Kotlin: error on converting mat to bitmap

Android/Java
1

Hey! I’ve been trying to convert Mat to bitmap but it’s not working.

I’m using the OpenCV’s Utils class to convert it to mat.


 fun detectPerson(img: Bitmap, folderName: String) {
        
        val frame = Mat()
        val originalMat = Mat()

        Utils.bitmapToMat(img, originalMat)
        Utils.bitmapToMat(img, frame)

        Imgproc.cvtColor(frame, frame, Imgproc.COLOR_RGBA2RGB)

        val blob = Dnn.blobFromImage(
            frame,
            0.00392,
            Size(416.0, 416.0),
            Scalar(0.0, 0.0, 0.0),/*swapRB*/
            true, /*crop*/
            false
        )



      writeToFileWithName("blob-file",convertMatToBitMap(blob)!!)

}


 private fun convertMatToBitMap(input: Mat): Bitmap? {
        var bmp: Bitmap? = null
        val rgb = Mat()
//        Imgproc.cvtColor(input, rgb, Imgproc.COLOR_BGR2RGB)
        try {
            bmp = Bitmap.createBitmap(rgb.cols(), rgb.rows(), Bitmap.Config.ARGB_8888)
            Utils.matToBitmap(rgb, bmp)
        } catch (e: CvException) {
            e.printStackTrace()
        }
        return bmp
    }

error received is this
 java.lang.IllegalArgumentException: width and height must be > 0
        at android.graphics.Bitmap.createBitmap(Bitmap.java:1060)
        at android.graphics.Bitmap.createBitmap(Bitmap.java:1027)
        at android.graphics.Bitmap.createBitmap(Bitmap.java:977)
        at android.graphics.Bitmap.createBitmap(Bitmap.java:938)
        at com.buttonwillow.app.views.main.camera.SeatBeltDemoActivity.convertMatToBitMap
2

rgb is empty / invalid, if you comment the cvtColor() away
(so, rgb.cols() == 0, no bitmap created)

next, the blob is a 4d tensor (a batch of seperated color planes), not an image, you can’t use that as an input for your Bitmap conversion

then, is that really all of the person detection ?
you’d still have to pipe the blob through the network, and parse rectangles from the output tensor, right ?
(then maybe you could draw some rects into the original image, and show that instead)

3

Yes. I need to draw line on the bitmap. How do I do that? I have bouding box values but I need to draw that on the image. If possible could you please show me how to do that?

  val imgRoi = Mat(
                            originalMat,
                            Rect(x, y, x1 - x, y1 - y)
                        )

I have to show lines how do I do that? I need to draw Rect(x, y, x1 - x, y1 - y) on my bitmap

val originalMat = Mat()
        Utils.bitmapToMat(img, originalMat)
4

don’t crop it, instead draw a rectangle into originalMat frame. convert to bitmap.

1 Like
5

I’m using openCV346. and I can not see the specific function that you have mentioned. Can you also let me know give an example to show the bounding? thanks.

6

Like how do I convert my rect to point1 and point2? any suggestions?

7

i updated the link, have a look

8 
public static void rectangle(Mat img,
                             Point pt1,
                             Point pt2,
                             Scalar color)

How do I convert a Rect object to point p1, point pt2. so that my bouding box get displayed?

9

no, i won’t answer this …

10

Anyhow thanks for your help. You’ve helped me enough already. :blush: