the dotted rectangle is accomplished by redrawing the rectangle at its previous position before drawing it at its new location. With srcXor mode, simply redrawing the rectangle at the same position restores the desktop to its original state. So by repeatedly drawing and redrawing the rectangle in its new position, you float a frame across the screen without damaging the contents of the desktop.

As a variation on the dotted rectangle, applications use what's called the "marching ants" effect. With this effect, the bounding frame gives the illusion that the dashed lines or "ants" are moving around the edges of the box, thereby producing an animated and more interesting visual appearance.

[IMAGE Graphics_col._final_rev2.GIF] The marching ants effect is simple to create. The most common way to do this is with a simple 8-by- 8-bit pattern. To create the illusion, you draw a bounding frame by calling FrameRect, with the PenMode set to srcXor and the PenPat set to a pattern defined with diagonal stripes (see the illustration below). Shifting the pattern up one row, and then wrapping the first row of the pattern to the last row, creates the effect. If the rows were shifted down rather than up, the ants would appear to move in the opposite direction. In either case, the ants typically start at one corner of the box and then end at the opposite corner.

As with the dotted rectangle, the frame is continually drawn and redrawn, but this time with each new updated pattern. Note the difference between the two effects when the frame is drawn: With the ants, the frame is constantly being drawn and redrawn even if the rectangle's coordinates haven't changed. With the dotted rectangle, the frame is redrawn only when its position has changed. Since no animation takes place when the dotted rectangle is sitting in the same position, it's not necessary to continually draw the frame in that case.

Another method of performing animation is to use off-screen drawing. With this method, the actual drawing is being done behind the user's back. The animation frames are prepared off-screen and quickly transferred to the screen with CopyBits to create the animation sequence. Regardless of what CPU you're running, this method can provide excellent animation effects. And with the advent of GWorlds to simplify the process of building off-screen environments, performing animation with this technique has become much easier.

In this section I'll provide some important points to consider when building your own off-screen world and describe how to apply these off-screen worlds to animation. For a detailed description of creating your own custom GDevices, cGrafPorts, and pixMaps, see the Macintosh Technical Note "Principia Off-Screen Graphics Environments."

Before even considering off-screen animation, you need to determine whether your Macintosh has enough memory for creating the off-screen environment. Without sufficient memory, you might as well forget it. Having high-performance, high-quality animation isn't cheap. Most of what determines the amount of required memory is the off-screen world's dimensions and pixel depth.

  • Typically, or at least for this method, the dimensions of the off-screen world are the same as those of the entire on-screen area.
  • For the depth of the off-screen world, you'll need to determine whether it's based on the depth of the images used in the window or on the depth of the GDevice intersecting the window. In the case where the GDevice is set to direct colors, you may want to create only an 8-bit off-screen world to save memory if your images use only 256 or fewer colors. On the other hand, you may want to create an off-screen world equal to the depth of the GDevice containing the window, for better data transfer performance.

Once you've determined the dimensions and depth for the off-screen world, you're ready to create the off-screen environment. Note that if you're using the GWorlds introduced with 32-Bit QuickDraw, many of the off-screen initialization procedures have been simplified. Also, with certain video display cards, the GWorlds can be cached into the NuBusTM card's memory, providing even better performance when off-screen worlds are used.

To create the off-screen environment, you pass NewGWorld the off-screen dimensions, depth, and color table, and the routine creates the environment or warns you if there wasn't sufficient memory. After you've made all the required memory checks and created your off-screen environment, either by hand or with NewGWorld, the next step is to create the animation sequence.

In the simplest case, the off-screen world is used to store an identical copy of what's displayed on the screen. Rather than erasing and drawing the moving object on-screen, you perform all this in the off- screen world. Once the moving object has been drawn in its new position, the off-screen image is transferred to the screen. By continually drawing the next frame of the moving object in the off-screen world before displaying it on the screen, you produce the animation effect. The following steps describe the process.

