Android如何从图片中切取人脸区域？

从一张图片中切出人脸区域是App开发中常用的场景，譬如，现在很多App用户上传头像的时候，喜欢随手自拍。自拍的图片往往在尺寸、位置上并不完美。而App需要在各种千奇百怪的UI场景下显示用户的头像。所以从原始头像图片中切取出人脸区域看起来是个刚需。这里介绍如何应用Android提供的人脸识别接口完成简单的切取人脸区域。

 

简单起见，输入为一个可能人脸的Bitmap，并且假定目标是识别出一个人脸而已。输出为一个以人脸为中心的原图的部分切图。如果需要自定义长宽比例，或者只需要定位人脸位置，来半侧切图，可以稍加改动实现。

 

Android官方提供的人脸识别api在android.media.FaceDetector，核心的api是

FaceDetector.findFaces()

主要的坑有两个：

（1）只能处理RGB_565格式的Bitmap，需要做转换。

（2）只能处理width为偶数的Bitmap，这里的解法是如果是奇数，则放弃最右侧的一列像素。

 

直接贴代码：

package com.example.testface;

import android.graphics.Bitmap;
import android.graphics.PointF;
import android.media.FaceDetector;
import android.util.Log;

public final class FaceHelper {

private static final String LOG_TAG = "FaceHelper";
private static final boolean DEBUG_ENABLE = false;

public static Bitmap genFaceBitmap(Bitmap sourceBitmap) {

if (!checkBitmap(sourceBitmap, "genFaceBitmap()")) {
return null;
}

// default algorithm of face detecting of Android can only handle
// RGB_565 bitmap, so copy it by RGB_565 here.
Bitmap cacheBitmap = sourceBitmap.copy(Bitmap.Config.RGB_565, false);

// gives up strong reference here. because this method may be
// time-consuming, so maybe run in work thread usually, we give up the
// strong reference of the source bitmap after it copied, so that it can
// be recycled and GC in another thread.
sourceBitmap = null;

if (DEBUG_ENABLE) {
Log.i(LOG_TAG,
"genFaceBitmap() : source bitmap width - "
+ cacheBitmap.getWidth() + " , height - "
+ cacheBitmap.getHeight());
}

int cacheWidth = cacheBitmap.getWidth();
int cacheHeight = cacheBitmap.getHeight();

// default algorithm of face detecting of Android can only handle the
// bitmap that width is even, so we give up the 1 pixel from right if
// not even.
if (cacheWidth % 2 != 0) {
if (0 == cacheWidth - 1) {
if (DEBUG_ENABLE) {
Log.e(LOG_TAG,
"genFaceBitmap() : source bitmap width is only 1 , return null.");
}
return null;
}
final Bitmap localCacheBitmap = Bitmap.createBitmap(cacheBitmap, 0,
0, cacheWidth - 1, cacheHeight);
cacheBitmap.recycle();
cacheBitmap = localCacheBitmap;
--cacheWidth;

if (DEBUG_ENABLE) {
Log.i(LOG_TAG,
"genFaceBitmap() : source bitmap width - "
+ cacheBitmap.getWidth() + " , height - "
+ cacheBitmap.getHeight());
}
}

final FaceDetector.Face[] faces = new FaceDetector.Face[1];
final int facefound = new FaceDetector(cacheWidth, cacheHeight, 1)
.findFaces(cacheBitmap, faces);
if (DEBUG_ENABLE) {
Log.i(LOG_TAG, "genFaceBitmap() : facefound - " + facefound);
}
if (0 == facefound) {
if (DEBUG_ENABLE) {
Log.e(LOG_TAG, "genFaceBitmap() : no face found , return null.");
}
return null;
}

final PointF p = new PointF();
faces[0].getMidPoint(p);
if (DEBUG_ENABLE) {
Log.i(LOG_TAG,
"getFaceBitmap() : confidence - " + faces[0].confidence());
Log.i(LOG_TAG, "genFaceBitmap() : face center x - " + p.x
+ " , y - " + p.y);
}
final int faceX = (int) p.x;
final int faceY = (int) p.y;
if (DEBUG_ENABLE) {
Log.i(LOG_TAG, "genFaceBitmap() : int faceX - " + faceX
+ " , int faceY - " + faceY);
}

// compute an area that face in middle of it.
int startX, startY, width, height;
if (faceX <= cacheWidth - faceX) {
startX = 0;
width = faceX * 2;
} else {
startX = faceX - (cacheWidth - faceX);
width = (cacheWidth - faceX) * 2;
}
if (faceY <= cacheHeight - faceY) {
startY = 0;
height = faceY * 2;
} else {
startY = faceY - (cacheHeight - faceY);
height = (cacheHeight - faceY) * 2;
}

final Bitmap result = Bitmap.createBitmap(cacheBitmap, startX, startY,
width, height);
cacheBitmap.recycle();
return result;
}

private static boolean checkBitmap(final Bitmap bitmap,
final String debugInfo) {
if (null == bitmap || bitmap.isRecycled()) {
if (DEBUG_ENABLE) {
Log.e(LOG_TAG, debugInfo
+ " : check bitmap , is null or is recycled.");
}
return false;
}
if (0 == bitmap.getWidth() || 0 == bitmap.getHeight()) {
if (DEBUG_ENABLE) {
Log.e(LOG_TAG, debugInfo
+ " : check bitmap , width is 0 or height is 0.");
}
return false;
}
return true;
}

}