CrossShade:Shading Concept Sketches Using

Cross-section Curves

Presenter: Feilong Yan

Introduction

SketchShaded Sketch:Production drawing

Introduction

SketchManually shading

• Manually shade with painting tools• Convert sketch to complete 3D Model

Introduction

• A natural question is “Is there a better way ?”• Fast & reusable

This Paper

• This paper facilitates the creation of 3D-looking shaded production drawing from sketches by the normal field estimated from cross section curves.

Sketch Normal fieldProduction drawing

Main Work & Contribution

• 1. An explicit mathematical formulation of the relationship between sketched cross-section curves and the 3D geometry they aim to convey

Main Work & Contribution

• 2. An algorithm for extracting a normal field from cross-section curve networks based on the formulation above.

Cross Section & Cross Hair

Formulation

1. Orthogonal Cross-Hair Planes:Perception studies indicate that designer consistently use orthogonal planes for intersecting cross-sections.

Formulation

2. Cross-Sections as Curvature Lines:Perception studies indicate that observers interpret intersecting curves as aligned with the principal lines of curvature.

Formulation

3. Cross-sections as Local Geodesics:

4. Minimal Foreshortening :

5. Orientation:

Human perceive intersecting cross-section curves as geodesics

When drawing a shape, designers favor informative viewpoints that convey most visible surface with minimal foreshortening

Geometry Estimation

• Cross-Section Plane Estimation : Use energy function constrained by 5 Formulations at the cross hair to

estimate the support plane of cross-section curves • Normal Propagation: curve normal estimation -> patch construction-> Coons interpolation

Cross-Section Plane EstimationAssumption : Orthographic projection

Orthogonality:)2(

)1(

jiij

ji

tt

nn

Local Geodesics &Minimal Foreshorening:

)3()()()(min 2222 zji

zijiji

ijiji

nttntnt

i

Inherent Constrained:

)4(0 iij nt

At each cross-hair, the intersecting 3D curves share the same depth z:

)5(0,0 jjii cnxcnx

Normal Propagation

• Propagate the normals of the cross-hairs along the cross-section curves

• Interpolate all the sketch

Result

Result

Result

Not a Conclusion

• While the automatic shading quality can never match the expressiveness of manual artwork, the result are aspired , and can express the major features in the production drawing.

Manually shading Automatic shadingSketch

Thank You