Smearing is an essential effect to expressively convey motion in stylized animations. In this paper, we extend the method of Basset et al. [2024] to better emphasize the main motion’s trajectory of an object when generating elongated in-betweens, i.e., when stretching a 3D object along its trajectory to cover adjacent frames. This limits visual artifacts such as intersections that typically occur when trajectories self-overlap due to local rotations or abrupt changes of direction (trajectories with high curvatures or even discontinuities at contacts). We address these cases with minor computational and memory overheads, and offer enhanced impact expressiveness by combining smear and squash-and-stretch effects at collisions.

Representing 3D motion and depth through 2D animated drawings is a notoriously difficult task, requiring time and expertise when done by hand. Artists must pay particular attention to occlusions and how they evolve through time, a tedious process. Computer-assisted inbetweening methods such as cut-out animation tools allow for such occlusions to be handled beforehand using a 2D rig, at the expense of flexibility and artistic expression. In this work, we extend the more flexible 2D animation framework of Even et al., (2023) to handle occlusions. We do so by retaining three key properties of their system that are crucial to speed-up the animation process: input rough drawings, real-time preview, and non-linear animation editing. Our contribution is two-fold: a fast method to compute 2D masks from rough drawings with a semi-automatic dynamic layout system for occlusions between drawing parts; and a method to both automatically and manually control the dynamic visibility of strokes for self-occlusions. Such controls are not available in any traditional 2D animation software especially with rough drawings. Our system helps artists produce convincing 3D-like 2D animations, including head turns, foreshortening effects, out-of-plane rotations, overlapping volumes and even transparency. • We extend the system of Even et al., (2023) to support 3D-like occlusions. • Masks for drawing parts are automatically computed and updated in real-time. • Dynamic layout changes between key drawings are controlled via semi-automatic tools. • Self-occlusions are produced via editable stroke visibility thresholds. • Our system is evaluated on several use cases and compared to Even et al., (2023).

Abstract Object‐space occluding contours of triangular meshes (a.k.a. mesh contours) are at the core of many methods in computer graphics and computational geometry. A number of hierarchical data‐structures have been proposed to accelerate their computation on the CPU, but they do not map well to the GPU for real‐time applications, such as video games. We show that a simple, flat data‐structure composed of patches bounded by a normal cone and a bounding sphere may reach this goal, provided it is constructed to maximize the probability for a patch to be culled over all viewpoints. We derive a heuristic metric to efficiently estimate this probability, and present a greedy, bottom‐up algorithm that constructs patches by grouping mesh edges according to this metric. In addition, we propose an effective way of computing their bounding sphere. We demonstrate through extensive experiments that this data‐structure achieves similar performance as the state‐of‐the‐art on the CPU but is also perfectly adapted to the GPU, leading to up to ×5 speedups.

Abstract In traditional 2D animation, sketches drawn at distant keyframes are used to design motion, yet it would be far too labor‐intensive to draw all the inbetween frames to fully visualize that motion. We propose a novel efficient interpolation algorithm that generates these intermediate frames in the artist's drawing style. Starting from a set of registered rough vector drawings, we first generate a large number of candidate strokes during a pre‐process, and then, at each intermediate frame, we select the subset of those that appropriately conveys the underlying interpolated motion, interpolates the stroke distributions of the key drawings, and introduces a minimum amount of temporal artifacts. In addition, we propose quantitative error metrics to objectively evaluate different stroke selection strategies. We demonstrate the potential of our method on various animations and drawing styles, and show its superiority over competing raster‐ and vector‐based methods.

Abstract Traditional 2D animation requires time and dedication since tens of thousands of frames need to be drawn by hand for a typical production. Many computer‐assisted methods have been proposed to automatize the generation of inbetween frames from a set of clean line drawings, but they are all limited by a rigid workflow and a lack of artistic controls, which is in the most part due to the one‐to‐one stroke matching and interpolation problems they attempt to solve. In this work, we take a novel view on those problems by focusing on an earlier phase of the animation process that uses rough drawings (i.e., sketches). Our key idea is to recast the matching and interpolation problems so that they apply to transient embeddings, which are groups of strokes that only exist for a few keyframes. A transient embedding carries strokes between keyframes both forward and backward in time through a sequence of transformed lattices. Forward and backward strokes are then cross‐faded using their thickness to yield rough inbetweens. With our approach, complex topological changes may be introduced while preserving visual motion continuity. As demonstrated on state‐of‐the‐art 2D animation exercises, our system provides unprecedented artistic control through the non‐linear exploration of movements and dynamics in real‐time.

