Question

i am working on a camera app, y need to extract some data of the preview each frame so y tried to do it using onPreviewFrame, the problem is that with my code onPreviewFrame is only called once and while debugging i don´t get errors so i don´t know what to do

here is my code

public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback {
    private static final String TAG = "CameraPreview";
    private SurfaceHolder mHolder;
    private Camera mCamera;
    public byte[] buffer;//for previewcallback

    public CameraPreview(Context context, Camera camera) {
        super(context);
        Log.d("Function", "CameraPreview constructor iniciado");
        mCamera = camera;

        // Install a SurfaceHolder.Callback so we get notified when the
        // underlying surface is created and destroyed.
        mHolder = getHolder();
        mHolder.addCallback(this);
        // deprecated setting, but required on Android versions prior to 3.0
        mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    }

    public void surfaceCreated(SurfaceHolder holder) {
        // The Surface has been created, now tell the camera where to draw the preview.
        Log.d("Function", "SurfaceCreated iniciado");
        try {
            mCamera.setPreviewDisplay(holder);

            buffer = previewBuffer();
            mCamera.addCallbackBuffer(buffer);
            mCamera.setPreviewCallbackWithBuffer(previewCallback);

            mCamera.startPreview();

        } catch (IOException e) {
            Log.d(TAG, "Error setting camera preview: " + e.getMessage());
        }
    }

    public void surfaceDestroyed(SurfaceHolder holder) {
        // Surface will be destroyed when we return, so stop the preview.
        Log.d("Function", "SurfaceDestroyed iniciado");

    }

    public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
        // If your preview can change or rotate, take care of those events here.
        // Make sure to stop the preview before resizing or reformatting it.
        Log.d("Function", "surfaceChanged iniciado");
        if (mHolder.getSurface() == null) {
            // preview surface does not exist
            return;
        }

        // stop preview before making changes
        try {
            mCamera.stopPreview();
        } catch (Exception e) {
            // ignore: tried to stop a non-existent preview
        }

        // set preview size and make any resize, rotate or
        // reformatting changes here


        // start preview with new settings
        try {
            mCamera.setPreviewDisplay(mHolder);

            buffer = previewBuffer();
            mCamera.addCallbackBuffer(buffer);
            mCamera.setPreviewCallbackWithBuffer(previewCallback);

            mCamera.startPreview();

        } catch (Exception e) {
            Log.d(TAG, "Error starting camera preview: " + e.getMessage());
        }
    }

    //Create the callback to access preview frames
    PreviewCallback previewCallback = new PreviewCallback() {

        public void onPreviewFrame(byte[] data, Camera camera) {
            // TODO Auto-generated method stub
            Log.d("Function", "onPreviewFrame iniciado");
            //Convert to jpg
            Size previewSize = camera.getParameters().getPreviewSize();
            Log.d("Function", "onPreviewFrame: preview size=" + previewSize.height + " " + previewSize.width);
            YuvImage yuvImage = new YuvImage(data, ImageFormat.NV21, previewSize.width, previewSize.height, null);
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            yuvImage.compressToJpeg(new Rect(0, 0, previewSize.width, previewSize.height), 80, baos);
            byte jpgData[] = baos.toByteArray();
        }
    };

    //To create a buffer of the preview bytes size
    private byte[] previewBuffer() {
        Log.d("Function", "previewBuffer iniciado");
        int bufferSize;
        byte buffer[];
        int bitsPerPixel;

        Camera.Parameters mParams = mCamera.getParameters();
        Camera.Size mSize = mParams.getPreviewSize();
        Log.d("Function", "previewBuffer: preview size=" + mSize.height + " " + mSize.width);
        int mImageFormat = mParams.getPreviewFormat();

        if (mImageFormat == ImageFormat.YV12) {
            int yStride = (int) Math.ceil(mSize.width / 16.0) * 16;
            int uvStride = (int) Math.ceil((yStride / 2) / 16.0) * 16;
            int ySize = yStride * mSize.height;
            int uvSize = uvStride * mSize.height / 2;
            bufferSize = ySize + uvSize * 2;
            buffer = new byte[bufferSize];
            Log.d("Function", "previewBuffer: buffer size=" + Integer.toString(bufferSize));
            return buffer;
        }

        bitsPerPixel = ImageFormat.getBitsPerPixel(mImageFormat);
        bufferSize = (int) (mSize.height * mSize.width * ((bitsPerPixel / (float) 8)));
        buffer = new byte[bufferSize];
        Log.d("Function", "previewBuffer: buffer size=" + Integer.toString(bufferSize));
        return buffer;
    }
}

Answer 1

Once you receive a preview buffer you added with addCallbackBuffer, you need to give it back to the camera once you’re done with it (otherwise, the camera might overwrite your data before you’re done using it). So once you’re done using the data byte array in your onPreviewFrame call, give it back to the camera with addCallbackBuffer again.

You may also want to consider adding two or more callback buffers when you set up your camera. The camera will just drop frames if it doesn’t have a free buffer to use, so having a few buffers free can smooth out your frame rate if you have an occasional glitch or other delay in your processing.