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Android ImageProxy 转 OpenCV Mat对象

Z同学
2022-08-07 / 0 评论 / 1 点赞 / 146 阅读 / 2,350 字
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1. 介绍

通过CameraXAnalyzer方法得到的图片对象是ImageProxy对象,而OpenCV的处理对象必须是Mat

在处理这几个转换过程中碰见过的问题有:

1. ImageProxy 如何转Mat?
2. 部分手机摄像头数据需要进行90°到270°的纠正,该如何处理?
3. Mat如何转Bitmap 给到View进行显示?

如果你也有以上的疑问,那么本篇内容应该会给你一点参考价值。

2. 转换

前期如何初始化CameraX并获取ImageProxy就不说了。我们从得到ImageProxy对象开始处理转换吧。

2.1 方案一:ImageProxy转Bitmap再转Mat

前期由于项目需求,处理过ImageProxyBitmap。而OpenCV SDK 本身就提供了BitmapMat的函数:Utils.bitmapToMat(Bitmap bmp, Mat mat)

所以开始的时候,没有自己造轮子,直接使用了现有的转换方法。下面贴一下相关代码:

/**
 * 用于转换Bitmap 的工具类
 */
public class BitmapUtils {
    /**
     * Converts a YUV_420_888 image from CameraX API to a bitmap.
     */
    @RequiresApi(VERSION_CODES.KITKAT)
    @Nullable
    @ExperimentalGetImage
    public static Bitmap getBitmap(ImageProxy image) {
        ByteBuffer nv21Buffer =
                yuv420ThreePlanesToNV21(image.getImage().getPlanes(), image.getWidth(), image.getHeight());
        return getBitmap(nv21Buffer, image.getWidth(), image.getHeight(), image.getImageInfo().getRotationDegrees());
    }

    /**
     * Converts NV21 format byte buffer to bitmap.
     */
    @Nullable
    public static Bitmap getBitmap(ByteBuffer data, int width, int height, int rotation) {
        data.rewind();
        byte[] imageInBuffer = new byte[data.limit()];
        data.get(imageInBuffer, 0, imageInBuffer.length);
        try {
            YuvImage image =
                    new YuvImage(
                            imageInBuffer, ImageFormat.NV21, width, height, null);
            ByteArrayOutputStream stream = new ByteArrayOutputStream();
            image.compressToJpeg(new Rect(0, 0, width, height), 80, stream);

            Bitmap bmp = BitmapFactory.decodeByteArray(stream.toByteArray(), 0, stream.size());

            stream.close();
            return rotateBitmap(bmp, rotation, false, false);
        } catch (Exception e) {
            Log.e("VisionProcessorBase", "Error: " + e.getMessage());
        }
        return null;
    }


    /**
     * Rotates a bitmap if it is converted from a bytebuffer.
     */
    private static Bitmap rotateBitmap(
            Bitmap bitmap, int rotationDegrees, boolean flipX, boolean flipY) {
        Matrix matrix = new Matrix();

        // Rotate the image back to straight.
        matrix.postRotate(rotationDegrees);

        // Mirror the image along the X or Y axis.
        matrix.postScale(flipX ? -1.0f : 1.0f, flipY ? -1.0f : 1.0f);
        Bitmap rotatedBitmap =
                Bitmap.createBitmap(bitmap, 0, 0, bitmap.getWidth(), bitmap.getHeight(), matrix, true);

        // Recycle the old bitmap if it has changed.
        if (rotatedBitmap != bitmap) {
            bitmap.recycle();
        }
        return rotatedBitmap;
    }


