hlsl unexpected acos result - c++

Question

I found a few strange HLSL bugs - or Pix is telling nonsense:
I have 2 orthogonal Vectors: A = { 0.0f, -1.0f, 0.0f } and B { 0.0f, 0.0f, 1.0f }
If I use the HLSL dot function, the output is (-0.0f) which makes sense BUT now the acos of that output is -0.0000675917 (that's what Pix says - and what the shader outputs) which is not what I had expected;
Even if I compute the dotproduct myself (A.x*B.x + A.y * B.y + etc.) the result is still 0.0f but the acos of my result isn't zero.
I do need the result of acos to be as precisely as possible because i want to color my vertices according to the angle between the triangle normal and a given vector.
float4 PS_MyPS(VS_OUTPUT input) : COLOR
{
float Light = saturate(dot(input.Normal, g_LightDir)) + saturate(dot(-input.Normal, g_LightDir)); // compute the lighting
if (dot(input.Vector, CameraDirection) < 0) // if the angle between the normal and the camera direction is greater than 90 degrees
{
input.Vector = -input.Vector; // use a mirrored normal
}
float angle = acos(0.0f) - acos(dot(input.Vector, Vector));
float4 Color;
if (angle > Angles.x) // Set the color according to the Angle
{
Color = Color1;
}
else if (angle > Angles.y)
{
Color = Color2;
}
else if (angle >= -abs(Angles.y))
{
Color = Color3;
}
else if (angle >= Angles.z)
{
Color = Color4;
}
else
{
Color = Color5;
}
return Light * Color;
}
It works fine for angles above 0.01 degrees, but gives wrong results for smaller values.
The other bugs I found are: The "length"-function in hlsl returns 1 for the vector (0, -0, -0, 0) in Pix and the HLSL function "any" on that vector returns true as well. This would mean that -0.0f != 0.0f.
Has anyone else encountered these and maybe has a workaround for my problem?
I tested it on an Intel HD Graphics 4600 and a Nvidia card with the same results.

Answer 1

One of the primary reasons why acos may return bad results is because always remember that acos takes values between -1.0 and 1.0.
Hence if the value exceeds even slightly(1.00001 instead of 1.0) , it may return incorrect result.
I deal with this problem by forced capping i.e putting in a check for
if(something>1.0)
something = 1.0;
else if(something<-1.0)
something = -1.0;