This article proposes a method for computing the visible occluding contours of subdivision surfaces. The article first introduces new theory for contour visibility of smooth surfaces. Necessary and sufficient conditions are introduced for when a sampled occluding contour is valid, that is, when it may be assigned consistent visibility. Previous methods do not guarantee these conditions, which helps explain why smooth contour visibility has been such a challenging problem in the past. The article then proposes an algorithm that, given a subdivision surface, finds sampled contours satisfying these conditions, and then generates a new triangle mesh matching the given occluding contours. The contours of the output triangle mesh may then be rendered with standard non-photorealistic rendering algorithms, using the mesh for visibility computation. The method can be applied to any triangle mesh, by treating it as the base mesh of a subdivision surface.

Abstract We present a novel temporally coherent stylized rendering technique working entirely at the compositing stage. We first generate a distribution of 3D anchor points using an implicit grid based on the local object positions stored in a G‐buffer, hence following object motion. We then draw splats in screen space anchored to these points so as to be motion coherent. To increase the perceived flatness of the style, we adjust the anchor points density using a fractalization mechanism. Sudden changes are prevented by controlling the anchor points opacity and introducing a new order‐independent blending function. We demonstrate the versatility of our method by showing a large variety of styles thanks to the freedom offered by the splats content and their attributes that can be controlled by any G‐buffer.

We present a purely geometric, time-independent deformer resolving local contacts between elastic objects, including self-collisions between adjacent parts of the same object that often occur in character skinning animation. Starting from multiple meshes in intersection, our deformer first computes the parts of the surfaces remaining in contact, and then applies a procedural displacement with volume preservation. Although our deformer processes each frame independently, it achieves temporally continuous deformations with artistic control of the bulge through few pseudo-stiffness parameters. The plausibility of the deformation is further enhanced by anisotropically spreading the volume-preserving bulge. The result is a robust, real-time deformer that can handle complex geometric configurations such as a ball squashed by a hand, colliding lips, bending fingers, etc.

We present a novel approach to facilitate the creation of stylized 2D rigid body animations. Our approach can handle multiple rigid objects following complex physically-simulated trajectories with collisions, while retaining a unique artistic style directly specified by the user. Starting with an existing target animation (e.g., produced by a physical simulation engine) an artist interactively draws over a sparse set of frames, and the desired appearance and motion stylization is automatically propagated to the rest of the sequence. The stylization process may also be performed in an off-line batch process from a small set of drawn sequences. To achieve these goals, we combine parametric deformation synthesis that generalizes and reuses hand-drawn exemplars, with non-parametric techniques that enhance the hand-drawn appearance of the synthesized sequence. We demonstrate the potential of our method on various complex rigid body animations which are created with an expressive hand-drawn look using notably less manual interventions as compared to traditional techniques.

Abstract Hardware tessellation is de facto the preferred mechanism to adaptively control mesh resolution with maximal performances. However, owing to its fixed and uniform pattern, leveraging tessellation for feature‐aware LOD rendering remains a challenging problem. We relax this fundamental constraint by introducing a new spatial and temporal blending mechanism of tessellation levels, which is built on top of a novel hierarchical representation of multi‐resolution meshes. This mechanism allows to finely control topological changes so that vertices can be removed or added at the most appropriate location to preserve geometric features in a continuous and artifact‐free manner. We then show how to extend edge‐collapse based decimation methods to build feature‐aware multi‐resolution meshes that match the tessellation patterns. Our approach is fully compatible with current hardware tessellators and only adds a small overhead on memory consumption and tessellation cost.