    /**
     * Converts YUV_420_888 to NV21 bytebuffer.
     *
     * <p>The NV21 format consists of a single byte array containing the Y, U and V values. For an
     * image of size S, the first S positions of the array contain all the Y values. The remaining
     * positions contain interleaved V and U values. U and V are subsampled by a factor of 2 in both
     * dimensions, so there are S/4 U values and S/4 V values. In summary, the NV21 array will contain
     * S Y values followed by S/4 VU values: YYYYYYYYYYYYYY(...)YVUVUVUVU(...)VU
     *
     * <p>YUV_420_888 is a generic format that can describe any YUV image where U and V are subsampled
     * by a factor of 2 in both dimensions. {@link Image#getPlanes} returns an array with the Y, U and
     * V planes. The Y plane is guaranteed not to be interleaved, so we can just copy its values into
     * the first part of the NV21 array. The U and V planes may already have the representation in the
     * NV21 format. This happens if the planes share the same buffer, the V buffer is one position
     * before the U buffer and the planes have a pixelStride of 2. If this is case, we can just copy
     * them to the NV21 array.
     */
    @RequiresApi(VERSION_CODES.KITKAT)
    private static ByteBuffer yuv420ThreePlanesToNV21(
            Plane[] yuv420888planes, int width, int height) {
        int imageSize = width * height;
        byte[] out = new byte[imageSize + 2 * (imageSize / 4)];

        if (areUVPlanesNV21(yuv420888planes, width, height)) {
            // Copy the Y values.
            yuv420888planes[0].getBuffer().get(out, 0, imageSize);

            ByteBuffer uBuffer = yuv420888planes[1].getBuffer();
            ByteBuffer vBuffer = yuv420888planes[2].getBuffer();
            // Get the first V value from the V buffer, since the U buffer does not contain it.
            vBuffer.get(out, imageSize, 1);
            // Copy the first U value and the remaining VU values from the U buffer.
            uBuffer.get(out, imageSize + 1, 2 * imageSize / 4 - 1);
        } else {
            // Fallback to copying the UV values one by one, which is slower but also works.
            // Unpack Y.
            unpackPlane(yuv420888planes[0], width, height, out, 0, 1);
            // Unpack U.
            unpackPlane(yuv420888planes[1], width, height, out, imageSize + 1, 2);
            // Unpack V.
            unpackPlane(yuv420888planes[2], width, height, out, imageSize, 2);
        }

        return ByteBuffer.wrap(out);
    }

    /**
     * Checks if the UV plane buffers of a YUV_420_888 image are in the NV21 format.
     */
    @RequiresApi(VERSION_CODES.KITKAT)
    private static boolean areUVPlanesNV21(Plane[] planes, int width, int height) {
        int imageSize = width * height;

        ByteBuffer uBuffer = planes[1].getBuffer();
        ByteBuffer vBuffer = planes[2].getBuffer();

        // Backup buffer properties.
        int vBufferPosition = vBuffer.position();
        int uBufferLimit = uBuffer.limit();

        // Advance the V buffer by 1 byte, since the U buffer will not contain the first V value.
        vBuffer.position(vBufferPosition + 1);
        // Chop off the last byte of the U buffer, since the V buffer will not contain the last U value.
        uBuffer.limit(uBufferLimit - 1);

        // Check that the buffers are equal and have the expected number of elements.
        boolean areNV21 =
                (vBuffer.remaining() == (2 * imageSize / 4 - 2)) && (vBuffer.compareTo(uBuffer) == 0);

        // Restore buffers to their initial state.
        vBuffer.position(vBufferPosition);
        uBuffer.limit(uBufferLimit);

        return areNV21;
    }

    /**
     * Unpack an image plane into a byte array.
     *
     * <p>The input plane data will be copied in 'out', starting at 'offset' and every pixel will be
     * spaced by 'pixelStride'. Note that there is no row padding on the output.
     */
    @TargetApi(VERSION_CODES.KITKAT)
    private static void unpackPlane(
            Plane plane, int width, int height, byte[] out, int offset, int pixelStride) {
        ByteBuffer buffer = plane.getBuffer();
        buffer.rewind();

        // Compute the size of the current plane.
        // We assume that it has the aspect ratio as the original image.
        int numRow = (buffer.limit() + plane.getRowStride() - 1) / plane.getRowStride();
        if (numRow == 0) {
            return;
        }
        int scaleFactor = height / numRow;
        int numCol = width / scaleFactor;

        // Extract the data in the output buffer.
        int outputPos = offset;
        int rowStart = 0;
        for (int row = 0; row < numRow; row++) {
            int inputPos = rowStart;
            for (int col = 0; col < numCol; col++) {
                out[outputPos] = buffer.get(inputPos);
                outputPos += pixelStride;
                inputPos += plane.getPixelStride();
            }
            rowStart += plane.getRowStride();
        }
    }
}

