onpreviewcallback byte array to mat opencv

Question

Hi I'm trying to use Android's PreviewCallback to perform some processing with opencv on the frames captured OnPreviewFrame. The problem I'm having is while testing that everything works before the processing, when setting the byte[] to a Mat and then the Mat to a bitmap, the bitmap looks like color noise.

mCamera.setPreviewCallback(new Camera.PreviewCallback() {
    @Override
    public void onPreviewFrame(byte[] data, Camera camera) {
    Mat color = new Mat(480,640, CvType.CV_8UC1);
    color.put(0, 0, data);
    Mat imageMat = new Mat(480,640,CvType.CV_8UC1);
    Bitmap bmpOut = Bitmap.createBitmap(color.width(),color.height(),
                                        Bitmap.Config.ARGB_8888);

    Imgproc.cvtColor(color,imageMat, Imgproc.COLOR_YUV420sp2RGBA);

    Utils.matToBitmap(imageMat,bmpOut,true);
    image.setImageBitmap(bmpOut);
}

Any help would be greatly appreciated!

Edson Menegatti · Accepted Answer · 2015-08-14 19:57:22Z

3

Actually, the size of the Mat is not supposed to be the size of the selected preview.

The width of your matrix needs to be 1.5 times bigger than the preview size as follows:

// Copy preview data to a new Mat element 
Mat mYuv = new Mat(frameHeight + frameHeight / 2, frameWidth, CvType.CV_8UC1);
mYuv.put(0, 0, data);

Afterwards, to create a Bitmap I usually do:

// Convert preview frame to rgba color space
final Mat mRgba = new Mat();
Imgproc.cvtColor(mYuv, mRgba, Imgproc.COLOR_YUV2BGR_NV12, 4);

// Converts the Mat to a bitmap.
Bitmap bitmap = Bitmap.createBitmap(mRgba.cols(), mRgba.rows(), Bitmap.Config.ARGB_8888);
Utils.matToBitmap(img, bitmap);

The matToBitmap method is a utility method that saves up a lot of effort in the conversion to Bitmap, so I strongly advise to use it as oppose to creating by yourself.

answered Aug 14, 2015 at 19:57

Edson Menegatti

4,0362 gold badges27 silver badges40 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Christina Tsangouri Over a year ago

I still seem to be getting color noise when displaying the bitmap in an ImageView.

Edson Menegatti Over a year ago

Did you copy the whole code or did you updated only the matrix dimensions?

Edson Menegatti Over a year ago

Can you paste the code that selects the preview size?

Christina Tsangouri Over a year ago

Do you mean this? Camera.Size size = params.getPictureSize(); int frameHeight = size.height; int frameWidth = size.width;

Edson Menegatti Over a year ago

Long shot, but can you check if getPictureSize() return is different from getPreviewSize()? I use the latter

Collectives™ on Stack Overflow

onpreviewcallback byte array to mat opencv

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related