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This is an addon for Godot 4.4+ that adds the Lambert node to the visual shader system. This node outputs the Diffuse Light based on the Lambertian reflectance model.

Lambert Diffuse Reflectance Model (Node).

GDSL:

shader_type spatial;

const float INV_PI = 0.31830988618379067154; // 1.0 / PI.

group_uniforms _Lambert;
uniform vec3 _DiffuseColor : source_color = vec3(1.0);

float diffuse_lambert(
    in vec3 n, // Normal.
    in vec3 l // Light.
)
{
    float NdotL = dot(n, l); // cos(theta_l) == cos(theta_i).

    if(NdotL < 0.0)
    {
        return 0.0;
    }

    return INV_PI * NdotL;
}

void fragment() {
    ALBEDO = _DiffuseColor;
}

void light() {
    vec3 n = normalize(NORMAL);
    vec3 l = normalize(LIGHT);

    /* Lambert */
    float fd = diffuse_lambert(n, l);

    // To compare with the Slice-Image.
    //vec3 radiance = (LIGHT_COLOR / PI) * ATTENUATION;

    // In PI light units.
    vec3 radiance = LIGHT_COLOR * ATTENUATION;

    DIFFUSE_LIGHT += radiance * fd;
}