Android Yuv420sp to ARGB in OpenCV

Question

Hi, I am trying to send a raw image from the previewcallback from the phone to PC. The PC will then process the image. I am use OpenCV library to do the image processing. At the moment i just write a function in previewcallback to save teh byte array to a file. and copy the file to pc and i worte a simple program to first read in the file and save it into memeory and then directly copy the data to the IplImage.imageData. But i always get a garbage image.

I use this function i found on net to do the converstion between YUV420SP to ARGB. Thanks for your help.

void decodeYUV420SP(int* rgb, char* yuv420sp, int width, int height) { 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);
        }
    }
}

Answer 1

YUV420 images are an odd shape, the Y (luminescence) part is half the width and heigth of the image but this is followed by the Cr and Cb parts at half width but quarter height.
Normally they aren't displayed directly so nobody cares about the odd packing.

The result is a color image with only 1.5bytes/pixel instead of for rgb

Answer 2

Thanks for your reply Martin. I just want to make sure a get the structure right. So lets say I got an image that is 640*480. In the YUV420 byte buffer, the first 320*240 bytes will be correspond to the Y part. And followed by 320*120 bytes of Cr and Cb, so the data will look something like, YYYYYYYYYYY.......CrCbCrCb...? Also can you point me out any good sources of how am I going to reconstruct this back to the original size of RGB image, since we lost so much info in YUV420 format?