Using a shader messes up my alpha in XNA

Question

I'm making a 2D game that ends up being too dark on some laptops, so I figured I'd use a shader so people could brighten it up if need be. Unfortunately, once I got it in there nothing would fade in or out, they'd just pop in and out.

Here's my FX file

float intensity;

sampler TextureSampler : register(s0);

float4 Glow(float2 texCoord:TEXCOORD0):COLOR0
{
    float4 Color;
    Color = tex2D(TextureSampler, texCoord.xy);
    Color.rgb *= intensity;
    return Color;
}

technique Technique1
{
    pass Glow
    {
        PixelShader=compile ps_2_0 Glow();
    }
}

Even after commenting out the Color.rgb *= intensity; line it still removes all fading effects.

I'm using the sprite batch to draw, and I begin and end it a few times because I use different BlendStates

brightnessShader.Parameters["intensity"].SetValue(brightness);
spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.AlphaBlend, null, null, null, brightnessShader, Matrix.CreateScale(scale));
...
spriteBatch.End();
spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.Additive, null, null, null, brightnessShader, Matrix.CreateScale(scale));
....
spriteBatch.End();
spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.AlphaBlend, null, null, null, brightnessShader, Matrix.CreateScale(scale));
...
spriteBatch.End();    

I'm using XNA 4.0.

Answer 1

SpriteBatch passes in both a colour (containing an alpha/transparency value) and a texture coordinate. You need to use the colour parameter. Here is the XNA SpriteBatch pixel shader (from XNA 2.0)

void SpritePixelShader(inout float4 color : COLOR0, float2 texCoord : TEXCOORD0)
{
    color *= tex2D(TextureSampler, texCoord);
}

So your shader would need to be something like this:

void SpritePixelShader(inout float4 color : COLOR0, float2 texCoord : TEXCOORD0)
{
    color *= tex2D(TextureSampler, texCoord);
    color.rgb *= intensity;
}

Also be aware of premultiplied alpha, which was added in XNA 4.0. Simply multiplying the RGB values may give you a result that is different to what you actually want to achieve.

Probably a better solution to your problem would be to render your entire scene as normal to a render target, and then gamma-correct that render target using a post-process pixel shader.

Answer 2

If your source bitmap is compressed with DXT1 you'll only get 1bit of alpha, on or off. DXT3 is a little better with 4bit so 16 total alpha values.