Coherent noise modifier for Grease Pencil

Currently the noise modifier, especially at high strength settings, gives an unnatural look. A new noise modifier based on coherent noise would be a better fit for:

• simulating hand-drawn lines
• simulating irregularities of natural media
• simulating [line boil]( tvtropes.org/pmwiki/pmwiki.php/Main/LineBoil

) in animation

The coherent noise modifier would use simplex noise . The position of each vertex being effected would be checked against the value of the simplex noise instance at that 2d position. The result of the value would be used to either modify the vertex's 2d position (an independent simplex field lookup for X and Y), strength, thickness or UV.

Because the values in simplex fields vary continuously over space, the displacement of one vertex would have some resemblance to the displacement of its neighbours. The result would be more wavy (less spiky) and vertex density wouldn't affect the result as much as it currently does.

Here's an interface mock-up, description below.

• Amplitude. Controls the intensity of the modification.
• Scale. Shrinks or enlarge the simplex noise field before the sampling takes place. Higher scale values give gentler, more wavy displacement, low scale values would give a rougher looking and more detailed displacement (more likely to be spiky or abrupt).
• Modify lines independently. Links different lines to different parts of the noise field, to deliberately break continuity of modification with neighbouring lines. Imagine a cel animation of small concentric circles. When simulating line boil on a scene like this, we wouldn't want all circles to appear to be being nudged as one, instead we'd want the displacement of each circle to be independent for added realism.
• Randomise offset each step. When enabled, a new random offset is applied to all noise field lookups every few frames. e.g. setting to 2 would be equivalent to 'animating on twos'.
• Frames per step. Specifies the interval between randomising noise field offset.
• Offset X, Offset Y. If the randomise offset switch isn't on, these values can be animated manually to create distortion that modulates smoothly over time. Think: looking through the water at the tiles at the bottom of a swimming bath.