opengl - Small artifacts with simple hard shadow mapping technique using OpenGL and GLSL

Question

I have a strange behaviour with my hard shadows I have never seen before because of the somber color of the textures I used (these artifacts was not visible). Here's some screenshots of my problem with 3 different shadowmap resolutions:

1) Shadowmap 160 * 120

As you can see, there are kind of teeth on the surface of the cube.

Here's the view of the scene from light position:

Here's the shadowmap:

2) Shadowmap 640 * 480

With a higher resolution the artifacts are less visible because the shadow map brings more precision.

Here's the view of the scene from light position:

Here's the shadowmap:

I tried with higher other resolution. Of course the artifacts are smaller and smaller but if you zoom on the surface they are still visible.

Here's the code I use in my fragment shader during the second pass:

[...]

uniform sampler2DShadow ShadowSampler[MAX_LIGHT_COUNT];

[...]

float GetDynamicShadowBias(vec3 normalDir_cs, vec3 lightDir_cs)
{
    float cosTheta = clamp(dot(normalDir_cs, lightDir_cs), 0.0f, 1.0f);
    float bias = 0.005f * tan(acos(cosTheta));

    return (clamp(bias, 0.0f, 0.01f));
}

float GetBiased_Hard_ShadowFactor(vec3 normalDir, vec3 lightDir, int idx)
{    
    if (ShadowCoords[idx].w > 0.0f)
    {
        float bias = GetDynamicShadowBias(normalDir, lightDir);

        float LightToOccluderClipDist = texture(
            ShadowSampler[idx], ShadowCoords[idx].xyz/ShadowCoords[idx].w);

        float LightToVertexClipDist = (ShadowCoords[idx].z - bias)/ShadowCoords[idx].w;

        if (LightToOccluderClipDist < LightToVertexClipDist)
            return (0.0f);
    }
    return (1.0f);
}

So I wonder if it's possible to remove these artifacts on the edges of the cube. I tried to play with the shadow bias without real success (for information I use FRONT_FACE culling during the first pass (filling of the depth texture)).