NV21 -> ARGB -> NV21 转换的缩小问题

Question

我必须向识别解决方案提供一个 YUV(NV21) 字节数组，我想减少处理时间，缩小预览帧。

根据在 SO 上到处收集的解决方案，我设法以 1:1 比率进行转换，并且获得了认可。但是如果我想缩小中间位图，我就得不到结果。即使我只将它缩小到 95%。

如有任何帮助，我们将不胜感激。

因此，我每隔 400 毫秒就对预览帧进行异步转换。我使用 RenderScript 将其转换为 ARGB，将其缩小然后再转换回来。

// Camera callback 
@Override
public void onPreviewFrame(byte[] frame, Camera camera) {
    if (camera != null) {
        // Debounce
        if ((System.currentTimeMillis() - mStart) > 400) {
            mStart = System.currentTimeMillis();

            Camera.Size size = camera.getParameters().getPreviewSize();
            new FrameScaleAsyncTask(frame, size.width, size.height).execute();
        }
    }

    if (mCamera != null) {
        mCamera.addCallbackBuffer(mBuffer);
    }
}

// In FrameScaleAsyncTask
@Override
protected Void doInBackground(Void... params) {
    // Create YUV type for in-allocation
    Type yuvType = new Type.Builder(mRenderScript, Element.U8(mRenderScript))
            .setX(mFrame.length)
            .create();
    mAllocationIn = Allocation.createTyped(mRenderScript, yuvType, Allocation.USAGE_SCRIPT);

    // Create ARGB-8888 type for out-allocation
    Type rgbType = new Type.Builder(mRenderScript, Element.RGBA_8888(mRenderScript))
            .setX(mWidth)
            .setY(mHeight)
            .create();
    mAllocationOut = Allocation.createTyped(mRenderScript, rgbType, Allocation.USAGE_SCRIPT);

    // Copy frame data into in-allocation
    mAllocationIn.copyFrom(mFrame);

    // Set script input and fire !
    mScript.setInput(mAllocationIn);
    mScript.forEach(mAllocationOut);

    // Create a bitmap of camera preview size (see camera setup) and copy out-allocation to it
    Bitmap bitmap = Bitmap.createBitmap(mWidth, mHeight, Bitmap.Config.ARGB_8888);
    mAllocationOut.copyTo(bitmap);

    // Scale bitmap down
    double scaleRatio = 1;
    Bitmap scaledBitmap = Bitmap.createScaledBitmap(
            bitmap,
            (int) (bitmap.getWidth() * scaleRatio),
            (int) (bitmap.getHeight() * scaleRatio),
            false
    );
    bitmap.recycle();

    int size = scaledBitmap.getRowBytes() * scaledBitmap.getHeight();
    int scaledWidth = scaledBitmap.getWidth();
    int scaledHeight = scaledBitmap.getHeight();
    int[] pixels = new int[scaledWidth * scaledHeight];
    // Put bitmap pixels into an int array
    scaledBitmap.getPixels(pixels, 0, scaledWidth, 0, 0, scaledWidth, scaledHeight);

    mFrame = new byte[pixels.length * 3 / 2];
    ImageHelper.encodeYUV420SPAlt(mFrame, pixels, scaledWidth, scaledHeight);

    return null;
}

RGB转YUV算法（见：this answer）：

public static void encodeYUV420SPAlt(byte[] yuv420sp, int[] argb, int width, int height) {
    final int frameSize = width * height;

    int yIndex = 0;
    int uvIndex = frameSize;

    int a, R, G, B, Y, U, V;
    int index = 0;
    for (int j = 0; j < height; j++) {
        for (int i = 0; i < width; i++) {

            a = (argb[index] & 0xff000000) >> 24; // a is not used obviously
            R = (argb[index] & 0xff0000) >> 16;
            G = (argb[index] & 0xff00) >> 8;
            B = (argb[index] & 0xff) >> 0;

            // well known RGB to YUV algorithm
            Y = ((66 * R + 129 * G + 25 * B + 128) >> 8) + 16;
            U = ((-38 * R - 74 * G + 112 * B + 128) >> 8) + 128;
            V = ((112 * R - 94 * G - 18 * B + 128) >> 8) + 128;

            // NV21 has a plane of Y and interleaved planes of VU each sampled by a factor of 2
            //    meaning for every 4 Y pixels there are 1 V and 1 U.  Note the sampling is every other
            //    pixel AND every other scanline.
            yuv420sp[yIndex++] = (byte) ((Y < 0) ? 0 : ((Y > 255) ? 255 : Y));
            if (j % 2 == 0 && index % 2 == 0) {
                yuv420sp[uvIndex++] = (byte) ((V < 0) ? 0 : ((V > 255) ? 255 : V));
                yuv420sp[uvIndex++] = (byte) ((U < 0) ? 0 : ((U > 255) ? 255 : U));
            }

            index++;
        }
    }
}

Answer 1

我最终直接在 C++ 中调整了图像大小（作为 OpenCV.Mat）。这更容易、更快捷。

Size size(correctedWidth, correctedHeight);
Mat dst;
resize(image, dst, size);

NV21 -> ARGB -> NV21 转换的缩小问题

Down scale issue on NV21 -> ARGB -> NV21 conversion

android

image-processing

yuv

argb