This article introduces a method for accurately computing the visible contours of a smooth 3D surface for stylization. This is a surprisingly difficult problem, and previous methods are prone to topological errors, such as gaps in the outline. Our approach is to generate, for each viewpoint, a new triangle mesh with contours that are topologically equivalent and geometrically close to those of the original smooth surface. The contours of the mesh can then be rendered with exact visibility. The core of the approach is Contour Consistency, a way to prove topological equivalence between the contours of two surfaces. Producing a surface tessellation that satisfies this property is itself challenging; to this end, we introduce a type of triangle that ensures consistency at the contour. We then introduce an iterative mesh generation procedure, based on these ideas. This procedure does not fully guarantee consistency, but errors are not noticeable in our experiments. Our algorithm can operate on any smooth input surface representation; we use Catmull-Clark subdivision surfaces in our implementation. We demonstrate results computing contours of complex 3D objects, on which our method eliminates the contour artifacts of other methods.

Skilled artists, using traditional media or modern computer painting tools, can create a variety of expressive styles that are very appealing in still images, but have been unsuitable for animation. The key difficulty is that existing techniques lack adequate temporal coherence to animate these styles effectively. Here we augment the range of practical animation styles by extending the guided texture synthesis method of Image Analogies [Hertzmann et al. 2001] to create temporally coherent animation sequences. To make the method art directable, we allow artists to paint portions of keyframes that are used as constraints. The in-betweens calculated by our method maintain stylistic continuity and yet change no more than necessary over time.

Abstract Non‐photorealistic rendering (NPR) algorithms allow the creation of images in a variety of styles, ranging from line drawing and pen‐and‐ink to oil painting and watercolour. These algorithms provide greater flexibility, control and automation over traditional drawing and painting. Despite significant progress over the past 15 years, the application of NPR to the generation of stylized animations remains an active area of research. The main challenge of computer‐generated stylized animations is to reproduce the look of traditional drawings and paintings while minimizing distracting flickering and sliding artefacts present in hand‐drawn animations. These goals are inherently conflicting and any attempt to address the temporal coherence of stylized animations is a trade‐off. This state‐of‐the‐art report is motivated by the growing number of methods proposed in recent years and the need for a comprehensive analysis of the trade‐offs they propose. We formalize the problem of temporal coherence in terms of goals and compare existing methods accordingly. We propose an analysis for both line and region stylization methods and discuss initial steps towards their perceptual evaluation. The goal of our report is to help uninformed readers to choose the method that best suits their needs, as well as motivate further research to address the limitations of existing methods.

Abstract We present a new solution for temporal coherence in non‐photorealistic rendering (NPR) of animations. Given the conflicting goals of preserving the 2D aspect of the style and the 3D scene motion, any such solution is a tradeoff. We observe that primitive‐based methods in NPR can be seen as texture‐based methods when using large numbers of primitives, leading to our key insight, namely that this process is similar to sparse convolution noise in procedural texturing. Consequently, we present a new primitive for NPR based on Gabor noise, that preserves the 2D aspect of noise, conveys the 3D motion of the scene, and is temporally continuous. We can thus use standard techniques from procedural texturing to create various styles, which we show for interactive NPR applications. We also present a user study to evaluate this and existing solutions, and to provide more insight in the trade‐off implied by temporal coherence. The results of the study indicate that maintaining coherent motion is important, but also that our new solution provides a good compromise between the 2D aspect of the style and 3D motion.

Stylized rendering methods, which aim at depicting 3D scenes with 2D marks such as pigments or strokes, are often faced with temporal coherence issues when applied to dynamic scenes. These issues arise from the difficulty of having to satisfy two contrary goals: ensuring that the style marks follow 3D motions while preserving their 2D appearance. In this paper we describe a new texture based method for real-time temporally coherent stylization called dynamic textures. A dynamic texture is a standard texture mapped on the object and enriched with an infinite zoom mechanism. This simple and fast mechanism maintains quasi-constant size and density of texture elements in screen space for any distance from the camera. We show that these dynamic textures can be used in many stylization techniques, enforcing the 2D appearance of the style marks while preserving the accurate 3D motion of the depicted objects.