  1. Assuming that the entire window is being used for the animation, create an off-screen environment of the same dimensions as the window, either by hand or with NewGWorld. When you're defining the depth and color table of the off-screen world, remember that QuickDraw requires extra time to map colors when the destination GDevice's depth and color table are different from those of the source.
  2. Switch to the off-screen grafPort and GDevice and draw the background image. This is the image that the object will be moved on top of; typically it won't change.
  3. Draw the object that will be moved or animated into the off-screen world. Actually, any image not part of the background image should be drawn at this time. Also, since the object overwrites the background image, the background under the object will eventually need to be restored.
  4. Switch back to the on-screen grafPort and GDevice and use CopyBits to transfer the off-screen pixMap to the screen.

These steps create just one frame of the animation sequence. To create the full sequence, repeat the last three steps until the animation is complete. In step 2, instead of redrawing the entire background, you may want to redraw just the areas that need to be restored, if that information is available. By redrawing just a portion of the damaged background, you'll notice improved performance, especially when working with higher pixel depths.

Besides providing a quick introduction to off-screen animation, this method has the advantage that it's simple and straightforward. Since all the objects and images are drawn at one time and in the same environment, it's easy to create your sequences and synchronize the animation for any moving object. However, as mentioned earlier, large off-screen images at higher pixel depths can really affect the performance of the animation. To overcome this problem, you need to use multiple off-screen worlds.

The concept of multilayer off-screen worlds isn't much different from the basics of off-screen animation. Rather than having just one off-screen environment, you've also got an intermediate off- screen layer in which all the actual drawing is completed, leaving the background layer undamaged. So unlike the previous method, where one off-screen world was used for storing the background and the moving object, this method uses two separate off-screen worlds to maintain this information. The following steps describe how the intermediate layer fits in.

  1. Again, create the background off-screen layer with the same dimensions as the window.
  2. Switch the current grafPort and GDevice to the background layer, then draw the background image. This layer will never change, since its main purpose is to restore the overwritten areas of the intermediate layer.
  3. Find the common rectangle containing the object's previous location and its new location. This can be calculated by passing UnionRect the object's bounding rectangle for both positions. Be sure the common rectangle uses coordinates local to the window.
  4. Create the intermediate off-screen layer with the dimensions of the common rectangle.
  5. Switch to the intermediate layer and transfer the area of the corresponding common rectangle of the background layer to the current layer. This will restore the area at which the object was last positioned. Rather than having to redraw the background for each frame, you simply replace the damaged area with the background image stored in memory.
  6. Draw the moving object at its new location in the intermediate layer. If multiple objects are within the same bounding region of this layer, they should be drawn at this time as well.
  7. Switch to the window layer and use CopyBits to transfer the contents of the intermediate layer to the screen. Finally, to create the entire animation sequence, repeat steps 3-7 until the animation is complete. The illustration below shows the process of creating one of the frames in the sequence. In this frame, the moving object is the sun, drawn on top of the background image of the mountains.

When moving multiple objects, you'll need to decide whether to handle the objects separately or in groups. In the case where objects are widely dispersed in the window, it would be more practical to create a separate intermediate layer for each object than to create one layer containing all the objects. Since no changes are occurring in places between widely spread objects, unnecessary time and memory would be spent updating these areas.

[IMAGE Graphics_col._final_rev3.GIF]

However, if the objects are closely spaced, grouping the objects and creating one intermediate layer would make more sense. Since objects can overlap each other, creating separate off-screen worlds would not be too practical or easily accomplished. So when determining the number of intermediate off-screen layers, you'll want to first check where the objects are located in the window.

The main advantage of using the intermediate layer is the performance improvement. As mentioned earlier, transferring large blocks of data at high pixel depths can be time consuming. As you can guess, the smaller the transfer image, the less time QuickDraw requires. Another advantage of using this layer is the ability to isolate the background image. Since all the drawing is taking place in the intermediate layer, there's no need to redraw the background image for each frame, which can be a real time saver for complex backgrounds. Though more memory is required with the addition of the intermediate layer, the performance gains can sometimes make the extra memory worth it.

