We Could Plant a House, We Could Build a Tree

Well we now have terra firma under our feet, but after walking around the world for a while, the countryside still feels rather empty. This is the point in the development cycle where the world artists commence production on all kinds of various landscape art assets – anything from flora, fauna, and rocks to waterfalls. From trees, even to their fallen ancestors, all in order to help us feel a little more grounded and at home within our world.

Cyrodiil, nestled in the heart of Tamriel, is a richly diverse and lush section of countryside. As Imperial naturalists, historians, and TES fans know alike, there are dense swaths of forests and rolling green pastures to be found in the hills and valleys surrounding the Imperial City. As artists, we couldn’t really get by with building just a few static trees and dotting them here and there across the land. We need veritable reams of trees, forests ripe for exploring, vast pastures, and foliage filled meadows teeming with flora to trek through.

How in the world were we to do this? Buildings and swords are one matter – we’ve tackled these types of models pretty successfully before, but lush, moving vegetation? High detail trees that sway and interact with dynamic wind? These are no small undertakings in 3D worlds that are generally constructed out of straight lines and hard angles. In order to pull off this kind of graphic effect successfully, we needed to add some new tools to our repertoire. Fortunately for us this time around, the Oblivion developers had a few tricks up their collective sleeves.

To tackle the problem of creating high detail, and fully animating foliage and tree art, we solicited the aid of IDV’s SpeedTree technology. Oblivion artists were able to quickly generate complex and organic tree shapes with relative ease, thanks to a highly streamlined, yet powerful and intuitive interface. Using parent/child hierarchies and iterative branch levels comprised of highly modifiable cylinder primitives, an entire tree shape can be created in a manner of minutes, just by adjusting numerical values and tweaking spline curve handles.

The resulting shapes are very similar to what the artists normally would painstakingly create by hand, polygon by polygon, within a modeling program. Perhaps best of all, the tree objects created save the artists even more time by being innately rigged for animation. After the programmers made our engine render and animate the trees, artists were able to quickly place their models in the game and carefully observe the results in real-time.

Well now that the tree shapes are established, and our world is looking a little less barren now, what about the greenery, foliage, and the very leaves on the trees themselves? Glancing up at an Earthly forest canopy, we are presented with an amazing array of dense, mottled shapes and complex, overlapping patterns of light and color. Rays of sunlight spill downward, filtering through layers of leaves and needles of varying thickness and weight, while the cell and chlorophyll filled vegetation reacts with and modulates the blanket of the starlight in subtle ways.

This is all well and good in description, or for film and pre-rendering, but real-time graphics of this variety and complexity can bring video cards and GPUs to their very knees. Surely we can’t render each leaf individually as that would be an inordinate strain on the computer, taxing the hardware to its limits. Even the interaction of light and foliage in a real-time environment is beyond our current technological reach. Nevertheless, we’d like to achieve these visual effects…somehow.

In the end, to circumvent this particular visual hurdle, the Oblivion artists put their 3D skills to the test and displayed their mettle in organic modeling. Typically, foliage portrayed within interactive entertainment is comprised largely of real-world photographs and Photoshop manipulation. The same is still true for Oblivion, but in this case, the artists really went the extra mile for detail and visual fidelity. In order to try and capture some of the natural translucency and light play that tree greenery and foliage displays, a method of modeling foliage in high detail 3D was developed.

We would start by building the foliage via relatively simple geometry meshes inside of a modeling program and gradually increase the complexity through instancing and clever arranging, until an entire tree bough was created. Using these shapes, combined with high detail leaf/vegetation images and a controlled, fixed camera perspective, we were able to render out crisp and colorful leaf textures that gave us better results than those we could hope to capture in the real world. These renderings were in turn converted into textures that were placed upon the trunk shapes as foliage. As with the branch meshes, these naturally animate and vary in motion - effectively giving the trees that you see within the game world a much more fluid and lifelike quality.