直接调用 Bitmap.getBitmap()ImageProxy对象传进去即可。转换完毕后的Bitmap是进行过角度校正的。也就是说竖着拍摄时相机旋转90°等问题,进行了matrix.postRotate 矩阵变化,将照片角度进行了纠正。

将得到的Bitmap对象给到OpenCV的Util对象进行转换得到Mat对象即可,实例:

Bitmap bitmap = Bitmap.getBitmap(imageProxy);
Mat mat = new Mat();
Utils.bitmapToMat(bitmap, mat);   // 注意 这个Bitmap对象只能是 ARGB_8888 和RGB_565 类型的。 如果转换失败是会崩溃出现异常的

到这里我们ImageProxy 就可以转为Mat对象了。

如果我们想将Mat在转换为Bitmap,实例:

Bitmap bitmap = Bitmap.createBitmap(mat.width(), mat.height(), Bitmap.Config.ARGB_8888);
Utils.matToBitmap(mat, bitmap);

就可以得到该Bitmap对象了。

但是这种效率明显不太高,因为要先生成Bitmap对象再进行转换。那么有没有直接转换的方法呢?

当然有。

2.2 方案二:ImageProxy 转Mat

我们参考OpenCV SDK中的JavaCamera2View的内部私有类JavaCamera2Frame的实现效果,它就是将Image对象转为了Mat对象。

那么ImageProxy也是有Image对象的:ImageProxy.getImage()

我们直接复制一份改改代码权限得到:

public class ImageUtil {
    public ImageUtil(Image image) {
        super();
        mImage = image;
        mRgba = new Mat();
        mGray = new Mat();
    }

    private Image mImage;
    private Mat mRgba;
    private Mat mGray;

    public Mat gray() {
        Image.Plane[] planes = mImage.getPlanes();
        int w = mImage.getWidth();
        int h = mImage.getHeight();
        assert (planes[0].getPixelStride() == 1);
        ByteBuffer y_plane = planes[0].getBuffer();
        int y_plane_step = planes[0].getRowStride();
        mGray = new Mat(h, w, CvType.CV_8UC1, y_plane, y_plane_step);
        return mGray;
    }

    public Mat rgba() {
        Image.Plane[] planes = mImage.getPlanes();
        int w = mImage.getWidth();
        int h = mImage.getHeight();
        int chromaPixelStride = planes[1].getPixelStride();
        if (chromaPixelStride == 2) { // Chroma channels are interleaved
            assert (planes[0].getPixelStride() == 1);
            assert (planes[2].getPixelStride() == 2);
            ByteBuffer y_plane = planes[0].getBuffer();
            int y_plane_step = planes[0].getRowStride();
            ByteBuffer uv_plane1 = planes[1].getBuffer();
            int uv_plane1_step = planes[1].getRowStride();
            ByteBuffer uv_plane2 = planes[2].getBuffer();
            int uv_plane2_step = planes[2].getRowStride();
            Mat y_mat = new Mat(h, w, CvType.CV_8UC1, y_plane, y_plane_step);
            Mat uv_mat1 = new Mat(h / 2, w / 2, CvType.CV_8UC2, uv_plane1, uv_plane1_step);
            Mat uv_mat2 = new Mat(h / 2, w / 2, CvType.CV_8UC2, uv_plane2, uv_plane2_step);
            long addr_diff = uv_mat2.dataAddr() - uv_mat1.dataAddr();
            if (addr_diff > 0) {
                assert (addr_diff == 1);
                Imgproc.cvtColorTwoPlane(y_mat, uv_mat1, mRgba, Imgproc.COLOR_YUV2RGBA_NV12);
            } else {
                assert (addr_diff == -1);
                Imgproc.cvtColorTwoPlane(y_mat, uv_mat2, mRgba, Imgproc.COLOR_YUV2RGBA_NV21);
            }
            return mRgba;
        } else { // Chroma channels are not interleaved
            byte[] yuv_bytes = new byte[w * (h + h / 2)];
            ByteBuffer y_plane = planes[0].getBuffer();
            ByteBuffer u_plane = planes[1].getBuffer();
            ByteBuffer v_plane = planes[2].getBuffer();

