# Transforming implicit surface (distance field) and their gradient

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Let $$f:\mathbb R^3 \rightarrow\mathbb R$$ be any distance field. We can transform any implicit surface $$f(x,y,z) = c$$ with a 4x4 transformation matrix as follows:

$$\hat f(\vec p) = f( \mathbf T^{-1} \vec p)$$

### Gradient

Sometimes you need to compute the normal of an implicit surface. You usually do this using the gradient $$\nabla f$$ which needs to be transformed as well. Similar to the normal of a mesh use the inverse transpose of $$\mathbf T$$:

$$\hat{ \nabla f}(\vec p) = \left ( \mathbf T^{-1} \right )^{T} . \nabla f ( \mathbf T^{-1} \vec p )$$

### Code

float transformed_value_and_gradient(Point pos, Vec3& grad)
{
Point p = T.inverse() * pos;
float field_values = distance_field(p, grad); // original field function value
grad = T.inverse().transpose() * grad;
return field_values;
}

### Special transformations

TODO

• bend
• twist
• ..

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