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javaimage-processingkotlinsobel

Sobel filter imlementation

I have trouble implementing Sobele filter for edge detection.

First of all, I make SOBEL_VERTICAL and SOBEL_HORIZONTAL convolution and then calculate color of pixel using this formula: G = sqrt(Gx * Gx + Gy * Gy)

Code: 

val log = KotlinLogging.logger { }

val width = fastImage.width
val height = fastImage.height

override fun filter(): FastImage {
    val convolution = Convolution(fastImage)
    val obsSobelHorizontal = Observable.fromCallable { convolution.convolve(Convolution.SOBEL_HORIZONTAL) }
    val obsSobelVertical = Observable.fromCallable { convolution.convolve(Convolution.SOBEL_VERTICAL) }

    var fastImageSobel: FastImage? = null

    Observable.zip(obsSobelHorizontal, obsSobelVertical, { r1, r2 ->
        log.info { Thread.currentThread() }
        val fast = FastImage(width, height)
        for (x in 0..width - 1) {
            for (y in 0..height - 1) {
                val argb1: Int? = r1.getARGB(x, y)
                val argb2: Int? = r2.getARGB(x, y)
                if (argb1 != null && argb2 != null) {
                    val G = sqrt(((argb1 * argb1) + (argb2 * argb2)).toDouble()).toInt().clamp()
                    val color = Color(G,G,G)
                    fast.setARGB(x, y, color.rgb)
                }
            }
        }
        return@zip fast

    }).subscribe({ fastImageSobel = it }, { log.error("Can`t do sobel!", it) })

    return fastImageSobel!!
}

enter image description here Right image is wrong

Solution

  • Ok, after some research I found out what was my problem. My pixels were stored in RGB int array. So I have to manually calculate G for each color channel:

                        val cx = Color(argb1)
                    val cy = Color(argb2)

                    val rx = cx.red
                    val gx = cx.green
                    val bx = cx.blue

                    val ry = cy.red
                    val gy = cy.green
                    val by = cy.blue

                    val R = Math.hypot(rx.toDouble(), ry.toDouble()).toInt().clamp()
                    val G = Math.hypot(gx.toDouble(), gy.toDouble()).toInt().clamp()
                    val B = Math.hypot(bx.toDouble(), by.toDouble()).toInt().clamp()

                    val color = Color(R, G, B)

                    fast.setARGB(x, y, color.rgb)