Question

Recently, I try to blur an image, that the radius is variable. I tried to implement it myseft, but It seem so slow. From this site, I got a fast blur method which is called stack blur:

  static Bitmap fastblur(Bitmap sentBitmap, int radius, int fromX, int fromY,
        int width, int height) {

    // Stack Blur v1.0 from
    // http://www.quasimondo.com/StackBlurForCanvas/StackBlurDemo.html
    //
    // Java Author: Mario Klingemann <mario at quasimondo.com>
    // http://incubator.quasimondo.com
    // created Feburary 29, 2004
    // Android port : Yahel Bouaziz <yahel at kayenko.com>
    // http://www.kayenko.com
    // ported april 5th, 2012

    // This is a compromise between Gaussian Blur and Box blur
    // It creates much better looking blurs than Box Blur, but is
    // 7x faster than my Gaussian Blur implementation.
    //
    // I called it Stack Blur because this describes best how this
    // filter works internally: it creates a kind of moving stack
    // of colors whilst scanning through the image. Thereby it
    // just has to add one new block of color to the right side
    // of the stack and remove the leftmost color. The remaining
    // colors on the topmost layer of the stack are either added on
    // or reduced by one, depending on if they are on the right or
    // on the left side of the stack.
    //
    // If you are using this algorithm in your code please add
    // the following line:
    //
    // Stack Blur Algorithm by Mario Klingemann <[e-mail]>

    Bitmap bitmap = sentBitmap.copy(sentBitmap.getConfig(), true);

    if (radius < 1) {
        return (null);
    }

    int w = width;
    int h = height;

    int[] pix = new int[w * h];

    bitmap.getPixels(pix, 0, w, fromX, fromY, w, h);

    int wm = w - 1;
    int hm = h - 1;
    int wh = w * h;
    int div = radius + radius + 1;

    int r[] = new int[wh];
    int g[] = new int[wh];
    int b[] = new int[wh];
    int rsum, gsum, bsum, x, y, i, p, yp, yi, yw;
    int vmin[] = new int[Math.max(w, h)];

    int divsum = (div + 1) >> 1;
    divsum *= divsum;
    int dv[] = new int[256 * divsum];
    for (i = 0; i < 256 * divsum; i++) {
        dv[i] = (i / divsum);
    }

    yw = yi = 0;

    int[][] stack = new int[div][3];
    int stackpointer;
    int stackstart;
    int[] sir;
    int rbs;
    int r1 = radius + 1;
    int routsum, goutsum, boutsum;
    int rinsum, ginsum, binsum;

    int originRadius = radius;
    for (y = 0; y < h; y++) {
        rinsum = ginsum = binsum = routsum = goutsum = boutsum = rsum = gsum = bsum = 0;
        for (i = -radius; i <= radius; i++) {
            p = pix[yi + Math.min(wm, Math.max(i, 0))];
            sir = stack[i + radius];
            sir[0] = (p & 0xff0000) >> 16;
            sir[1] = (p & 0x00ff00) >> 8;
            sir[2] = (p & 0x0000ff);
            rbs = r1 - Math.abs(i);
            rsum += sir[0] * rbs;
            gsum += sir[1] * rbs;
            bsum += sir[2] * rbs;
            if (i > 0) {
                rinsum += sir[0];
                ginsum += sir[1];
                binsum += sir[2];
            } else {
                routsum += sir[0];
                goutsum += sir[1];
                boutsum += sir[2];
            }
        }
        stackpointer = radius;

        for (x = 0; x < w; x++) {

            r[yi] = dv[rsum];
            g[yi] = dv[gsum];
            b[yi] = dv[bsum];

            rsum -= routsum;
            gsum -= goutsum;
            bsum -= boutsum;

            stackstart = stackpointer - radius + div;
            sir = stack[stackstart % div];

            routsum -= sir[0];
            goutsum -= sir[1];
            boutsum -= sir[2];

            if (y == 0) {
                vmin[x] = Math.min(x + radius + 1, wm);
            }
            p = pix[yw + vmin[x]];

            sir[0] = (p & 0xff0000) >> 16;
            sir[1] = (p & 0x00ff00) >> 8;
            sir[2] = (p & 0x0000ff);

