How to blur some portion of Image in Android?

do a masked blur few times ….

1. create mask

   0 means blur (black) and >=1 means not blur (white). Init this part by big enough value for example w=100 pixels

   

2. create masked blur function

   just a common convolution with some matrix like

   0.0 0.1 0.0
   0.1 0.6 0.1
   0.0 0.1 0.0
   

   but do it only for target pixels where mask is ==0 after image is blurred blur also the mask. This should enlarge the white area a bit (by pixel per iteration but losing magnitude on borders that is why w>1).

3. loop bullet #2 N times

   N determines blur/non-blur gradient depth the w is only to assure that burred mask will grow… Each time the blur mask will increase its white part

That should do the trick, You can also use dilatation of the mask instead of blurring it.

[edit1] implementation

Have played with this a bit today and found out that the mask is not growing enough with smooth so I change the algo a bit (here mine code C++):

picture pic0,pic1,pic2;
    // pic0 - source
    // pic1 - output
    // pic2 - mask
int x0=400,y0=330,r0=100,dr=200;
    // x0,y0,r0 - masked area
    // dr - blur gradient size
int i,r;

// init output as sourceimage
pic1=pic0;
// init mask (size of source image) with gradient circles
pic2.resize(pic0.xs,pic0.ys);
pic2.clear(0);
for (i=1;i<=255;i++)
    {
    r=r0+dr-((dr*i)>>8);
    pic2.bmp->Canvas->Brush->Color=TColor(i<<16); // shifted because GDI has inverse channel layout then direct pixel access
    pic2.bmp->Canvas->Pen  ->Color=TColor(i<<16);
    pic2.bmp->Canvas->Ellipse(x0-r,y0-r,x0+r,y0+r);
    }
for (i=1;i<255;i+=10) pic1.rgb_smooth_masked(pic2,i);

here the smooth function:

//---------------------------------------------------------------------------
void picture::rgb_smooth_masked(const picture &mask,DWORD treshold)
    {
    int i,x,y;
    color *q0,*q1,*m0,c0,c1,c2;
    if ((xs<2)||(ys<2)) return;
    for (y=0;y<ys-1;y++)
        {
        q0=p[y  ]; m0=mask.p[y];
        q1=p[y+1];
        for (x=0;x<xs-1;x++)
         if (m0[x].dd<treshold)
            {
            c0=q0[x];
            c1=q0[x+1];
            c2=q1[x];
            for (i=0;i<4;i++)
             q0[x].db[i]=DWORD((DWORD(c0.db[i])+DWORD(c0.db[i])+DWORD(c1.db[i])+DWORD(c2.db[i]))>>2);
            }
        }
    }
//---------------------------------------------------------------------------

1. create gradient mask with circles increasing in color from 1 to 255

   rest is black the gradient width is dr and determine the smoothing sharpness.

2. create smooth masked with mask and threshold

   smooth all pixels where mask pixel is < threshold. See the function rgb_smooth_masked. It uses 2x2 convolution matrix

   0.50,0.25
   0.25,0.00
   

3. loop threshold from 1 to 255 by some step

   the step determines the image blur strength.

And finally here some visual results this is source image I taken with my camera:

And here the output on the left and mask on the right:

the blue color means values < 256 (B is lowest 8 bits of color)

I use my own picture class for images so some members are:

• xs,ys size of image in pixels
• p[y][x].dd is pixel at (x,y) position as 32 bit integer type
• clear(color) – clears entire image
• resize(xs,ys) – resizes image to new resolution