direct3d fade effect leaves artifacts

Question

I have a Direct3D program that draws with trails by instead of clearing every frame, it draws a black square over the screen with alpha blending. After rendering, It goes:

renderstate.alphablendenable = true;
renderstate.blendoperation = add; 
renderstate.sourceblend = zero; 
renderstate.destinationblend = invblendfactor; 
renderstate.blendfactor = rgb(8,8,8);

then it renders the squares (please forgive the pseudo code). this works nicely except that it doesn't completely clear the screen. it leaves permanent trails, I can't figure out why. Proper blending should after enough frames fade it completely to black, but this leaves gray trails. anyone know why or a better fade method in Direct3D?

Answer 1

There are plenty of ways you can approach this depending on how the rest of your code is structured. For example you could render everything to a texture and render that with declining alpha over the top of a black background instead of the other way around.

If you don't mind using shaders, one quick and relatively efficient way would be to apply a pixel shader to whatever it is you're 'fading'.

A shader equivalent of what you're doing at the moment (rendering low-alpha black over the top) would be this:

float4 FadeEffectShader(PixelInputType input) : SV_Target
{
   float4 color;    

    color = shaderTexture.Sample(SampleType, input.tex);
    color = color * 0.01;

    return color;
}

You end up with colours fading out along an exponential curve which, for the purpose of this example, will never equal zero and you'll be left with a nice graceful fadeout until almost-black where the artifacts occur. What you actually want is more like:

float4 FadeEffectShader(PixelInputType input) : SV_Target
{
    float4 color;   

    color = shaderTexture.Sample(SampleType, input.tex);
    color = color * 0.01;
    if(color.r<0.01) color.r = 0;
    if(color.g<0.01) color.g = 0;
    if(color.b<0.01) color.b = 0;

    return color;
}

The clamping of values below 0.01 will ensure they fade to black instead of to close-but-not-close-enough-to-black.