Getting frames from Video Image in Android

Question

I've implemented a simple application which shows the camera picture on the screen. What I like to do now is grab a single frame and process it as bitmap. From what I could find out to this point it is not an easy thing to do.

I've tried using the onPreviewFrame method with which you get the current frame as a byte array and tried to decode it with the BitmapFactory class but it returns null. The format of the frame is a headerless YUV which could be translated to bitmap but it takes too long on a phone. Also I've read that the onPreviewFrame method has contraints on the runtime, if it takes too long the application could crash.

So what is the right way to do this?

Answer 1

Using ffmpeg we can do it in native code.

Answer 2

Ok what we ended up doing is using the onPreviewFrame method and decoding the data in a seperate Thread using a method which can be found in the android help group.

decodeYUV(argb8888, data, camSize.width, camSize.height);
Bitmap bitmap = Bitmap.createBitmap(argb8888, camSize.width,
                    camSize.height, Config.ARGB_8888);

...

// decode Y, U, and V values on the YUV 420 buffer described as YCbCr_422_SP by Android 
// David Manpearl 081201 
public void decodeYUV(int[] out, byte[] fg, int width, int height)
        throws NullPointerException, IllegalArgumentException {
    int sz = width * height;
    if (out == null)
        throw new NullPointerException("buffer out is null");
    if (out.length < sz)
        throw new IllegalArgumentException("buffer out size " + out.length
                + " < minimum " + sz);
    if (fg == null)
        throw new NullPointerException("buffer 'fg' is null");
    if (fg.length < sz)
        throw new IllegalArgumentException("buffer fg size " + fg.length
                + " < minimum " + sz * 3 / 2);
    int i, j;
    int Y, Cr = 0, Cb = 0;
    for (j = 0; j < height; j++) {
        int pixPtr = j * width;
        final int jDiv2 = j >> 1;
        for (i = 0; i < width; i++) {
            Y = fg[pixPtr];
            if (Y < 0)
                Y += 255;
            if ((i & 0x1) != 1) {
                final int cOff = sz + jDiv2 * width + (i >> 1) * 2;
                Cb = fg[cOff];
                if (Cb < 0)
                    Cb += 127;
                else
                    Cb -= 128;
                Cr = fg[cOff + 1];
                if (Cr < 0)
                    Cr += 127;
                else
                    Cr -= 128;
            }
            int R = Y + Cr + (Cr >> 2) + (Cr >> 3) + (Cr >> 5);
            if (R < 0)
                R = 0;
            else if (R > 255)
                R = 255;
            int G = Y - (Cb >> 2) + (Cb >> 4) + (Cb >> 5) - (Cr >> 1)
                    + (Cr >> 3) + (Cr >> 4) + (Cr >> 5);
            if (G < 0)
                G = 0;
            else if (G > 255)
                G = 255;
            int B = Y + Cb + (Cb >> 1) + (Cb >> 2) + (Cb >> 6);
            if (B < 0)
                B = 0;
            else if (B > 255)
                B = 255;
            out[pixPtr++] = 0xff000000 + (B << 16) + (G << 8) + R;
        }
    }

}

Link: http://groups.google.com/group/android-developers/browse_thread/thread/c85e829ab209ceea/3f180a16a4872b58?lnk=gst&q=onpreviewframe#3f180a16a4872b58

Answer 3

I actually tried the code given the previous answer found that the Colorvalues are not exact. I checked it by taking both the preview and the camera.takePicture which directly returns a JPEG array. And the colors were very different. After a little bit more searching I found another example to convert the PreviewImage from YCrCb to RGB:

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);
        }
    }
}

The color values given by this and the takePicture() exactly match. I thought I should post it here. This is where I got this code from. Hope this helps.