Bitmap to Mat gives wrong colors back

Question

So I make a bitmap from a blob with the next code:

byte[] blob = contact.getMP();
ByteArrayInputStream inputStream = new ByteArrayInputStream(blob);
Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
Bitmap scalen = Bitmap.createScaledBitmap(bitmap, 320, 240, false);

and it gives back the next output, which is good

Then I do the following to make the bitmap into a Mat, but then my colors just change...

//Mat ImageMat = new Mat();
Mat ImageMat = new Mat(320, 240, CvType.CV_32F);
Utils.bitmapToMat(scalen, ImageMat);

I have no idea why, nor another way to make the bitmap into a Mat. What is wrong?

Cleat

Looks like the blue and red channels are swapped. Are you sure your Mat is storing image as RGB (Java's default) instead of BGR (Opencv's default) – Trowel

any updates on this question ? – Cirenaica

Have you found solution for this? i am facing same problem. – Frendel

As a side note: 1) I believe your commented line is better 2) Mat ImageMat = new Mat(320, 240, CvType.CV_32F); will allocate a matrix which is not suitable to receive the output of bitmapToMat, the type should be CV_8UC4. Coincidentally the same number of bytes :) See doc iopencv.com/docs/java/2.4.9/org/opencv/android/Utils.html – Scandal

Answer 1

The format of color channels in Android Bitmap are RGB But in opencv Mat, the channels are BGR by default.

So when you do Utils.bitmapToMat(), [B,G,R] values are stored in [R,G,B] channels. The red and blue channels are interchanged.

One possible solution is to apply cvtcolor on the opencv Mat you got as below: Imgproc.cvtColor(ImageMat, ImageMat, Imgproc.COLOR_BGR2RGB);

It worked for me.

Worldling

Fun fact: Android.camera2 jpegs converted to bitmaps and then converted to mats keep their RGB channels in tact. – Motheaten

Need to be double-confirmed, but it seems Utils.bitmapToMat creates RGB Mat. ( github.com/opencv/opencv/blob/master/modules/java/generator/src/… ) – Darn

Imgproc.cvtColor(ImageMat, ImageMat, Imgproc.COLOR_BGR2RGB); will a eat lot of cpu time – Suburbanite