Png over jpeg (water mark effect) bad quality?

Question

Hi i have two Writablebitmap , one from jpg and another from png and use this method to mix color in a loop:

private static Color Mix(Color from, Color to, float percent)
{
    float amountFrom = 1.0f - percent;
    return Color.FromArgb(
        (byte)(from.A * amountFrom + to.A * percent),
        (byte)(from.R * amountFrom + to.R * percent),
        (byte)(from.G * amountFrom + to.G * percent),
        (byte)(from.B * amountFrom + to.B * percent));
}

My problem is in alpha channel, my watermark effect result is bad (quality)!

This is the original png.

This is the original jpg.

Any Help?

Answer 1

In this case you probably don't want the result to take on any alpha from the watermark, you want it to retain 100% of the opacity of the JPEG. Instead of setting the new alpha to from.A * amountFrom + to.A * percent, just use from.A.

Edit: In addition you want the percent to be adjusted according to the alpha of the PNG. Here's your sample, updated:

private static Color Mix(Color from, Color to, float percent) 
{
    float amountTo = percent * to.A / 255.0;
    float amountFrom = 1.0f - amountTo; 
    return Color.FromArgb( 
        from.A, 
        (byte)(from.R * amountFrom + to.R * amountTo), 
        (byte)(from.G * amountFrom + to.G * amountTo), 
        (byte)(from.B * amountFrom + to.B * amountTo)); 
}

I converted this code to Python and ran your sample images through it with 0.5 percent, here's the result: