如何保存 YUV_420_888 图像？

Question

我使用 camera2 API 构建了自己的相机应用。我从示例“camera2Raw”开始，添加了 YUV_420_888 支持而不是 JPEG。但是现在我想知道如何将图像保存在 ImageSaver 中！？

这是我的运行方法代码：

@Override
    public void run() {
        boolean success = false;
        int format = mImage.getFormat();
        switch(format) {
            case ImageFormat.RAW_SENSOR:{
                DngCreator dngCreator = new DngCreator(mCharacteristics, mCaptureResult);
                FileOutputStream output = null;
                try {
                    output = new FileOutputStream(mFile);
                    dngCreator.writeImage(output, mImage);
                    success = true;
                } catch (IOException e) {
                    e.printStackTrace();
                } finally {
                    mImage.close();
                    closeOutput(output);
                }
                break;
            }
            case ImageFormat.YUV_420_888:{
                ByteBuffer buffer = mImage.getPlanes()[0].getBuffer();
                byte[] bytes = new byte[buffer.remaining()];
                buffer.get(bytes);
                FileOutputStream output = null;
                try {
                    output = new FileOutputStream(mFile);
                    output.write(bytes);
                    success = true;
                } catch (IOException e) {
                    e.printStackTrace();
                } finally {
                    mImage.close();
                    closeOutput(output);
                }
                break;
            }
            default:
                Log.e(TAG, "Cannot save image, unexpected image format:" + format);

        }
        // Decrement reference count to allow ImageReader to be closed to free up resources.
        mReader.close();

        // If saving the file succeeded, update MediaStore.
        if (success) {
            MediaScannerConnection.scanFile(mContext, new String[] { mFile.getPath()},
            /*mimeTypes*/ null, new MediaScannerConnection.MediaScannerConnectionClient() {
                @Override
                public void onMediaScannerConnected() {
                    // Do nothing
                }

                @Override
                public void onScanCompleted(String path, Uri uri) {
                    Log.i(TAG, "Scanned " + path + ":");
                    Log.i(TAG, "-> uri=" + uri);
                }
            });
        }
    }

我尝试像 JPEG 一样保存 YUV 图像，但这样我只能得到一个平面，并且保存的数据对我没有任何意义......

保存 YUV 图像的正确方法是什么？转换成RGB（那YUV是什么意思？）？还是使用 YuvImage 类？

Answer 1

通常，您不将 YUV 图像保存为文件，因此没有内置函数可以这样做。此外，此类 YUV 数据没有标准的图像格式编码。 YUV 通常是一种中间形式的数据，便于相机流水线和之后转换为其他格式。

如果您真的打算这样做，您可以将三个通道的缓冲区作为未编码的字节数据写入文件，然后在其他地方打开并重建它。不过，请确保您也保存其他重要信息，例如步幅数据。这就是我所做的。以下是我使用的文件格式切换语句中的相关行，以及推理中的 cmets：

File file = new File(SAVE_DIR, mFilename);
FileOutputStream output = null;
ByteBuffer buffer;
byte[] bytes;
boolean success = false;

switch (mImage.getFormat()){

    (... other image data format cases ...)

    // YUV_420_888 images are saved in a format of our own devising. First write out the
    // information necessary to reconstruct the image, all as ints: width, height, U-,V-plane
    // pixel strides, and U-,V-plane row strides. (Y-plane will have pixel-stride 1 always.)
    // Then directly place the three planes of byte data, uncompressed.
    //
    // Note the YUV_420_888 format does not guarantee the last pixel makes it in these planes,
    // so some cases are necessary at the decoding end, based on the number of bytes present.
    // An alternative would be to also encode, prior to each plane of bytes, how many bytes are
    // in the following plane. Perhaps in the future.
    case ImageFormat.YUV_420_888:
        // "prebuffer" simply contains the meta information about the following planes.
        ByteBuffer prebuffer = ByteBuffer.allocate(16);
        prebuffer.putInt(mImage.getWidth())
        .putInt(mImage.getHeight())
        .putInt(mImage.getPlanes()[1].getPixelStride())
        .putInt(mImage.getPlanes()[1].getRowStride());

        try {
            output = new FileOutputStream(file);
            output.write(prebuffer.array()); // write meta information to file
            // Now write the actual planes.
            for (int i = 0; i<3; i++){
                buffer = mImage.getPlanes()[i].getBuffer();
                bytes = new byte[buffer.remaining()]; // makes byte array large enough to hold image
                buffer.get(bytes); // copies image from buffer to byte array
                output.write(bytes);    // write the byte array to file
            }
            success = true;
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            Log.v(appFragment.APP_TAG,"Closing image to free buffer.");
            mImage.close(); // close this to free up buffer for other images
            if (null != output) {
                try {
                    output.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        break;
    }

由于数据的交错方式完全取决于设备，因此稍后从该编码信息中提取 Y、U、V 通道可能具有挑战性。要查看如何读取和提取此类文件的 MATLAB 实现，请参阅here。

Answer 2

如果你有 YuvImage 对象，那么你可以像这样使用 compressToJpeg 函数将其转换为 Jpeg。

ByteArrayOutputStream out = new ByteArrayOutputStream();
YuvImage yuvImage = new YuvImage(data, ImageFormat.NV21, width, height, null);
yuvImage.compressToJpeg(new Rect(0, 0, width, height), 50, out);
byte[] imageBytes = out.toByteArray();
Bitmap image = BitmapFactory.decodeByteArray(imageBytes, 0, imageBytes.length);
iv.setImageBitmap(image);

您必须明确设置图像的宽度和高度。