How can I fade something to clear instead of white?

Question

I've got an XNA game which essentially has "floating combat text": short-lived messages that display for a fraction of a second and then disappear.

I've recently added a gradual "fade-away" effect, like so:

 public void Update()
        {
            color.A -= 10;
            position.X += 3;
            if (color.A <= 10) isDead = true;
        }

Where color is the Color int the message displays as. This works as expected, however, it fades the messages to white, which is very noticeable on my indigo background.

Is there some way to fade it to transparent, rather than white? Lerp-ing towards the background color isn't an option, as there's a possibility there will be something between the text and the background, which would simply be the inverse of the current problem.

Answer 1

If you work with premultiplied alpha, the default behaviour, you have to multiply the alpha....

public void Update()
{
      color*= 0.95f;
      position.X += 3;
      if (color.A <= 10) isDead = true;
}

The difference here is you're modifying the color, i.e. all values simultaneously and uniformly, rather than just the alpha value.

Answer 2

You're doing exactly what you should do - changing the alpha channel of the color you use when drawing the sprite. That isn't where the problem lies.

The problem is that if you just use the default SpriteBatch.Begin() behaviour, your sprite blends to its color (normally: white) rather than blending to actual transparency. What you need to do is set your BlendState to BlendState.NonPremultiplied, then everything will be fine.

SpriteBatch.Begin(SpriteSortMode.FrontToBack, BlendState.NonPremultiplied)

Blau's answer provides a different method (the premultiplied one) which apparently has a few advantages over this one.