Computer graphics & visualization Real-Time Pencil Rendering Marc Treib

computer graphics & visualization

Real-Time Pencil Rendering

Marc Treib


Real-Time Pencil Rendering – 2007-07-25Marc Treib

Contents1. Introduction: Pencil Drawing2. Challenges3. Contours4. Interior Shading: Hatching

1. Object vs. Screen Space2. Implementation in Object Space3. Implementation in Screen Space

5. Results6. Summary



1 Introduction: Pencil Drawing









2 Challenges• Contours:– Errors in contours– Multiple „trial-and-error“ lines

• Hatching:– Maintain constant stroke size– Continuously adjust stroke density– Align stroke direction with shape (curvature)



3.1 Contour Detection• Object Space Approach:

– But how to implement this?



3.1 Contour Detection• Screen Space Approach:– Render depth and normals to textures

– Apply edge detection algorithm (in a pixel shader)

– Optionally: Object IDs (colors)



3.2 Contour Shaking• Divide screen into regularly sized rectangles• Perturb texture coordinates– E.g. with sine function + random offset

• Map contour image onto this distorted plane• Repeat for multiple lines



4.1 Hatching: Object vs. Screen Space• Object space approach:

+ Strokes stick to objects- Need to adapt stroke widths and densities with distance- Need continuous global parameterization

Map textures containing pencil strokes onto objects, like conventional texture mapping



4.1 Hatching: Object vs. Screen Space• Screen space approach:

Map stroke textures onto objects in screen space

+ Easier control of stroke widths and densities- No object space continuity- „Shower-door-effect“



4.2 Implementation in Object Space• Tonal Art Maps:

• Strokes in one image appear in all images to the right and down from it

• Store in 3d texture



4.2 Implementation in Object Space• Rendering:

6-way blending



4.2 Implementation in Object Space• To circumvent the need for a continuous global

parameterization:Use Lapped Textures



4.3 Implementation in Screen Space• Stroke textures:– Similar to object space approach, but don‘t need to

bother with mipmaps



4.3 Implementation in Screen Space• In the vertex shader:– Project 3d curvature direction onto screen space– Compute brightness

• In the pixel shader:– For each vertex, rotate texture coordinates to

projected curvature direction– Blend 3 stroke textures (one per vertex)



5 Results



5 Results



5 Results



5 Results

Demo!



6 Summary• Contours:– Edge detection using 2d image processing algorithm– Errors in contours by projecting onto distorted plane

• Interiors:– In object space:• Tonal Art Maps, 6-way blending, Lapped Textures

– In screen space:• Rotate and map textures in screen space, 3-way blending



Thanks for listening!Questions?



Bibliography• Emil Praun, Adam Finkelstein and Hugues Hoppe. Lapped textures. In

SIGGRAPH '00: Proceedings of the 27th annual conference on Computer graphics and interactive techniques, 2000.

• Emil Praun, Hugues Hoppe, Matthew Webb and Adam Finkelstein. Real-time hatching. In SIGGRAPH '01: Proceedings of the 28th annual conference on Computer graphics and interactive techniques, 2001.

• Matthew Webb, Emil Praun, Adam Finkelstein and Hugues Hoppe. Fine tone control in hardware hatching. In NPAR '02: Proceedings of the 2nd international symposium on Non-photorealistic animation and rendering, 2002.

• Hyunjun Lee, Sungtae Kwon and Seungyong Lee. Real-time pencil rendering. In NPAR '06: Proceedings of the 4th international symposium on Non-photorealistic animation and rendering, 2006.

Documents

Computer graphics & visualization Real-Time Pencil Rendering Marc Treib