sampling from a cube map as though it were a 3d texture

ShaderEffect.fx

float ComputeShadow(float4 positionLightSpace, int directionalLightIndex)
{
    float bias = 0.005;

    // maps to [-1, 1]
    float3 projectionCoords = positionLightSpace.xyz / positionLightSpace.w;

    // map our UV sample coordinates to a cube map sample vector
    float3 cubeMapSampleVector;
    if (directionalLightIndex == 0)
    {
        cubeMapSampleVector = float3(1.0f, projectionCoords.y, -projectionCoords.x);
    }
    else if (directionalLightIndex == 1)
    {
        cubeMapSampleVector = float3(-1.0f, projectionCoords.y, projectionCoords.x);
    }
    else if (directionalLightIndex == 2)
    {
        cubeMapSampleVector = float3(projectionCoords.x, 1.0f, projectionCoords.y);
    }
    else if (directionalLightIndex == 3)
    {
        cubeMapSampleVector = float3(projectionCoords.x, -1.0f, -projectionCoords.y);
    }
    else if (directionalLightIndex == 4)
    {
        cubeMapSampleVector = float3(projectionCoords.x, projectionCoords.y, 1.0);
    }
    else
    {
        cubeMapSampleVector = float3(-projectionCoords.x, projectionCoords.y, -1.0);
    }

    float closestDepth = SAMPLE_CUBEMAP(shadowMap, cubeMapSampleVector).r;
    float currentDepth = projectionCoords.z;

    if (projectionCoords.z > 1.0) { return 0.0; }

    float shadow = currentDepth - bias > closestDepth ? 1.0 : 0.0;

    return shadow;
}