StackBlur.java

import android.graphics.Bitmap;
import androidx.annotation.IntRange;
import androidx.annotation.NonNull;

import java.util.Arrays;

/**
 * This is copied from https://stackoverflow.com/a/10028267/3050249
 * and optimized for less memory consumption & better locality, and creates less GC pressure.
 * {@code int[] dv} division memoization was removed: on my load it had 700k elements and gave no speedup.
 *
 * This gist: https://gist.github.com/Miha-x64/3fb489d13dbf69e1611a8fb688b57d3d
 * Superseded by my library: https://github.com/Miha-x64/Lubricant
 *
 * This class is *not* thread safe: it reuses some buffers.
 * @author Mike Gorünov
 */
public final class StackBlur {

  private int[] pix;
  private int[] rgb;
  private int[] vmin;
  private int[] stack;

  public StackBlur() {
  }

  /**
   * 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 <mario@quasimondo.com>
   */
  public void blurRgb(@NonNull Bitmap bitmap, @IntRange(from=1) int radius) {
    int w = bitmap.getWidth(); // implicit null-check
    int h = bitmap.getHeight();
    if (radius < 1) throw new IllegalArgumentException("radius < 1: " + radius);

    int[] pix, rgb, vmin, stack;

    int wh = w * h;
    if ((pix = this.pix) == null || pix.length < wh) {
      this.pix = pix = new int[wh];
      this.rgb = rgb = new int[wh];
    } else {
      // Bitmap#getPixels will fill pix[]
      Arrays.fill(rgb = this.rgb, 0);
    }

    if ((vmin = this.vmin) == null || vmin.length < Math.max(w, h)) this.vmin = vmin = new int[Math.max(w, h)];
    else Arrays.fill(vmin, 0);

    int div = radius + radius + 1;
    if ((stack = this.stack) == null || stack.length < div) this.stack = stack = new int[div];
    else Arrays.fill(stack, 0);

    int rsum, gsum, bsum, x, y, i, p, yp, yi, yw;
    bitmap.getPixels(pix, 0, w, 0, 0, w, h);

    yw = yi = 0;

    int prt, start;
    int sir, rsir, gsir, bsir;
    int rbs;
    int r1 = radius + 1;
    int routsum, goutsum, boutsum;
    int rinsum, ginsum, binsum;
    int wm = w - 1, hm = h - 1;

    int divsum = (radius + radius + 2) >> 1;
    divsum *= divsum;

    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))];
        stack[i + radius] = p;
        rbs = r1 - Math.abs(i);
        rsir = (p & 0xff0000) >> 16;
        gsir = (p & 0x00ff00) >> 8;
        bsir = (p & 0x0000ff);
        rsum += rsir * rbs;
        gsum += gsir * rbs;
        bsum += bsir * rbs;
        if (i > 0) {
          rinsum += rsir;
          ginsum += gsir;
          binsum += bsir;
        } else {
          routsum += rsir;
          goutsum += gsir;
          boutsum += bsir;
        }
      }
      prt = radius;

      for (x = 0; x < w; x++) {
        rgb[yi] = ((rsum / divsum) << 16) | ((gsum / divsum) << 8) | (bsum / divsum);

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

        start = prt - radius + div;
        sir = stack[start % div];

        routsum -= (sir & 0xFF0000) >> 16;
        goutsum -= (sir & 0x00FF00) >> 8;
        boutsum -= (sir & 0x0000FF);

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

        stack[start % div] = p;
        rsir = (p & 0xff0000) >> 16;
        gsir = (p & 0x00ff00) >> 8;
        bsir = (p & 0x0000ff);

        rinsum += rsir;
        ginsum += gsir;
        binsum += bsir;

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

        prt = (prt + 1) % div;
        sir = stack[prt % div];
        rsir = (sir & 0xff0000) >> 16;
        gsir = (sir & 0x00ff00) >> 8;
        bsir = (sir & 0x0000ff);

        routsum += rsir;
        goutsum += gsir;
        boutsum += bsir;

        rinsum -= rsir;
        ginsum -= gsir;
        binsum -= bsir;

        yi++;
      }
      yw += w;
    }
    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;

        int c = rgb[yi];
        stack[i + radius] = c;

        rsir = (c & 0xFF0000) >> 16;
        gsir = (c & 0x00FF00) >> 8;
        bsir = (c & 0x0000FF);

        rbs = r1 - Math.abs(i);

        rsum += rsir * rbs;
        gsum += gsir * rbs;
        bsum += bsir * rbs;

        if (i > 0) {
          rinsum += rsir;
          ginsum += gsir;
          binsum += bsir;
        } else {
          routsum += rsir;
          goutsum += gsir;
          boutsum += bsir;
        }

        if (i < hm) yp += w;
      }
      yi = x;
      prt = radius;
      for (y = 0; y < h; y++) {
        // Preserve alpha channel: ( 0xff000000 & pix[yi] )
        pix[yi] = (0xff000000 & pix[yi]) | ((rsum / divsum) << 16) | ((gsum / divsum) << 8) | (bsum / divsum);

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

        start = prt - radius + div;
        sir = stack[start % div];

        routsum -= (sir & 0xFF0000) >> 16;
        goutsum -= (sir & 0x00FF00) >> 8;
        boutsum -= (sir & 0x0000FF);

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

        int c = rgb[p];
        stack[start % div] = c;

        rinsum += (c & 0xFF0000) >> 16;
        ginsum += (c & 0x00FF00) >> 8;
        binsum += (c & 0x0000FF);

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

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

        rsir = (sir & 0xFF0000) >> 16;
        gsir = (sir & 0x00FF00) >> 8;
        bsir = (sir & 0x0000FF);

        routsum += rsir;
        goutsum += gsir;
        boutsum += bsir;

        rinsum -= rsir;
        ginsum -= gsir;
        binsum -= bsir;

        yi += w;
      }
    }

    bitmap.setPixels(pix, 0, w, 0, 0, w, h);
  }
}