Complex 3D curves can be created by directly drawing mid-air in immersive environments (Augmented and Virtual Realities). Drawing mid-air strokes precisely on the surface of a 3D virtual object, however, is difficult, necessitating a projection of the mid-air stroke onto the user “intended” surface curve. We present the first detailed investigation of the fundamental problem of 3D stroke projection in VR. An assessment of the design requirements of real-time drawing of curves on 3D objects in VR is followed by the definition and classification of multiple techniques for 3D stroke projection. We analyze the advantages and shortcomings of these approaches both theoretically and via practical pilot testing. We then formally evaluate the two most promising techniques spraycan and mimicry with 20 users in VR. The study shows a strong qualitative and quantitative user preference for our novel stroke mimicry projection algorithm. We further illustrate the effectiveness and utility of stroke mimicry to draw complex 3D curves on surfaces for various artistic and functional design applications.

中文翻译：

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虚拟现实中 3D 表面曲线的半空中绘制

可以通过在沉浸式环境（增强现实和虚拟现实）中直接绘制半空中来创建复杂的 3D 曲线。然而，在 3D 虚拟对象的表面上精确地绘制空中笔画是很困难的，需要将空中笔画投影到用户“预期”的曲面曲线上。我们首次详细研究了 VR 中 3D 笔画投影的基本问题。在评估了在 VR 中实时绘制 3D 对象曲线的设计要求之后，对 3D 笔画投影的多种技术进行了定义和分类。我们从理论上和通过实际试点测试分析了这些方法的优点和缺点。然后我们正式评估两种最有前途的技术喷雾罐和模仿有 20 个 VR 用户。研究表明，用户对我们的新中风有强烈的定性和定量偏好模仿投影算法。我们进一步说明了笔画模仿的有效性和实用性，以在表面上为各种艺术和功能设计应用绘制复杂的 3D 曲线。