echofilter.java

一个类似超级玛丽的Java游戏 来自《Developing Games in Java》的一个游戏编程实例

package com.brackeen.javagamebook.sound;

/**
    The EchoFilter class is a SoundFilter that emulates an echo.
    @see FilteredSoundStream
*/
public class EchoFilter extends SoundFilter {

    private short[] delayBuffer;
    private int delayBufferPos;
    private float decay;

    /**
        Creates an EchoFilter with the specified number of delay
        samples and the specified decay rate.
        <p>The number of delay samples specifies how long before
        the echo is initially heard. For a 1 second echo with
        mono, 44100Hz sound, use 44100 delay samples.
        <p>The decay value is how much the echo has decayed from
        the source. A decay value of .5 means the echo heard is
        half as loud as the source.
    */
    public EchoFilter(int numDelaySamples, float decay) {
        delayBuffer = new short[numDelaySamples];
        this.decay = decay;
    }


    /**
        Gets the remaining size, in bytes, of samples that this
        filter can echo after the sound is done playing.
        Ensures that the sound will have decayed to below 1%
        of maximum volume (amplitude).
    */
    public int getRemainingSize() {
        float finalDecay = 0.01f;
        // derived from Math.pow(decay,x) <= finalDecay
        int numRemainingBuffers = (int)Math.ceil(
            Math.log(finalDecay) / Math.log(decay));
        int bufferSize = delayBuffer.length * 2;

        return bufferSize * numRemainingBuffers;
    }


    /**
        Clears this EchoFilter's internal delay buffer.
    */
    public void reset() {
        for (int i=0; i<delayBuffer.length; i++) {
            delayBuffer[i] = 0;
        }
        delayBufferPos = 0;
    }


    /**
        Filters the sound samples to add an echo. The samples
        played are added to the sound in the delay buffer
        multipied by the decay rate. The result is then stored in
        the delay buffer, so multiple echoes are heard.
    */
    public void filter(byte[] samples, int offset, int length) {

        for (int i=offset; i<offset+length; i+=2) {
            // update the sample
            short oldSample = getSample(samples, i);
            short newSample = (short)(oldSample + decay *
                delayBuffer[delayBufferPos]);
            setSample(samples, i, newSample);

            // update the delay buffer
            delayBuffer[delayBufferPos] = newSample;
            delayBufferPos++;
            if (delayBufferPos == delayBuffer.length) {
                delayBufferPos = 0;
            }
        }
    }

}