// We compress horizontally first, then vertically
// which I think may be the fastest way.
//
// Up to 7 pixel columns and 7 pixel rows may be discarded
// (those not fitting in an 8x8 block). The discarded pixels are
// along the right and bottom border of the image, respectively.

sclass FastDownscale8x8 {
  int w, h, w2, h2;
  int[] horizontallyScaledPixels, finalPixels;
  
  void scaleHorizontally(BufferedImage img) {
    scaleHorizontally(grabbableIntPixels(img));
  }
    
  void scaleHorizontally(GrabbableIntPixels gp) {
    int scaleX = 8, scaleY = scaleX;
    w = gp.w;
    int h = this.h = gp.h;
    int w2 = this.w2 = w/scaleX, h2 = this.h2 = h/scaleY;
    
    int[] newPixels = horizontallyScaledPixels = new int[w2*h];
  
    int iRow = gp.offset, iOut = 0;
    int[] pixels = gp.data;
    
    for (int y = 0; y < h; y++) {
      int iIn = iRow;
    
      for x to w2: {
        int r = 0, g = 0, b = 0;
    
        for subX to 8: {
          int rgb = pixels[iIn+subX];
          r += (rgb >> 16) & 0xFF;
          g += (rgb >> 8) & 0xFF;
          b += rgb & 0xFF;
        }
    
        iIn += 8;
        newPixels[iOut++] = rgbIntFullAlpha(r/scaleX, g/scaleX, b/scaleX);
      }
    
      iRow += gp.scanlineStride;
    } // for y
  }
  
  void scaleVertically {
    int scaleX = 8, scaleY = scaleX;
    int[] pixels = horizontallyScaledPixels;
    int[] newPixels = finalPixels = new int[w2*h2];
  
    int iRow = 0, iOut = 0;
    
    for (int y = 0; y < h2; y++) {
      int iIn = iRow;
      
      for x to w2: {
        int r = 0, g = 0, b = 0;
        int iIn2 = iIn;
        
        for subY to 8: {
          int rgb = pixels[iIn2];
          iIn2 += w2;
          r += (rgb >> 16) & 0xFF;
          g += (rgb >> 8) & 0xFF;
          b += rgb & 0xFF;
        }
        
        iIn++;
        newPixels[iOut++] = rgbIntFullAlpha(r/scaleX, g/scaleX, b/scaleX);
      }
      
      iRow += w2*scaleY;
    } // for y
  }
  
  BufferedImage get(BufferedImage img) {
    scaleHorizontally(img);
    scaleVertically();
    ret get();
  }
  
  BufferedImage get() {
    ret bufferedImage(w2, h2, finalPixels);
  }
}

Began life as a copy of #1035216