Mobile Phone Game Programming : Analyzing 2D Sprite Animation

11/29/2011 9:01:30 AM
Frame-Based Animation

The most simple animation technique is frame-based animation, which finds a lot of usage in nongaming animations. Frame-based animation involves simulating movement by displaying a sequence of pregenerated, static frame images. A movie is a perfect example of frame-based animation: Each frame of the film is a frame of animation, and when the frames are shown in rapid succession, they create the illusion of movement.

Frame-based animation has no concept of a graphical object distinguishable from the background; everything appearing in a frame is part of that frame as a whole. The result is that each frame image contains all the information necessary for that frame in a static form. This is an important point because it distinguishes frame-based animation from cast-based animation, which you learn about in the next section. Figure 1 shows a few frames in a frame-based animation.

Figure 1. In frame-based animation, the entire frame changes to achieve the effect of animation.

Figure 5.1 shows how a paratrooper is drawn directly onto each frame of animation, so there is no separation between the paratrooper object and the sky background. This means that the paratrooper cannot be moved independently of the background. The illusion of movement is achieved as each frame is redrawn with the paratrooper in a slightly different position. This type of animation is of limited use in games because games typically require the ability to move objects around independently of the background.

Cast-Based Animation

A more powerful animation technique employed by many games is cast-based animation, which is also known as sprite animation. Cast-based animation involves graphical objects that move independently of a background. At this point, you might be a little confused by the usage of the term “graphical object” when referring to parts of an animation. In this case, a graphical object is something that logically can be thought of as a separate entity from the background of an animation image. For example, in the animation of a space shoot-em-up game, the aliens are separate graphical objects that are logically independent of the star field background.

Gamer’s Garage

The term “cast-based animation” comes from the fact that sprites can be thought of as cast members moving around on a stage. This analogy of relating computer animation to theatrical performance is very useful. By thinking of sprites as cast members and the background as a stage, you can take the next logical step and think of an animation as a theatrical performance. In fact, this isn’t far from the mark because the goal of theatrical performances is to entertain the audience by telling a story through the interaction of the cast members. Likewise, cast-based animations use the interaction of sprites to entertain the user, while often telling a story.

Each graphical object in a cast-based animation is referred to as a sprite, and has a position that can vary over time. In other words, a sprite can have a velocity associated with it that determines how its position changes over time. Almost every video game uses sprites to some degree. For example, every object in the classic Asteroids game is a sprite that moves independently of the background; even though Asteroids relies on vector graphics, the objects in the game are still sprites. Figure 2 shows an example of how cast-based animation simplifies the paratrooper example you saw in the previous section.

Figure 2. In cast-based animation, a graphical object can move independently of the background to achieve the effect of animation.

In this example, the paratrooper is now a sprite that can move independently of the background sky image. So, instead of having to draw every frame manually with the paratrooper in a slightly different position, you can just move the paratrooper image around on top of the background.

Even though the fundamental principle behind sprite animation is the positional movement of a graphical object, there is no reason you can’t incorporate frame-based animation into a sprite. This enables you to change the image of the sprite as well as alter its position. This hybrid type of animation is actually built into the sprite support in the MIDP 2.0 API, as you soon learn.

I mentioned in the frame-based animation discussion that television is a good example of frame-based animation. But can you think of something on television that is created in a manner similar to cast-based animation (other than animated movies and cartoons)? Have you ever wondered how weather people magically appear in front of a computer-generated map showing the weather? The news station uses a technique known as blue-screening or green-screening, which enables them to overlay the weatherperson on top of the weather map in real time. It works like this: The person stands in front of a solid colored backdrop (blue or green), which serves as a transparent background. The image of the weatherperson is overlaid onto the weather map; the trick is that the colored background is filtered out when the image is overlaid so that it is effectively transparent. In this way, the weatherperson is acting exactly like a sprite!

Seeing Through Objects with Transparency

The weatherperson example brings up a very important point regarding sprites: transparency. Because bitmapped images are rectangular by nature, a problem arises when sprite images aren’t rectangular in shape. In sprites that aren’t rectangular in shape, which is the majority of them, the pixels surrounding the sprite image are unused. In a graphics system without transparency, these unused pixels are drawn just like any others. The end result is sprites that have visible rectangular borders around them, which completely destroys the effectiveness of having sprites overlaid on a background image.

What’s the solution? Well, one solution is to make all your sprites rectangular. Because this solution isn’t very practical, a more realistic solution is transparency, which allows you to define a certain color in an image as unused, or transparent. When drawing routines encounter pixels of this color, they simply skip them, leaving the original background showing through. Transparent colors in images act exactly like the weatherperson’s colored screen in the earlier example.

