Paint.NET

int Amount1=7;   //[0,200]Depth
int Amount2=20;   //[0,100]Strength

void Render(Surface dst, Surface src, Rectangle rect)
{
    PdnRegion selectionRegion = EnvironmentParameters.GetSelection(src.Bounds);

    // Delete any of these lines you don't need
    Rectangle selection = this.EnvironmentParameters.GetSelection(src.Bounds).GetBoundsInt();

    long CenterX = (long)(((selection.Right - selection.Left) / 2)+selection.Left);
    long CenterY = (long)(((selection.Bottom - selection.Top) / 2)+selection.Top);
    int left = (int)selection.Left;
    int top = (int)selection.Top;
    int right = (int)selection.Right;
    int bottom = (int)selection.Bottom;
    ColorBgra PrimaryColor = (ColorBgra)EnvironmentParameters.PrimaryColor;
    ColorBgra SecondaryColor = (ColorBgra)EnvironmentParameters.SecondaryColor;
    int BrushWidth = (int)EnvironmentParameters.BrushWidth;

    ColorBgra CurrentPixel;
    for(int y = rect.Top; y < rect.Bottom; y++)
    {
        for (int x = rect.Left; x < rect.Right; x++)
        {
            if (selectionRegion.IsVisible(x, y))
            {
                CurrentPixel = src[x,y];
                //top
                if ((x-left) >= (y-top) && (y-top) <= Amount1 && x+(y-top) < right)
                {
                    CurrentPixel = transformPixel(y-top, CurrentPixel, PrimaryColor);
                }
                //left
                else if ((y-top) >= (x-left) && (x-left) <= Amount1 && (x-left)+y < bottom)
                {
                    CurrentPixel = transformPixel(x-left, CurrentPixel, PrimaryColor);
                }
                //bottom
                else if ((x-left) >= (bottom-y) && (bottom-y) <= Amount1 && x+(bottom-y) <= right)
                {
                    CurrentPixel = transformPixel(bottom-y, CurrentPixel, SecondaryColor);
                }
                //right
                else if ((y-top) >= (right-x) && (right-x) <= Amount1 && (right-x)+y < bottom)
                {
                    CurrentPixel = transformPixel(right-x, CurrentPixel, SecondaryColor);
                }
                dst[x,y] = CurrentPixel;
            }
        }
    }
}
ColorBgra transformPixel(float px_from_edge, ColorBgra CurrentPixel, ColorBgra NewColor)
{
    //set amount of background interference 0~1 (ie how much it blends to the background)
    float diff = (float)(0.05 + px_from_edge/Amount1);
    float strength = (float)((float)(100 - Amount2) / 100 + (float)0.35);
    diff = diff * strength;
    if (diff > 1)
    {
        diff = 1;
    }
    else if (diff < 0)
    {
        diff = 0;
    }
   
    //some vars just for convenience, more readable
    float div = diff + 1;
    float invrt_diff = 1 - diff;
   
    //we set the RGB to primary color if it is completely transparent (black is the default)
    //That should solve
    if (CurrentPixel.A == 0)
    {
        CurrentPixel.R = NewColor.R;
        CurrentPixel.G = NewColor.G;
        CurrentPixel.B = NewColor.B;
    }
   
    //and we are ready to color the pixels
    //ex: if 70% background interference
    //we have 70% of original color's R plus 30% of the new color's R
    CurrentPixel.R = (byte)((invrt_diff * (float)NewColor.R) + (diff * (float)CurrentPixel.R));
    CurrentPixel.G = (byte)((invrt_diff * (float)NewColor.G) + (diff * (float)CurrentPixel.G));
    CurrentPixel.B = (byte)((invrt_diff * (float)NewColor.B) + (diff * (float)CurrentPixel.B));
   
    //transparency values need special manipulation to prevent bad-looking renders
    float temp = CurrentPixel.A + NewColor.A;
    if (temp > 255)
    {
        temp = 255;
    }
    temp = ((int)CurrentPixel.A + temp) / 2;
    if (CurrentPixel.A < 255)
    {
        temp = temp * (1 + (float)invrt_diff);
    }
    if (temp > 255)
    {
        temp = 255;
    }
    CurrentPixel.A = (byte)temp;
   
    //and we are done
    return CurrentPixel;
}