            int yuv_bytes_offset = 0;

            int y_plane_step = planes[0].getRowStride();
            if (y_plane_step == w) {
                y_plane.get(yuv_bytes, 0, w * h);
                yuv_bytes_offset = w * h;
            } else {
                int padding = y_plane_step - w;
                for (int i = 0; i < h; i++) {
                    y_plane.get(yuv_bytes, yuv_bytes_offset, w);
                    yuv_bytes_offset += w;
                    if (i < h - 1) {
                        y_plane.position(y_plane.position() + padding);
                    }
                }
                assert (yuv_bytes_offset == w * h);
            }

            int chromaRowStride = planes[1].getRowStride();
            int chromaRowPadding = chromaRowStride - w / 2;

            if (chromaRowPadding == 0) {
                // When the row stride of the chroma channels equals their width, we can copy
                // the entire channels in one go
                u_plane.get(yuv_bytes, yuv_bytes_offset, w * h / 4);
                yuv_bytes_offset += w * h / 4;
                v_plane.get(yuv_bytes, yuv_bytes_offset, w * h / 4);
            } else {
                // When not equal, we need to copy the channels row by row
                for (int i = 0; i < h / 2; i++) {
                    u_plane.get(yuv_bytes, yuv_bytes_offset, w / 2);
                    yuv_bytes_offset += w / 2;
                    if (i < h / 2 - 1) {
                        u_plane.position(u_plane.position() + chromaRowPadding);
                    }
                }
                for (int i = 0; i < h / 2; i++) {
                    v_plane.get(yuv_bytes, yuv_bytes_offset, w / 2);
                    yuv_bytes_offset += w / 2;
                    if (i < h / 2 - 1) {
                        v_plane.position(v_plane.position() + chromaRowPadding);
                    }
                }
            }

            Mat yuv_mat = new Mat(h + h / 2, w, CvType.CV_8UC1);
            yuv_mat.put(0, 0, yuv_bytes);
            Imgproc.cvtColor(yuv_mat, mRgba, Imgproc.COLOR_YUV2RGBA_I420, 4);
            return mRgba;
        }
    }
    
    public void release() {
        mRgba.release();
        mGray.release();
    }

}

使用实例:

ImageUtil util = new ImageUtil(imageProxy.getImage());
Mat rgb = util.rgba();
//TODO 处理我们要对Mat做的业务计算
util.release();
imageProxy.close();

但是,直接转换后会发现图片的角度不对,需要进行旋转。否则竖着拍摄就是横着的照片了。简单的方法就是直接对Mat进行角度处理。

在ImageUtil中添加:

    public void rotation(int rotation, Mat mat) {
        //  旋转90°
        if (rptation == 90) {
            Core.transpose(mat, mat); // 将图像逆时针旋转90°,然后再关于x轴对称
            Core.flip(mat, mat, 1); // 然后再绕Y轴旋转180° (顺时针)
        } else if (rptation == 180) {
            Core.flip(mat, mat, 0); //将图片绕X轴旋转180°(顺时针)
            Core.flip(mat, mat, 1); //将图片绕Y轴旋转180°(顺时针)
        } else if (rptation == 270) {
            Core.transpose(mat, mat); // 将图像逆时针旋转90°,然后再关于x轴对称
            Core.flip(mat, mat, 0); // //将图片绕X轴旋转180°(顺时针)
        }
    }

而rotation的角度值,在ImageProxy中有。获取方法为:int rotation = imageProxy.getImageInfo().getRotationDegrees();

所以完整实例为:

 int rotation = imageProxy.getImageInfo().getRotationDegrees();
ImageUtil util = new ImageUtil(imageProxy.getImage());
Mat rgb = util.rgba();
util.rotation(rotation, rgb);
//TODO 处理我们要对Mat做的业务计算
util.release(); //要释放资源哦
imageProxy.close();

到这里,转换的就结束了。

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