Good day everyone! I found an interesting example of generating normals on the net. Here is the code in the example. But it doesn't work. Tell me please where is the mistake?

[mihas49]

float2 _SamplingDistance;
float _HeightScale;

float3 HeightToNormal(float2 uv, sampler2D tex)
{
float2 uv_smpx = uv + float2(_SamplingDistance.x, 0);
float2 uv_smpy = uv + float2(0, _SamplingDistance.y);

// Sample source and each axis with a delta
float smp_source = tex2D(tex, uv).x;
float smp_x = tex2D(tex, uv_smpx).x;
float smp_y = tex2D(tex, uv_smpy).x;

// compute dervatives
float2 derivative = float2(smp_x - smp_source, smp_y - smp_source);

// Scale the derivative
derivative *= _HeightScale / _SamplingDistance;

// Pack into a normal, then re-normalize
float3 normal = normalize(float3(derivative, 1.0));

return normal;   

}