Finally, to fully optimize the animation performance, you'll want to be sure the data transfer from the off-screen layers is as fast as possible. Since you can influence the speed of CopyBits, here are a few points you'll want to keep in mind when creating and using off-screen layers:

  • For indexed GDevices, the same color table should be used for the window and the off-screen layers. Since no color mapping should be required when the source and destination share the same color table, less time is needed for the data transfer.
  • Be sure no nonrectangular clipping is involved in the CopyBits operation. Having to check which pixels should or shouldn't be clipped can really slow down the data transfer.
  • Use srcCopy as the transfer mode for CopyBits. Any other mode takes extra time to perform the logical operations on the source and destination pixels.
  • Set the current port's foreground color to black and background color to white before calling CopyBits. This will ensure that no colorizing (which can be slow) takes place.
  • Make sure no dithering takes place. Unless you have your own rippin' fast method for dithering, try to stay away from it. If possible, prepare the images in the off-screen layers in such a way that dithering isn't needed.
  • Keep the same alignment of pixels for the source and destination pixMaps. Having to shift unaligned pixels can take time.
  • The source and destination rectangles should be the same size. Scaling requires extra work.

By following as many of these points as possible, you'll improve the performance that you'll get out of CopyBits and waste less time in the on-screen updates.

I've presented several methods of animation; which method to use depends on your application. In fact, you may choose to use several methods or switch between methods under different system requirements. Say your application uses multiple layering for optimal animation; under low-memory conditions, you may want to switch to just one off-screen world to provide at least some type of off- screen animation. But if that isn't even an option, you may have to do all the animation on-screen. For an example that does exactly that, see DTS.Draw in the Sample Code folder on theDeveloper CD Series disc. If sufficient memory is available to create the off-screen worlds, the application uses the multilayer method; otherwise, the application decides on the next best method based on the current available memory.

This column has described different animation techniques, but the principle behind them is basically the same, even if the results don't show it. Given a set of slightly different images, all the methods involve stepping through the series of images, where each object in the image is erased before the next object in the series is displayed.

Animation provides excellent visual effects, more fun for the programmer, and most important, an enhanced experience for the user. Now that you've got the basics of animation on the Macintosh, I hope you'll be inspired to animate your own applications!


  • "Macintosh Display Card 8*24 GC: The Naked Truth" by Guillermo Ortiz, develop  Issue 3.
  • Macintosh Technical Notes "Principia Off-Screen Graphics Environments" (formerly #120) and "Of Time and Space and _CopyBits" (formerly #277).
  • Computer Graphics: Principles and Practice, 2nd ed., by J. D. Foley, A. Van Dam, S. K. Feiner, and J. F. Hughes (Addison-Wesley, 1990), Chapter

EDGAR LEE (AppleLink EDGAR) Recently spared from the traumas of big city living, Edgar enjoys the relaxing and granola-like atmosphere of sunny Cupertino. When asked what he likes most about the area, he proudly points to his car stereo in disbelief that it's still there. Besides adjusting to his newly found appreciation of suburban living, Edgar enjoys a good challenge of doubles volleyball, an excellent head-to-head game of Tetris, and learning the newest and latest human tricks from his faithful companion, Sunny. Though Edgar realizes Sunny is only a dog, he still believes some of the engineers here at Apple could stand to learn a lot from her. Of course these engineers don't seem to agree. *

For more information on caching GWorlds into NuBus memory and improving drawing performance, see "Macintosh Display Card 8*24 GC: The Naked Truth" in develop  Issue 3.*

For source-code routines that create and manage off-screen layers, see GWLayers in the Sample Code folder on theDeveloper CD Series  disc. To see how these routines are actually used, check out the Kibitz and DTS.Draw samples on the CD as well. (GWLayers is brought to you by Forrest Tanaka, and Kibitz and DTS.Draw are from Eric Soldan.) *


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