Alpha values below 0.5f are invisible

Question

I am currently developing on a new mechanism to visualize laser beam hits on spaceships' shields. The development on the CPU side is done and the Vertex Shader is working fine, but I have an issue while creating the Pixel Shader: Any transparency value below 0.5 is invisible.

The following Pixel Shader is incredible simple: If the pixel is inside the hit radius, I show a semi-transparent blue pixel, otherwise nothing is shown.

float4 PS(PS_IN input) : SV_Target
{
    if (input.impactDistances.x > 0.0f && input.impactDistances.x < 2.0f)
    {
        return float4(0.0f, 0.0f, 1.0f, 0.5f);
    }

    return float4(1.0f, 1.0f, 1.0f, 0.0f);
}

This results in something like this (see blue area above the yellow arrow).

Now, if I change the line

    return float4(0.0f, 0.0f, 1.0f, 0.5f);

to

    return float4(0.0f, 0.0f, 1.0f, 0.4f);

then the impact areas are completely invisible and I can't think of anything that causes this behaviour. Do you have any idea?

If it helps, here are my settings for the blend state that I use for the entire game.

var blendStateDescription = new SharpDX.Direct3D11.BlendStateDescription();
blendStateDescription.RenderTarget[0].IsBlendEnabled = true;
blendStateDescription.RenderTarget[0].RenderTargetWriteMask = SharpDX.Direct3D11.ColorWriteMaskFlags.All;

blendStateDescription.RenderTarget[0].SourceBlend = SharpDX.Direct3D11.BlendOption.SourceAlpha;
blendStateDescription.RenderTarget[0].BlendOperation = SharpDX.Direct3D11.BlendOperation.Add;
blendStateDescription.RenderTarget[0].DestinationBlend = SharpDX.Direct3D11.BlendOption.InverseSourceAlpha;

blendStateDescription.RenderTarget[0].SourceAlphaBlend = SharpDX.Direct3D11.BlendOption.Zero;
blendStateDescription.RenderTarget[0].AlphaBlendOperation = SharpDX.Direct3D11.BlendOperation.Add;
blendStateDescription.RenderTarget[0].DestinationAlphaBlend = SharpDX.Direct3D11.BlendOption.One;

blendStateDescription.AlphaToCoverageEnable = true;

_blendState = new SharpDX.Direct3D11.BlendState(_device, blendStateDescription);

_deviceContext.OutputMerger.SetBlendState(_blendState);

Answer 1

blendStateDescription.AlphaToCoverageEnable = true;

will definitely create an issue in your case, setting it to false will apply correct alpha blending.