Png over jpeg (watermark effect) poor quality?

Question

Hi, I have two Writablebitmap, one from jpg and the other from png, and use this method to mix colors 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 the alpha channel, my result of the watermark effect is bad (quality)!

This is the original png.

This is the original jpg.

Any help?

Answer 1

In this case, you probably do not want the result to accept any alpha from the watermark, you want to preserve 100% opacity of the JPEG. Instead of setting a new alpha, from.A * amountFrom + to.A * percentjust use from.A.

: , , percent PNG. , :

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)); 
}

Python 0,5%, :