在Android上用Canvas绘制音频波形图

最近在研究VAD算法，但调整参数时无法实时看到效果，于是决定将音频波形实时绘制出来，并且语音部分和噪音部分用不同颜色的线条显示，这样就能立即看到VAD算法对各种噪音类型的鲁棒性

为了简化问题规模，先研究出怎么在Android下实时绘制随机生成波形的功能，后面再加入语音获取、根据VAD算法的结果用不同颜色线条显示等功能

查了下Canvas类，发现drawLines方法跟drawLine方法都是根据4个浮点数绘制一条直线，前者还需要开辟额外的空间，所以选择了后者，结果绘制速度很慢，在我的小米2s上绘制2s单通道16KHz采样率16bit位宽的音频，耗时达到700ms，无奈尝试前者，其他条件不变的情况下降至20ms（最快1ms）

由此也能推测出，后者开销这么大主要是花在drawLine的函数调用，而不是坐标点的生成。

代码

package com.happen23.games.waveviewer;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.view.ViewTreeObserver;

import java.util.Random;

/**
 * Created by Administrator on 2016/5/14.
 */
public class VadView extends View {
    private static final String TAG = "VadView";
    Paint paint;
    int audioSampleNum = 2 * 16000;
    int widthPixels;
    int heightPixels;
    float points[];
    short audio[];

    public VadView(Context context, AttributeSet attrs) {
        super(context, attrs);
        paint = new Paint();
        paint.setColor(Color.BLUE);
        paint.setStrokeJoin(Paint.Join.ROUND);
        paint.setStrokeCap(Paint.Cap.ROUND);
        paint.setStrokeWidth(1);

        getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
            @Override
            public void onGlobalLayout() {
                widthPixels = getWidth();
                heightPixels = getHeight();
            }
        });
    }
    protected void drawWave2(Canvas canvas, short audio[]) {
        for (int i = 0; i < audioSampleNum-1; i++){
            points[4*i] = (float)i/audioSampleNum * widthPixels;
            points[4*i+1] = heightPixels/2 + (float)audio[i]/32768 * heightPixels/2;
            points[4*i+2] = (float)(i+1)/audioSampleNum * widthPixels;
            points[4*i+3] = heightPixels/2 + (float)audio[i+1]/32768 * heightPixels/2;
        };
        canvas.drawLines(points, paint);
    }
    protected void drawWave1(Canvas canvas, short audio[]){
        float startX, startY, stopX, stopY;
        for (int i = 0; i < audio.length-1; i++){
            startX = (float)i/audioSampleNum * widthPixels;
            startY = heightPixels/2 + (float)audio[i]/32768 * heightPixels/2;
            stopX = (float)(i+1)/audioSampleNum * widthPixels;
            stopY = heightPixels/2 + (float)audio[i+1]/32768 * heightPixels/2;
            canvas.drawLine(startX, startY, stopX, stopY, paint);
        }
    }
    protected void onDraw(Canvas canvas) {
        if (points == null) {
            points = new float[audioSampleNum * 4];
            audio = new short[audioSampleNum];
        }
        Random r = new Random();
        for (int i = 0; i < audioSampleNum; i++){
            audio[i] = (short)(r.nextInt(65536) - 32768);
        }
        Log.w(TAG, "start drawLine");
        drawWave2(canvas, audio);
        Log.w(TAG, "stop drawLine");
        postDelayed(new Runnable() {
            @Override
            public void run() {
                invalidate();
            }
        }, 200);
    }
}