Simulating Scratches

We are interested in the simulation of visible scratches, in which the change of the surface aspect is visible but not its geometry change. This assumes that scratches will be viewed from normal distances and not very close, as in the case of vision systems.

To simulate this kind of scratches we use the model of S.Mérillou [Mérillou 00], who realistically simulates their anisotropic behaviour, which means that their reflection change depending on the point of view and on the lights positions. The modelling of these scratches consists on the definition of their position and trajectory on the object surface and on the definition of their reflection properties. The first step is done using an image (2D texture) as the scratch pattern, mapping it onto the object surface with classical texture mapping operations. Next image shows an example of a scratch pattern:

Scratch pattern with many scratches

The scratch pattern can be created with any image processor or obtained from a picture made to the real scratched object. We allow also the definition of scratches using the painting tool from Maya, painting the scratches directly onto the object surface. The resulting pattern must be treated after that to obtain fine scratches. We use a thinning method. Next images show an example of pattern after painting (left) and after thinning (right):

Painted scratch
Thinned scratch

The next step, the definition of the reflection properties, is done according to the material properties and the scratches specific properties. The reflection of a scratch depends on its geometric profile onto the surface, and this can be measured by specific instruments. Next image shows a measure of a scratch profile:

Scanned measure of a scratch profile

If this instrument is not available we can define its properties empirically to approximate its behaviour. Finally, after mapping the pattern onto the object and defining the reflection properties we obtain the next result onto a plane:

Plane with scratches

We have to apply these ideas to simulate a scratched metallic part like this one:

Part 5036 with a scratch

We have created the next pattern for the scratch and we have obtained the next measure from the real object surface:

Scratch pattern
Profile measure

The result of the simulation is this:

Final simulation