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// 通过BitmapFactory.Options的这几个参数可以调整缩放系数
public class BitmapFactory {    public static class Options {        public boolean inScaled;     // 默认true
        public int inDensity;        // 无dpi的文件夹下默认160
        public int inTargetDensity;  // 取决具体屏幕
    }
}

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BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = false;
options.inSampleSize = 1;
options.inDensity = 160;
options.inTargetDensity = 160;
bitmap = BitmapFactory.decodeResource(getResources(),
        R.drawable.origin, options);
// MX4上，虽然density = 3
// 但是通过设置inTargetDensity / inDensity = 160 / 160 = 1
// 解码后图片大小为720x720
System.out.println("w:" + bitmap.getWidth()
        + ", h:" + bitmap.getHeight());

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/**
 * ... This is an advanced call, and normally need not be called,
 * since the normal GC process will free up this memory when
 * there are no more references to this bitmap.
 */
public void recycle() {}

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private static void addInBitmapOptions(BitmapFactory.Options options,
        ImageCache cache) {    // inBitmap only works with mutable bitmaps, so force the decoder to
    // return mutable bitmaps.
    options.inMutable = true;

    if (cache != null) {        // Try to find a bitmap to use for inBitmap.
        Bitmap inBitmap = cache.getBitmapFromReusableSet(options);

        if (inBitmap != null) {            // If a suitable bitmap has been found,
            // set it as the value of inBitmap.
            options.inBitmap = inBitmap;
        }
    }
}

static boolean canUseForInBitmap(
        Bitmap candidate, BitmapFactory.Options targetOptions) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {        // From Android 4.4 (KitKat) onward we can re-use
        // if the byte size of the new bitmap is smaller than
        // the reusable bitmap candidate
        // allocation byte count.
        int width = targetOptions.outWidth / targetOptions.inSampleSize;
        int height =
            targetOptions.outHeight / targetOptions.inSampleSize;
        int byteCount = width * height
            * getBytesPerPixel(candidate.getConfig());
        return byteCount <= candidate.getAllocationByteCount();
    }

    // On earlier versions,
    // the dimensions must match exactly and the inSampleSize must be 1
    return candidate.getWidth() == targetOptions.outWidth
        && candidate.getHeight() == targetOptions.outHeight
        && targetOptions.inSampleSize == 1;
}

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// 根据maxWidth, maxHeight计算最合适的inSampleSize
public static int $sampleSize(BitmapFactory.Options options,
        int maxWidth, int maxHeight) {    // raw height and width of image
    int rawWidth = options.outWidth;
    int rawHeight = options.outHeight;

    // calculate best sample size
    int inSampleSize = 0;
    if (rawHeight > maxHeight || rawWidth > maxWidth) {        float ratioWidth = (float) rawWidth / maxWidth;
        float ratioHeight = (float) rawHeight / maxHeight;
        inSampleSize = (int) Math.min(ratioHeight, ratioWidth);
    }
    inSampleSize = Math.max(1, inSampleSize);

    return inSampleSize;
}

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/**
 * 获取缩略图
 * 支持自动旋转
 * 某些型号的手机相机图片是反的，可以根据exif信息实现自动纠正
 * @return
 */
public static Bitmap $thumbnail(String path,
        int maxWidth, int maxHeight, boolean autoRotate) {
    int angle = 0;
    if (autoRotate) {        angle = ImageLess.$exifRotateAngle(path);
    }

    BitmapFactory.Options options = new BitmapFactory.Options();
    options.inJustDecodeBounds = true;
    // 获取这个图片的宽和高信息到options中, 此时返回bm为空
    Bitmap bitmap = BitmapFactory.decodeFile(path, options);
    options.inJustDecodeBounds = false;
    // 计算缩放比
    int sampleSize = $sampleSize(options, maxWidth, maxHeight);
    options.inSampleSize = sampleSize;
    options.inPreferredConfig = Bitmap.Config.RGB_565;
    options.inPurgeable = true;
    options.inInputShareable = true;

    if (bitmap != null && !bitmap.isRecycled()) {        bitmap.recycle();
    }
    bitmap = BitmapFactory.decodeFile(path, options);

    if (autoRotate && angle != 0) {        bitmap = $rotate(bitmap, angle);
    }

    return bitmap;
}

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Matrix matrix = new Matrix();
// angle为旋转的角度
matrix.postRotate(angle);
Bitmap rotatedBitmap = Bitmap.createBitmap(originBitmap,
        0,
        0,
        originBitmap.getWidth(),
        originBitmap.getHeight(),
        matrix,
        true);

