Supersampling implementation for high quality downscaling

Question

I want to create down-scaled Bitmaps ( original Bitmap loaded from asset) with high quality comparable to the quality of bitmaps scaled with professional software like Paint.net (in Paint.NET the algorithm for scaling is choosen in a field called Interpolation. The highest setting uses supersampling)?

I understand how to implement supersampling for anti-aliasing. For anti-aliasing-purpose the original image is rendered in higher resolution and then gets downsampled. For Example to get a target image of 100x100 you would render the scene to 200x200 and then downsampling it with a 2x2 grid.

But, how the algorithm can handle downsampling from let's say 400x400 to 175x175 for scaling-purpose. The grid must be ~ 2.285x2.285 in this case. So how can supersampling be implemented for scaling-purpose?

thx

EDIT: My current algorithm looks like this:

private Bitmap downscale(Bitmap src, int targetWidth, int targetHeight){
    Bitmap target = Bitmap.createBitmap(targetWidth, targetHeight, Config.ARGB_8888);
    float w = src.getWidth()/(float)targetWidth;
    float s = src.getHeight()/(float)targetHeight;
    int color = 0;
    int g = 0;
    for(int i=0;i<target.getWidth();++i){
        for(int j=0;j<target.getHeight();++j){
            color = 0;
            g = 0;
            for(float k =i*w;k<roundUp((i+1)*w);++k){
                for(float l=j*w;l<roundUp((j+1)*s);++l){
                    ++g;
                    color+=src.getPixel((int)k, (int)l);
                }
            }
            target.setPixel(i, j, color/g);
        }
    }
    return target;

}

The image shows the same 100x100 bitmap scaled to 54x54. The left one is scaled by Paint.Net and the right one by my algorithm. Doesn't look good... How can I improve my code?

Answer 1

To calculate the average color you have to calculate each a-,r-,g-,b-value seperately

I changed my code and it works pretty good now.

    public static Bitmap downscaleBitmap(Bitmap src, int targetWidth, int targetHeight, int off){
    float r = (src.getWidth()-off)/(float)targetWidth;
    float s = (src.getHeight()-off)/(float)targetHeight;
    Bitmap target = Bitmap.createBitmap(Math.round(src.getWidth()/r), Math.round(src.getHeight()/s), Config.ARGB_8888);
    r = src.getWidth()/(float)target.getWidth();
    s = src.getHeight()/(float)target.getHeight();
    int argb;
    int red;
    int green;
    int blue;
    int alpha;
    float wx;
    float wy;
    float n;
    for(int i=0;i<target.getWidth();++i){
        for(int j=0;j<target.getHeight();++j){
            red = 0;
            green = 0;
            blue = 0;
            alpha = 0;
            n=0;
            for(int k =(int)(i*r);k<roundUp((i+1)*r);++k){
                if(k<i*r){
                    wx = k-i*r+1;
                }else{
                    if(k+1>(i+1)*r)
                        wx = (i+1)*r-k;
                    else
                        wx = 1;
                }
                for(int l=(int)(j*s);l<roundUp((j+1)*s);++l){
                    if(l<j*s){
                        wy = l-j*s+1;
                    }else{
                        if(l+1>(j+1)*s)
                            wy = (j+1)*s-l;
                        else
                            wy = 1;
                    }
                    n+=wy*wx;
                    argb=src.getPixel(k, l);
                    red += wx*wy*Color.red(argb);
                    green += wx*wy*Color.green(argb);
                    blue += wx*wy*Color.blue(argb);
                    alpha += wx*wy*Color.alpha(argb);

                }
            }
            target.setPixel(i, j, Color.argb((int)(alpha/n), (int)(red/n), (int)(green/n), (int)(blue/n)));
        }
    }
    return target;
}