            rinsum += sir[0];
            ginsum += sir[1];
            binsum += sir[2];

            rsum += rinsum;
            gsum += ginsum;
            bsum += binsum;

            stackpointer = (stackpointer + 1) % div;
            sir = stack[(stackpointer) % div];

            routsum += sir[0];
            goutsum += sir[1];
            boutsum += sir[2];

            rinsum -= sir[0];
            ginsum -= sir[1];
            binsum -= sir[2];

            yi++;
        }
        yw += w;
    }

    radius = originRadius;

    for (x = 0; x < w; x++) {
        rinsum = ginsum = binsum = routsum = goutsum = boutsum = rsum = gsum = bsum = 0;
        yp = -radius * w;
        for (i = -radius; i <= radius; i++) {
            yi = Math.max(0, yp) + x;

            sir = stack[i + radius];

            sir[0] = r[yi];
            sir[1] = g[yi];
            sir[2] = b[yi];

            rbs = r1 - Math.abs(i);

            rsum += r[yi] * rbs;
            gsum += g[yi] * rbs;
            bsum += b[yi] * rbs;

            if (i > 0) {
                rinsum += sir[0];
                ginsum += sir[1];
                binsum += sir[2];
            } else {
                routsum += sir[0];
                goutsum += sir[1];
                boutsum += sir[2];
            }

            if (i < hm) {
                yp += w;
            }
        }
        yi = x;
        stackpointer = radius;
        for (y = 0; y < h; y++) {
            pix[yi] = 0xff000000 | (dv[rsum] << 16) | (dv[gsum] << 8)
                    | dv[bsum];

            rsum -= routsum;
            gsum -= goutsum;
            bsum -= boutsum;

            stackstart = stackpointer - radius + div;
            sir = stack[stackstart % div];

            routsum -= sir[0];
            goutsum -= sir[1];
            boutsum -= sir[2];

            if (x == 0) {
                vmin[y] = Math.min(y + r1, hm) * w;
            }
            p = x + vmin[y];

            sir[0] = r[p];
            sir[1] = g[p];
            sir[2] = b[p];

            rinsum += sir[0];
            ginsum += sir[1];
            binsum += sir[2];

            rsum += rinsum;
            gsum += ginsum;
            bsum += binsum;

            stackpointer = (stackpointer + 1) % div;
            sir = stack[stackpointer];

            routsum += sir[0];
            goutsum += sir[1];
            boutsum += sir[2];

            rinsum -= sir[0];
            ginsum -= sir[1];
            binsum -= sir[2];

            yi += w;
        }
    }

    bitmap.setPixels(pix, 0, w, fromX, fromY, w, h);

    return (bitmap);
}

Actually, I want this blur method can be variable blur (such as: when the distance to image’s center decrease, the radius is decreased too). However, I’m not good at bit shifting, it’s too hard for me to modify those code!
So, I want to ask for a way to implements those code like radial blur! And I wonder that is there any blur library which contains radial blur, greyscale, …. in android! It’s really thankful if anyone can help me! Thanks you!

Answer 1

Due to its optimization for speed, this algorithm is not well suited to be adapted for a varying radius. You can still use it if you take a different approach:

The principle is that you create multiple temporary maps each with an increasing (uniform) blur radius and then blend two of them together based on how big the radius at that point should be. Let’s say you prepare 3 temp maps, one with radius 4, one with 8 and one with 16. Now you want a blur radius of 12 at one pixel. What you do is you blend map 2 and 3 with about 50%. The more temp maps you use the better the quality, but 3 (plus the original unblurred map) are usually sufficient.

I am using this technique in my Compound Blur for Canvas: http://www.quasimondo.com/CompoundBlurForCanvas/CompoundBlurDemo.html – this allows you to do tilt shift effects or vignettes.

If you are looking for a radial blur that looks more like a camera zoom effect you have to use a different approach. In that case you first transform the bitmap from cartesian mapping to polar mapping, then you do a horizontal blur and finally you transform the map back to cartesian space – there is a Flash demo and some source code on my site, also a more detailed explanation: http://www.quasimondo.com/archives/000697.php