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Matrix matrix = new Matrix();
// scaleX，scaleY分别为为水平和垂直方向上缩放的比例
matrix.postScale(scaleX, scaleY);
Bitmap scaledBitmap = Bitmap.createBitmap(originBitmap,
        0,
        0,
        originBitmap.getWidth(),
        originBitmap.getHeight(),
        matrix,
        true);

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// 水平和宽度缩放到指定大小，注意，这种情况下图片很容易变形
Bitmap scaledBitmap = Bitmap.createScaledBitmap(originBitmap,
        dstWidth,
        dstHeight,
        true);

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Canvas canvas = new Canvas(originBitmap);
draw(canvas);
// 确定裁剪的位置和裁剪的大小
Bitmap clipBitmap = Bitmap.createBitmap(originBitmap,
        left, top,
        clipWidth, clipHeight);

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// 准备画笔
Paint paint = new Paint();
paint.setAntiAlias(true);

// 准备裁剪的矩阵
Rect rect = new Rect(0, 0,
        originBitmap.getWidth(), originBitmap.getHeight());
RectF rectF = new RectF(new Rect(0, 0,
        originBitmap.getWidth(), originBitmap.getHeight()));

Bitmap roundBitmap = Bitmap.createBitmap(originBitmap.getWidth(),
        originBitmap.getHeight(), Bitmap.Config.ARGB_8888);
Canvas canvas = new Canvas(roundBitmap);
// 圆角矩阵，radius为圆角大小
canvas.drawRoundRect(rectF, radius, radius, paint);

// 关键代码，关于Xfermode和SRC_IN请自行查阅
paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_IN));
canvas.drawBitmap(originBitmap, rect, rect, paint);

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int min = originBitmap.getWidth() > originBitmap.getHeight() ?
originBitmap.getHeight() : originBitmap.getWidth();
Paint paint = new Paint();
paint.setAntiAlias(true);
Bitmap circleBitmap = Bitmap.createBitmap(min, min,
    Bitmap.Config.ARGB_8888);
Canvas canvas = new Canvas(circleBitmap);
// 圆形
canvas.drawCircle(min / 2, min / 2, min / 2, paint);
paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_IN));

// 居中显示
int left = - (originBitmap.getWidth() - min) / 2;
int top = - (originBitmap.getHeight() - min) / 2;
canvas.drawBitmap(originBitmap, left, top, paint);

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public static String $save(Bitmap bitmap,
        Bitmap.CompressFormat format, int quality, File destFile) {    try {        FileOutputStream out = new FileOutputStream(destFile);
        if (bitmap.compress(format, quality, out)) {            out.flush();
            out.close();
        }

        if (bitmap != null && !bitmap.isRecycled()) {            bitmap.recycle();
        }

        return destFile.getAbsolutePath();
    } catch (FileNotFoundException e) {        e.printStackTrace();
    } catch (IOException e) {        e.printStackTrace();
    }
    return null;
}

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// 保存到本地，默认路径/mnt/sdcard/[package]/save/，用随机UUID命名文件
public static String $save(Bitmap bitmap,
        Bitmap.CompressFormat format, int quality, Context context) {    if (!Environment.getExternalStorageState()
            .equals(Environment.MEDIA_MOUNTED)) {        return null;
    }

    File dir = new File(Environment.getExternalStorageDirectory()
            + "/" + context.getPackageName() + "/save/");
    if (!dir.exists()) {        dir.mkdirs();
    }
    File destFile = new File(dir, UUID.randomUUID().toString());
    return $save(bitmap, format, quality, destFile);
}

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public static BitmapRegionDecoder newInstance(byte[] data, int offset,
        int length, boolean isShareable) throws IOException {}
public static BitmapRegionDecoder newInstance(
        FileDescriptor fd, boolean isShareable) throws IOException {}
public static BitmapRegionDecoder newInstance(InputStream is,
        boolean isShareable) throws IOException {}
public static BitmapRegionDecoder newInstance(String pathName,
        boolean isShareable) throws IOException {}

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public Bitmap decodeRegion(Rect rect, BitmapFactory.Options options) {}

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Bitmap grayBitmap = Bitmap.createBitmap(originBitmap.getWidth(),
        originBitmap.getHeight(), Bitmap.Config.RGB_565);
Canvas canvas = new Canvas(grayBitmap);
Paint paint = new Paint();
ColorMatrix colorMatrix = new ColorMatrix();
// 设置饱和度为0，实现了灰阶效果
colorMatrix.setSaturation(0);
ColorMatrixColorFilter colorMatrixColorFilter =
        new ColorMatrixColorFilter(colorMatrix);
paint.setColorFilter(colorMatrixColorFilter);
canvas.drawBitmap(originBitmap, 0, 0, paint);