在android中解码流时保留图像质量

我在SD卡上有一个图像,需要在图像视图上显示

问题是,在解码之后,质量似乎恶化了.有没有什么方法可以保持质量,同时保留记忆？

或者,如果我使用更大的图像,是否有任何方法可以保留内存(避免加载太大的位图)和缩放？ (我需要保持原始图像的大小)

谢谢你的帮助.

public Bitmap decodeFile(String pubKey, int bookPageID, int type)
        throws IOException {
    Bitmap b = null;
    File f = null;
    String uri = null;
    FileInputStream fis = null;

    Log.d(TAG,"pageID to read: " + bookPageID);

    IRIssue issue = Broker.model.issueDataStore.getIRIssue(pubKey);

    String imageFolder = IRConstant.issueFolder(issue.year, issue.month, issue.day, issue.pubKey);

    // pageID - 1 since the page is an array (start at 0) , but page ID start at 1
    if (type == 2){
        uri = imageFolder + issue.vol[0].pages[bookPageID - 1].graphicUri;
    }else {
        uri = imageFolder + issue.vol[0].pages[bookPageID - 1].textUri;
    }

    f = new File(uri);

    Log.d(TAG,"is file: " + uri + " exist?" + f.exists());

    BitmapFactory.Options options = new BitmapFactory.Options();
    options.inPurgeable = true;
    options.inInputShareable = true;
    options.inJustDecodeBounds = false;
    options.inPreferredConfig = Bitmap.Config.ARGB_8888;
    fis = new FileInputStream(f);
    b = BitmapFactory.decodeStream(fis, null, options);
    fis.close();

    return b;
}

以下代码使用 Displaying Bitmaps Efficiently中的几个概念
首先关闭位图读取是在后台线程中完成的,我在inputStream上使用mark / reset(用BufferedInputstream包装),当我们试图找出计算尺度时要使用的图像大小时,不要从流中读取更多内容因子.下面的示例代码对图像进行子采样,以匹配320×240像素的大小.在非示例代码中,可以使用简单的回调接口将位图从onPostExecute发送到实现类(回调接口实现器).或者直接将视图作为AsyncTask的成员提供,并在onPostExecute中设置位图.

使用(我的设备上的示例下载图像)调用代码：

BitmapTask task = new BitmapTask(getContentResolver());
task.execute(Uri.parse("file:///storage/emulated/0/Download/download.jpg"));

有问题的课程

private static class BitmapTask extends AsyncTask<Uri, Void, Bitmap> {

    // prevent mem leaks
    private WeakReference<ContentResolver> mWeakContentResolver;

    public BitmapTask(ContentResolver resolver) {
        mWeakContentResolver = new WeakReference<ContentResolver>(resolver);
    }

    @Override
    protected Bitmap doInBackground(Uri... params) {
        Bitmap bitmap = null;
        ContentResolver resolver = mWeakContentResolver.get();
        if (resolver != null) {
            BufferedInputStream stream = null;
            try {
                stream = new BufferedInputStream(
                        resolver.openInputStream(params[0]));
                stream.mark(1 * 1024);
                BitmapFactory.Options options = new BitmapFactory.Options();
                options.inJustDecodeBounds = true;
                // Find out size of image
                BitmapFactory.decodeStream(stream, null, options);
                try {
                    stream.reset();
                } catch (IOException e) {
                    Log.d(TAG, "reset failed");
                }
                int imageHeight = options.outHeight;
                int imageWidth = options.outWidth;
                String imageType = options.outMimeType;
                Log.d(TAG, "w, h, mime " + imageWidth + " , " + imageHeight
                        + " , " + imageType);
                options.inJustDecodeBounds = false;
                // Calculate down scale factor
                options.inSampleSize = calculateInSampleSize(options, 320,
                        240);
                return BitmapFactory.decodeStream(stream, null, options);
            } catch (FileNotFoundException e) {
                bitmap = null;
            } finally {
                IOUtils.closeStreamSilently(stream);
            }
        }
        return bitmap;
    }

    @Override
    protected void onPostExecute(Bitmap result) {
        Log.d(TAG,
                "bitmap result: "
                        + ((result != null) ? "" + result.getByteCount()
                                : "0"));
        result.recycle();
    }
}

public static int calculateInSampleSize(BitmapFactory.Options options,
        int reqWidth, int reqHeight) {
    // Raw height and width of image
    final int height = options.outHeight;
    final int width = options.outWidth;
    int inSampleSize = 1;

    if (height > reqHeight || width > reqWidth) {

        final int halfHeight = height / 2;
        final int halfWidth = width / 2;

        // Calculate the largest inSampleSize value that is a power of 2 and
        // keeps both
        // height and width larger than the requested height and width.
        while ((halfHeight / inSampleSize) > reqHeight
                && (halfWidth / inSampleSize) > reqWidth) {
            inSampleSize *= 2;
        }
    }

    return inSampleSize;
}

编辑：对于大型输入流可能存在标记/重置技术的问题,SkImageDecoder :: Factory返回null有时可以在日志中看到,导致空位图,关于此问题的其他SO问题：SkImageDecoder::Factory returned null.它可以通过以下方式修复：再次重新启动流变量stream = new resolver.openInputStream(params [0]));在doInBackground中返回之前

编辑2：如果你必须保留图像大小但不想限制内存使用,你可以使用options.inPreferredConfig = Bitmap.Config.RGB_565;将每个像素的内存减半,但要记住图像可能不再具有高质量(实验！).