Adding Depth with Z-Order

In many instances, you will want some sprites to appear on top of others. For example, in a war game you might have planes flying over a battlefield dropping bombs on everything in sight. If a plane sprite happens to fly over a tank sprite, you obviously want the plane to appear above the tank and, therefore, hide the tank as it passes over. You handle this problem by assigning each sprite a screen depth, which is also referred to as Z-order.

Z-order is the relative depth of sprites on the screen. The depth of sprites is called Z-order because it works sort of like another dimension—like a z axis. You can think of sprites moving around on the screen in the XY axis. Similarly, the z axis can be thought of as another axis projected into the screen that determines how the sprites overlap each other. To put it another way, Z-order determines a sprite’s depth within the screen. Because they make use of a z axis, you might think that Z-ordered sprites are 3D. The truth is that Z-ordered sprites can’t be considered 3D because the z axis is a hypothetical axis only used to determine how sprite objects hide each other.

Construction Cue

The easiest way to control Z-order in a game is to pay close attention to the order in which you draw the game graphics. Fortunately, the MIDP API provides a class called LayerManager that simplifies the task of managing multiple graphics objects (layers) and their respective Z-orders.

Just to make sure that you get a clear picture of how Z-order works, let’s go back for a moment to the good old days of traditional animation. You learned earlier that traditional animators, such as those at Disney, used celluloid sheets to draw animated objects. They drew on celluloid sheets because the sheets could be overlaid on a background image and moved independently; cel animation is an early version of sprite animation. Each cel sheet corresponds to a unique Z-order value, determined by where in the pile of sheets the sheet is located. If a sprite near the top of the pile happens to be in the same location on the cel sheet as any lower sprites, it conceals them. The location of each sprite in the stack of cel sheets is its Z-order, which determines its visibility precedence. The same thing applies to sprites in cast-based animations, except that the Z-order is determined by the order in which the sprites are drawn, rather than the cel sheet location.

Detecting Collisions between Objects

No discussion of animation as it applies to games would be complete without covering collision detection. Collision detection is the method of determining whether sprites have collided with each other. Although collision detection doesn’t directly play a role in creating the illusion of movement, it is tightly linked to sprite animation and extremely crucial in games.

Collision detection is used to determine when sprites physically interact with each other. In an Asteroids game, for example, if the ship sprite collides with an asteroid sprite, the ship is destroyed and an explosion appears. Collision detection is the mechanism employed to find out whether the ship collided with the asteroid. This might not sound like a big deal; just compare their positions and see whether they overlap, right? Correct, but consider how many comparisons must take place when a lot of sprites are moving around—each sprite must be compared to every other sprite in the system. It’s not hard to see how the processing overhead of effective collision detection can become difficult to manage.

Not surprisingly, there are many approaches to handling collision detection. The simplest approach is to compare the bounding rectangles of each sprite with the bounding rectangles of all the other sprites. This method is efficient, but if you have objects that are not rectangular, a certain degree of error occurs when the objects brush by each other. Corners might overlap and indicate a collision when really only the transparent areas are overlapping. The less rectangular the shape of the sprites, the more error typically occurs. Figure 3 shows how simple rectangle collision works.

Figure 3. Collision detection using rectangle collision simply involves checking to see whether the bounding rectangles of two objects overlap.

In the figure, the areas determining the collision detection are shaded. You can see how simple rectangle collision detection isn’t very accurate unless you’re dealing with sprites that are rectangular in shape. An improvement upon this technique is to shrink the collision rectangles a little, which reduces the error. This method improves things a little, but it has the potential of causing error in the reverse direction by allowing sprites to overlap in some cases without signaling a collision. Figure 4 shows how shrinking the collision rectangles can improve the error on simple rectangle collision detection. Shrunken rectangle collision is just as efficient as simple rectangle collision because all you are doing is comparing rectangles for intersection.

Figure 4. Collision detection using shrunken rectangle collision involves checking to see whether shrunken versions of the bounding rectangles of two objects overlap.

The most accurate collision detection technique is to detect collision based on the sprite image data, which involves actually checking to see whether transparent parts of the sprite or the sprite images themselves are overlapping. In this case, you get a collision only if the actual sprite images are overlapping. This is the ideal technique for detecting collisions because it is exact and enables objects of any shape to move by each other without error. Figure 5 shows collision detection that uses the sprite image data.

Figure 5. Collision detection that uses image data collision involves checking the specific pixels of the images for two objects to see whether they overlap.

Unfortunately, the technique shown in Figure 5 requires more processing overhead than rectangle collision detection and can be a bottleneck in game performance. It really depends on the importance of extremely accurate collision detection in your specific game, and how much room you have to carry out the processing without killing your frame rate. You’ll find that shrunken rectangle collision detection is sufficient in a lot of games.

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