Extract black and white image from android camera's NV21 format

Question

I have done some google-ing around and couldn\'t find enough information about this format. It is the default format for camera preview. Can anyone suggest good sources of i

Answer 1

NV21 is basically YUV420 but instead of planar format where Y, U and V have independent planes, NV21 has 1 plane for Luma and 2nd plane for Chroma. The format looks like

YYYYYYYYYYYYYYYYYYYYYYYYYYYYY YYYYYYYYYYYYYYYYYYYYYYYYYYYYY . . . .

VUVUVUVUVUVUVUVUVUVUVUVUVUVUVU VUVUVUVUVUVUVUVUVUVUVUVUVUVUVU . . . . .

Answer 2

When you only need a grayscale camera preview, you could use a very simple renderscript:

# pragma version(1)
# pragma rs java_package_name(com.example.name)
# pragma rs_fp_relaxed

rs_allocation gIn;   // Allocation filled with camera preview data (byte[])
int previewwidth;    // camera preview width (int)

// the parallel executed kernel
void root(uchar4 *v_out, uint32_t x,uint32_t y){
   uchar c = rsGetElementAt_uchar(gIn,x+y*previewwidth); 
   *v_out = (uchar4){c,c,c,255};
}

Note : This is not faster than ScriptIntrinsicYuvToRGB (and a following ScriptIntrinsicColorMatrix to do the RGBA-> gray), but it runs with API 11+ (where the Intrinsics need Api 17+).

Answer 3

I also had lots of headache because of this preview format.

The best I could find are these:

http://www.fourcc.org/yuv.php#NV21

http://v4l2spec.bytesex.org/spec/r5470.htm

It seems that the Y component is the first width*height bytes int the array you get.

Some more informational links:

http://msdn.microsoft.com/en-us/library/ms867704.aspx#yuvformats_yuvsampling

http://msdn.microsoft.com/en-us/library/ff538197(v=vs.85).aspx

Hope this helps.

Answer 4

The data is in YUV420 format.
If you are only interested in the monochrome channel, i.e. "black and white", then this the first width x height bytes of the data buffer you already have.
The Y channel is the first image plane. It is exactly the grey/intensity/luminosity etc. channel.

Answer 5

Here's code to just extract the greyscale image data:

private int[] decodeGreyscale(byte[] nv21, int width, int height) {
    int pixelCount = width * height;
    int[] out = new int[pixelCount];
    for (int i = 0; i < pixelCount; ++i) {
        int luminance = nv21[i] & 0xFF;
        out[i] = Color.argb(0xFF, luminance, luminance, luminance);
    }
    return out;
}

Answer 6

I developed the following code to convert the NV21 to RGB, and it is working.

/**
 * Converts YUV420 NV21 to RGB8888
 * 
 * @param data byte array on YUV420 NV21 format.
 * @param width pixels width
 * @param height pixels height
 * @return a RGB8888 pixels int array. Where each int is a pixels ARGB. 
 */
public static int[] convertYUV420_NV21toRGB8888(byte [] data, int width, int height) {
    int size = width*height;
    int offset = size;
    int[] pixels = new int[size];
    int u, v, y1, y2, y3, y4;

    // i percorre os Y and the final pixels
    // k percorre os pixles U e V
    for(int i=0, k=0; i < size; i+=2, k+=2) {
        y1 = data[i  ]&0xff;
        y2 = data[i+1]&0xff;
        y3 = data[width+i  ]&0xff;
        y4 = data[width+i+1]&0xff;

        u = data[offset+k  ]&0xff;
        v = data[offset+k+1]&0xff;
        u = u-128;
        v = v-128;

        pixels[i  ] = convertYUVtoRGB(y1, u, v);
        pixels[i+1] = convertYUVtoRGB(y2, u, v);
        pixels[width+i  ] = convertYUVtoRGB(y3, u, v);
        pixels[width+i+1] = convertYUVtoRGB(y4, u, v);

        if (i!=0 && (i+2)%width==0)
            i+=width;
    }

    return pixels;
}

private static int convertYUVtoRGB(int y, int u, int v) {
    int r,g,b;

    r = y + (int)(1.402f*v);
    g = y - (int)(0.344f*u +0.714f*v);
    b = y + (int)(1.772f*u);
    r = r>255? 255 : r<0 ? 0 : r;
    g = g>255? 255 : g<0 ? 0 : g;
    b = b>255? 255 : b<0 ? 0 : b;
    return 0xff000000 | (b<<16) | (g<<8) | r;
}

This image helps to understand.

If you wanna just grayscale image is easer. You can discard all the U and V info, and take just the Y info. The code would can be like this:

/**
 * Converts YUV420 NV21 to Y888 (RGB8888). The grayscale image still holds 3 bytes on the pixel.
 * 
 * @param pixels output array with the converted array o grayscale pixels
 * @param data byte array on YUV420 NV21 format.
 * @param width pixels width
 * @param height pixels height
 */
public static void applyGrayScale(int [] pixels, byte [] data, int width, int height) {
    int p;
    int size = width*height;
    for(int i = 0; i < size; i++) {
        p = data[i] & 0xFF;
        pixels[i] = 0xff000000 | p<<16 | p<<8 | p;
    }
}

To create your Bitmap just:

Bitmap bm = Bitmap.createBitmap(pixels, width, height, Bitmap.Config.ARGB_8888);

Where pixels is your int [] array.