Computer-Generated Pen-and-Ink Illustration of Trees Oliver Deussen & Thomas Strothotte University of Magdeburg

The Idea Standard NPR papers have not specifically focused on foliage Current NPR plant models are generic… …Even though we can create specific species models Let’s start with an exact model and remove unnecessary geometry

Giving Away the Ending Tree model generated with the xfrog systemPen-and-Ink illustration of the complex model

Standard Issues in Traditional Tree Illustration (1) The tree skeleton (trunk) is usually drawn up to the 2 nd branching level –Prominent silhouette lines –Crosshatching for shading How do you determine silhouette? Where do you place the crosshatches? How high is high enough?

Standard Issues in Traditional Tree Illustration (2) Foliage is composed of 3 areas: –Top is usually well-lit, so minimal detail shown –Half-shadow areas show more foliage detail –Deep shadow regions (many techniques used) What shape are the leaves? What shading techniques to use (black, increasing leaf silhouette detail, thicker lines)?

And So, To Begin... 2 Files for stored model –One for the trunk (to 2nd branching level) –One for the leaves (pos. & normal) Trunk is drawn using traditional NPR algorithms Leaves are drawn with some primitive and assigned detail based on depth-difference algorithm

The Tree Skeleton Uses Markosian’s or Rakar & Cohen’s silhouette algorithms Crosshatches placed with the Difference Image algorithm or a Floyd Steinberg variant

The Fun Part: Leaves (!) System uses Zero-Order derivatives for determining important lines on a surface Outlines are drawn if the maximal depth difference of the leaf to neighboring leaves is above a certain threshold –Draw the leaves as solids & get depth buffer –For each pixel, find out how far it is in front of its neighbors –Use that data to draw the leaves

Big Ol’ Equation Time d 0 & d 1 = min and max values represented in the depth buffer z 0 & z 1 = corresponding depth values of near & far clipping planes in camera projection d = depth value [0..1] z = depth in camera coordinate system

More Equation Stuff Depth differences can be computed for eye coords or directly for depth buffer –Latter method results in larger changes Depth difference threshold is determined by a percentage of the depth range of the tree Other numbers: –d 0 = 0 & d 1 = –z 0 = 1 & z 1 = 11 (approx. for real trees)

Varying Equation Inputs - Disc size = Threshold = 1000 Tree 1 with varying primitive size & threshold - Disc size = Threshold = 2000

Essentially, here’s what you get: –Since depth buffer differences are non-linear, you get detail up front & less in the back –Changing ratio of z 1 to z 0 alters non-linearity –Primitive size based on the depth of the tree Free LOD Primitive size & threshold constantPrimitives enlarged with distance

Primitive Choices Primitive shapes can also be altered for a more accurate representation of real leaves – one could use an actual 3D model... – …or use a subset of possible views Shadow in black, threshold = 1000Shadow by detail, threshold = 6000

The Software Framework 1: Determine depth differences –Interactive: both stem & foliage done together 2: Software shadows are created & stored 3: Draw the pixels above the threshold For higher-quality images: –Vectorize the stem & foliage bitmaps separately least-square fitting index buffer (stores primitive ID) –Draw polygons by spline interpolation

Results & Measures (a): 13,200 elliptic primitives - 10 seconds (SGI Octane (Maximum Impact)) (c): 200,000 leaves reduce to 16,200 particles (f): 90,000 tree particles, 23,000 grass particles - 1 minute Interactive: 3 trees of 20,000 particles each and 25,000 ground particles at 3 frames/sec (SGI Onyx 2)

Conclusion & Questions Future Work: –Crosshatching on the leaves –Needs continuous LOD –Cartoons & other non-realistic effects My Questions: –What, exactly, is d? –Advantages over previous (non-species- specific) efforts? –3 frames-per-second. Gee, that’s helpful.