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            天行健 君子當自強而不息

            Timing in Animation and Movement(1)

            Games are packed with movement. Characters running here, bullets flying there−basically there's a slew of objects moving about your game world. The smooth motion of these objects is a very important aspect that can't be overlooked. Have you ever thought about the use of movement and animation based on time? Using time−based motion is hot, and to keep up with the rest of the world you must fully understand how it can help your game project. In fact, you need to understand not only using time−based motion, but also motion in general. Do you think only characters move about in your game? Nope, your in−game cinematic cameras need your guidance as well.

             

            Using Time−Based Motion

            Although it might not seem important at first, timing plays a crucial role in your game projects. I'm not talking about the time of day; rather, I'm referring to down−to−the−millisecond timing of animation. This sort of precise timing is required for smooth animation and movement of objects in your project. Although it is a basic topic, it is one that all game programmers should understand quite well.

            When it comes to moving your meshes, time−based motion is the best of the bunch.

            The whole basis of using time−based motion is simple−movement and animation always take the same amount of time to perform on any system, regardless of the computer's actual speed. For example, a 2−GHz system processing a 20−second animation will undoubtedly do so quickly and produce a very smooth motion, whereas the same animation running on a 500−MHz system will be choppy but will still maintain the 20−second animation length.

            More than likely, the slower computer will drop specific frames of the animation to keep up with the speed of the faster computer. Therein lies the secret−slower computers can drop more frames and still maintain a somewhat reasonable representation of the actual motion.

            Okay, I think you're starting to get the idea. As simple as the techniques sound, time−based animation and movement remain a slight mystery to fledgling game programmers. For that reason, I want to introduce (or re−introduce) you to the world of time−based motion. I'll start with a brief look at reading time in Windows.

             

            Reading Time in Windows

            The easiest method of reading time in Windows is by using the timeGetTime function. This function does not take any parameters. As the following code demonstrates, timeGetTime only returns the number of milliseconds that have passed since Windows was started.

            DWORD TimeSinceStarted = timeGetTime();

            What good does this do you? Typically, you call the timeGetTime function once per frame that your game processes. For each frame, you then subtract the time from the last frame processed to obtain the number of milliseconds that have elapsed since your last frame. You can use this elapsed time to compute your time−based motion.

            Storing the time of the last frame update is as easy as using a static variable. At the beginning of your frame's update function, insert a static variable that stores the current time.

            void FrameUpdate()
            {
              static DWORD LastTime = timeGetTime();

            At the end of the FrameUpdate function, you can then store the current time in LastTime.

            LastTime = timeGetTime();

            In this manner (storing the current time at the end of your frame's update function), you have managed to store the time at which the frame update ended. During the next call to FrameUpdate, the LastTime variable will still contain the time at which the last frame update ended. Using this time value, you can calculate the amount of time that has elapsed since you last called FrameUpdate by subtracting LastTime from the current time.

            DWORD ElapsedTime = timeGetTime() − LastTime;

            Now what about those instances when you want to calculate the number of elapsed milliseconds based on a specific time, such as when an animation starts, instead of counting the elapsed time or when the FrameUpdate function was first called?

            Just as you used a static variable to store the last frame's update time, you can store the time at which the function was first called. Using that static variable, you can determine how many milliseconds have passed since the function was first called.

            void FrameUpdate()
            {
              static DWORD StartTime = timeGetTime();
              DWORD ElapsedTime = timeGetTime() − StartTime;

              // ElapsedTime is the number of milliseconds that has passed since you first called FrameUpdate.
            }

            In the same way the previous bit of code tracked the total amount of time that has passed since the first call to the FrameUpdate function, you can embed the starting time of an animation inside your data structures.

            Combined with key frames, time−based animation is a perfect solution for creating smooth animation. Read on to take a closer look at using time−based animation.


            posted on 2008-04-15 15:50 lovedday 閱讀(355) 評論(0)  編輯 收藏 引用

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