如何将YUV高效转换为RGB

Question

``I want to use RGB in the camera's preview.I used JNI to do the YUV to RGB conversion.I changed the data in RGB,then I show RGB on preview by using drawBitmap.But it shows the very slow,how could I improve it

    public void onPreviewFrame(final byte[] data, Camera camera) {

    Thread showPic = new Thread(new Runnable() {

        @Override
        public void run() {
            // TODO Auto-generated method stub
            Canvas c = mHolder.lockCanvas(null);
            try {

                synchronized (mHolder) {

                    // TODO Auto-generated method stub
                    int imageWidth = mCamera.getParameters()
                            .getPreviewSize().width;
                    int imageHeight = mCamera.getParameters()
                            .getPreviewSize().height;
                    int RGBData[] = new int[imageWidth * imageHeight];
                    int RGBDataa[] = new int[imageWidth * imageHeight];
                    int RGBDatab[] = new int[imageWidth * imageHeight];
                    int center = imageWidth * imageHeight / 2;

                    Jni.decodeYUV420SP(RGBData, data, imageWidth,
                            imageHeight); // decode
                    for (int i = 0; i < center; i++)
                        RGBDataa[i] = RGBData[i];
                    for (int i = center; i < imageWidth * imageHeight; i++)
                        RGBDatab[i - center] = RGBData[i];
                    for (int i = 0; i < center; i++)
                        RGBData[i] = RGBDatab[i];
                    for (int i = center; i < imageWidth * imageHeight; i++)
                        RGBData[i] = RGBDataa[i - center];

                    c.drawBitmap(RGBData, 0, imageWidth, 0, 0, imageWidth,
                            imageHeight, false, new Paint());

                    // Bitmap bm = Bitmap.createBitmap(RGBData, imageWidth,
                    // imageHeight, Config.ARGB_8888);

                }

            } finally {
                if (data != null)
                    mHolder.unlockCanvasAndPost(c);
            }
        }
    });
    showPic.run();

}

The following code is Jni

    public class Jni {
public native static void decodeYUV420SP(int[] rgb, byte[] yuv420sp, int width,
        int height);

} The method decodeYUV420SP is completed by C.

Answer 1

I came across this thread searching for YUV420sp to RGB565 conversion. Using decodeYUV420SP as suggested above works fine, but I want to add some runtime considerations.

My first test using decodeYUV420SP in Java took 74 seconds decoding and converting a 10 second .mp4 FullHD video (using an Nexus 6 Android device). The profiler reported 96% of the overall process has been spent in decodeYUV420SP.

Moving decodeYUV420SP to JNI / C made made overall time spent go down to 32 seconds. Turning on GCC code optimization, it fell to 12 seconds.

Although the OP suggested this already, I wanted to emphasizes it will not be an improvement to move the routine to Java. While it is correct a context change between Java and C comes at a cost, this is a good example an expensive operation performed in the native function will easily compensate this overhead. The statement on overhead costs is valid for very simple JNI functions and in case method and field lookups back and forth are required. Nothing of this applies here.

To improve the performance of the OP's code: get rid of the memory allocation done for each frame and store it globally, get rid of the arithmetic on RGBData* and move that stuff to JNI too. Arrays can be accessed on JNI level at no cost using the Critical functions.

Answer 2

It's not recommended to use jni because of slow speed for compiler to change java code to C++. I think you should use direct method like this

public void decodeYUV420SP(int[] rgb, byte[] yuv420sp, int width, int height) {
    final int frameSize = width * height;

    for (int j = 0, yp = 0; j < height; j++) {
        int uvp = frameSize + (j >> 1) * width, u = 0, v = 0;
        for (int i = 0; i < width; i++, yp++) {
            int y = (0xff & ((int) yuv420sp[yp])) - 16;
            if (y < 0) y = 0;
            if ((i & 1) == 0) {
                v = (0xff & yuv420sp[uvp++]) - 128;
                u = (0xff & yuv420sp[uvp++]) - 128;
            }

            int y1192 = 1192 * y;
            int r = (y1192 + 1634 * v);
            int g = (y1192 - 833 * v - 400 * u);
            int b = (y1192 + 2066 * u);

            if (r < 0) r = 0; else if (r > 262143) r = 262143;
            if (g < 0) g = 0; else if (g > 262143) g = 262143;
            if (b < 0) b = 0; else if (b > 262143) b = 262143;

            rgb[yp] = 0xff000000 | ((r << 6) & 0xff0000) | ((g >> 2) & 0xff00) | ((b >> 10) & 0xff);
        }
    }
}

OR simply use this

Camera.Parameters.setPreviewFormat(ImageFormat.RGB_565);

to change the format output to rgb directly