decodeyuv420sp single pixel version

Question

I'm trying to convert several pixels from a YUV (nv21) image format to RGB format(yes, just some pixels, not the whole image because the run time constrains) Currently I'm using the decodeyuv420SP function from internet:

static 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);
        }
    }
    }

Now I want to make some modification s.t it can return the R,G,B values of a specific pixel(X,Y), but I can't understand what exactly it does, looks like a YUV pixel corresponds to more than one RGB pixel. Can someone help me with this issue? Thank you!

Answer 1

This is just quick and dirty, but I think it will work. All I'm doing is removing the loops and replacing them with the given X,Y (represented herein as "row" and "column" because we're using "y" to mean something else).

    int uvp = frameSize + (row >> 1) * width, u = 0, v = 0;
    int yp = row*width + column;
    int y = (0xff & ((int) yuv420sp[yp])) - 16;
    if (y < 0) y = 0;
    if ((column & 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;

    result = 0xff000000 | ((r << 6) & 0xff0000) | ((g >> 2) & 0xff00) | ((b >> 10) & 0xff);

Hope it helps.