I'm making an algorithm that converts a bitmap leaving it grayscale.

I managed to do using for but would like to do it recursively.

Using for (this is working fine)

//BUTTON - ON CLICK . . .
public void go(View v){
    //BM_3 É UM BITMAP, VARIAVEL GLOBAL QUE JÁ POSSUI UMA IMAGEM EM SEU CONTEUDO . . .
    bm_3 = bm_3.copy(Bitmap.Config.ARGB_8888, true);

    for (int x = 0; x < bm_3.getWidth(); x++){
        for (int y = 0; y < bm_3.getHeight(); y++) {

            String hex = Integer.toHexString(bm_3.getPixel(x, y));

            if(hex.length() == 8) {
                String hexX = "#" + convert_Hex_to_grey("" + hex.charAt(0) + hex.charAt(1),
                                                        "" + hex.charAt(2) + hex.charAt(3),
                                                        "" + hex.charAt(4) + hex.charAt(5),
                                                        "" + hex.charAt(6) + hex.charAt(7));

                bm_3.setPixel(x, y, Color.parseColor(hexX));
            }
        }
    }
    //IMG É UMA IMAGEVIEW . . .
    img.setImageBitmap(bm_3);
}

Based on the algorithm above, I tried to implement the same recursively. However, it is showing error of java.lang.StackOverflowError: stack size 1038KB

Using recursion (java.lang.StackOverflowError)

Recursion

private Bitmap grey_scale(int x, int y, Bitmap bm_3) {
        //IF PARA PERCORRER TODO Y . . .
        if (y < bm_3.getHeight()) {
            String hex = Integer.toHexString(bm_3.getPixel(x, y));
            if (hex.length() == 8) {
                String hexX = "#" + convert_Hex_to_grey("" + hex.charAt(0) + hex.charAt(1),
                                                        "" + hex.charAt(2) + hex.charAt(3),
                                                        "" + hex.charAt(4) + hex.charAt(5),
                                                        "" + hex.charAt(6) + hex.charAt(7));


                bm_3.setPixel(x, y, Color.parseColor(hexX));
            }
            //O ERRO OCORRE NESTE RETURN ! ! !
            return grey_scale(x, y+1, bm_3);

        }
        //IF PARA PERCORRER TODO X, ZERANDO Y A CADA TROCA DE X.
        if (x < bm_3.getWidth()) {
            return grey_scale(x+1, 0, bm_3);
        }
    //RETORNA BITMAP
    return bm_3;
}

BUTTON ON CLICK THAT CALLS RECURSION

public void go(View v){

    bm_3 = bm_3.copy(Bitmap.Config.ARGB_8888, true);
    //FAZ RECURSAO DENTRO DE UMA THREAD . . .
    new Thread(new Runnable() {
        @Override
        public void run() {
            //CHAMA A RECURSAO
            bm_3 = grey_scale(0, 0, bm_3);

            //VOLTA PRA THREAD PRINCIPAL PARA ATUALIZAR O IMAGEVIEW . . .
            runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    img.setImageBitmap(bm_3);
                }
            });
        }
    }).start();
}