In this paper, we describe a non-photorealistic rendering system that conveys shape using lines. We go beyond contours and creases by developing a new type of line to draw: the suggestive contour. Suggestive contours are lines drawn on clearly visible parts of the surface, where a true contour would first appear with a minimal change in viewpoint. We provide two methods for calculating suggestive contours, including an algorithm that finds the zero crossings of the radial curvature. We show that suggestive contours can be drawn consistently with true contours, because they anticipate and extend them. We present a variety of results, arguing that these images convey shape more effectively than contour alone.

http://www.cs.northwestern.edu/~ago820/cs395/Papers/W03_NPR/Decarlo_2003.pdf

Suggestive contours for conveying shape

Neural Style Transfer has shown very exciting results enabling new forms of image manipulation. Here we extend the existing method to introduce control over spatial location, colour information and across spatial scale. We demonstrate how this enhances the method by allowing high-resolution controlled stylisation and helps to alleviate common failure cases such as applying ground textures to sky regions. Furthermore, by decomposing style into these perceptual factors we enable the combination of style information from multiple sources to generate new, perceptually appealing styles from existing ones. We also describe how these methods can be used to more efficiently produce large size, high-quality stylisation. Finally we show how the introduced control measures can be applied in recent methods for Fast Neural Style Transfer.

/pdf/controlling-perceptual-factors-in-neural-style-transfer-zn37ye7zy5.pdf

Controlling Perceptual Factors in Neural Style Transfer

The computation of curvature and other differential properties of surfaces is essential for many techniques in analysis and rendering. We present a finite-differences approach for estimating curvatures on irregular triangle meshes that may be thought of as an extension of a common method for estimating per-vertex normals. The technique is efficient in space and time, and results in significantly fewer outlier estimates while more broadly offering accuracy comparable to existing methods. It generalizes naturally to computing derivatives of curvature and higher-order surface differentials.

Estimating curvatures and their derivatives on triangle meshes

Three-dimensional shape can be drawn using a variety of feature lines, but none of the current definitions alone seem to capture all visually-relevant lines. We introduce a new definition of feature lines based on two perceptual observations. First, human perception is sensitive to the variation of shading, and since shape perception is little affected by lighting and reflectance modification, we should focus on normal variation. Second, view-dependent lines better convey smooth surfaces. From this we define view-dependent curvature as the variation of the surface normal with respect to a viewing screen plane, and apparent ridges as the loci of points that maximize a view-dependent curvature. We present a formal definition of apparent ridges and an algorithm to render line drawings of 3D meshes. We show that our apparent ridges encompass or enhance aspects of several other feature lines.

/pdf/apparent-ridges-for-line-drawing-2ytb1c1hrn.pdf

Apparent ridges for line drawing

We present a system that lets a designer directly annotate a 3D model with strokes, imparting a personal aesthetic to the non-photorealistic rendering of the object. The artist chooses a "brush" style, then draws strokes over the model from one or more viewpoints. When the system renders the scene from any new viewpoint, it adapts the number and placement of the strokes appropriately to maintain the original look.

/pdf/wysiwyg-npr-drawing-strokes-directly-on-3d-models-u68jmm1v8b.pdf

WYSIWYG NPR: drawing strokes directly on 3D models

Recent advances in NPR have enabled real-time rendering of 3D models shaded with hatching strokes for use in interactive applications. The key challenges in real-time hatching are to convey tone by dynamically adjusting stroke density, while controlling stroke size and maintaining frame-to-frame coherence. In this paper, we introduce two new real-time hatching schemes that leverage recent advances in texture mapping hardware. Both schemes provide enhanced control of tone, thereby avoiding blending or aliasing artifacts present in previous systems. The first scheme, which relies on volume rendering hardware, admits the use of color. The second scheme, which uses pixel shaders, allows per-pixel lighting operations such as texture modulation. Both schemes run at interactive rates on inexpensive PC graphics cards.

/pdf/fine-tone-control-in-hardware-hatching-44xds3gdxq.pdf

Fine tone control in hardware hatching

The Good Dinosaur is the first Pixar film without matte painted skies; all 800 shots with visible skies were fully modeled, dressed, lit and volume rendered. In the film, the environment served as an adversary for Arlo to struggle against. Early concept art made it clear there would be multiple sequences during thunderstorms as well as others taking place above the clouds. Volumes were well suited to those settings, but even in quieter moments we wanted to capture constantly changing weather with the perspective and parallax of a true dimensional sky. Finally, 3D clouds enabled the lighting department to treat the environment as a single whole, from foreground all the way to the horizon.

Making a dinosaur seem small: cloudscapes in The Good Dinosaur

We developed workflows for Cars 3 and Incredibles 2 that maintain most benefits of volumetric clouds while leveraging the render speed and artistic control of traditional texture based approaches. For Cars 3, we rendered volumetric clouds using Houdini and RIS to create layered, reference plates that matte painters used to quickly paint sky texture maps. The Houdini based workflow let us intuitively scout sequences and set up paintings. We detail artist-friendly tools in Photoshop to allow for intuitive composition adjustments. On Incredibles 2, we replaced the Photoshop portion of the pipeline with Nuke and rendered three passes: key, fill, and world position. Standardizing the workflow around world aligned, high resolution, hemispherical textures greatly streamlined the process and provided downstream lighting department with greater control integrating the sky with the foreground.

Efficient hybrid volume and texture based clouds

Drawing surfaces using hatching strokes simultaneously conveys material, tone, and form. We present a real-time system for non-photorealistic rendering of hatching strokes over arbitrary surfaces. During an automatic preprocess, we construct a sequence of mipmapped hatch images corresponding to different tones, collectively called a tonal art map. Strokes within the hatch images are scaled to attain appropriate stroke size and density at all resolutions, and are organized to maintain coherence across scales and tones. At runtime, hardware multitexturing blends the hatch images over the rendered faces to locally vary tone while maintaining both spatial and temporal coherence. To render strokes over arbitrary surfaces, we build a lapped texture parametrization where the overlapping patches align to a curvature-based direction field. We demonstrate hatching strokes over complex surfaces in a variety of styles.

Real-time hatching

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