Convert bitmap array to YUV (YCbCr NV21)

Question

How to convert Bitmap returned by BitmapFactory.decodeFile() to YUV format (simillar to what camera's onPreviewFrame() returns in byte array)?

Answer 1

Here is some code that actually works:

    // untested function
    byte [] getNV21(int inputWidth, int inputHeight, Bitmap scaled) {

        int [] argb = new int[inputWidth * inputHeight];

        scaled.getPixels(argb, 0, inputWidth, 0, 0, inputWidth, inputHeight);

        byte [] yuv = new byte[inputWidth*inputHeight*3/2];
        encodeYUV420SP(yuv, argb, inputWidth, inputHeight);

        scaled.recycle();

        return yuv;
    }

    void encodeYUV420SP(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 2

Following is the code for converting Bitmap to Yuv(NV21) Format.

void yourFunction(){

    // mBitmap is your bitmap

    int mWidth = mBitmap.getWidth();
    int mHeight = mBitmap.getHeight();

    int[] mIntArray = new int[mWidth * mHeight];

    // Copy pixel data from the Bitmap into the 'intArray' array
    mBitmap.getPixels(mIntArray, 0, mWidth, 0, 0, mWidth, mHeight);

    // Call to encoding function : convert intArray to Yuv Binary data
    encodeYUV420SP(data, intArray, mWidth, mHeight);

}

static public void encodeYUV420SP(byte[] yuv420sp, int[] rgba,
        int width, int height) {
    final int frameSize = width * height;

    int[] U, V;
    U = new int[frameSize];
    V = new int[frameSize];

    final int uvwidth = width / 2;

    int r, g, b, y, u, v;
    for (int j = 0; j < height; j++) {
        int index = width * j;
        for (int i = 0; i < width; i++) {

            r = Color.red(rgba[index]);
            g = Color.green(rgba[index]);
            b = Color.blue(rgba[index]);

            // rgb to yuv
            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;

            // clip y
            yuv420sp[index] = (byte) ((y < 0) ? 0 : ((y > 255) ? 255 : y));
            U[index] = u;
            V[index++] = v;
        }
    }

Answer 3

If using java to convert Bitmap to YUV byte[] is too slow for you, you can try libyuv by Google

Answer 4

Via OpenCV library you can replace encodeYUV420SP java function with one native OpenCV line and it is ~4x more fastest:

Mat mFrame = Mat(height,width,CV_8UC4,pFrameData).clone();

Complete example:

Java side:

    Bitmap bitmap = mTextureView.getBitmap(mWidth, mHeight);
    int[] argb = new int[mWidth * mHeight];
    // get ARGB pixels and then proccess it with 8UC4 opencv convertion
    bitmap.getPixels(argb, 0, mWidth, 0, 0, mWidth, mHeight);
    // native method (NDK or CMake)
    processFrame8UC4(argb, mWidth, mHeight);

Native side (NDK):

JNIEXPORT jint JNICALL com_native_detector_Utils_processFrame8UC4
    (JNIEnv *env, jobject object, jint width, jint height, jintArray frame) {

    jint *pFrameData = env->GetIntArrayElements(frame, 0);
    // it is the line:
    Mat mFrame = Mat(height,width,CV_8UC4,pFrameData).clone();
    // the next only is a extra example to gray convertion:
    Mat mout;
    cvtColor(mFrame, mout,CV_RGB2GRAY);
    int objects = face_detection(env, mout);
    env->ReleaseIntArrayElements(frame, pFrameData, 0);
    return objects;
}

Answer 5

first you calculate the rgb data:

r=(p>>16) & 0xff;
g=(p>>8) & 0xff;
b= p & 0xff;
y=0.2f*r+0.7*g+0.07*b;
u=-0.09991*r-0.33609*g+0.436*b;
v=0.615*r-0.55861*g-0.05639*b;

y, u and v are the composants of the yuv matrix.

Answer 6

The bmp file will be in RGB888 format, so you will need to convert it to YUV. I have not come across any api in Android that will do this for you.

But you can do this yourself, see this link on how to..