Instant Animated Grass

Ralf Habel Michael Wimmer Stefan JeschkeInstitute of Computer Graphics and Algorithms

Vienna University of Technology, Austria{habel,wimmer,jeschke}@cg.tuwien.ac.at

ABSTRACTThis paper introduces a technique for rendering animated grass in real time. The technique uses front-to-backcompositing of implicitly defined grass slices in a fragment shader and therefore significantly reduces the overheadassociated with common vegetation rendering systems. We also introduce a texture-based animation scheme thatcombines global wind movements with local turbulences. Since the technique is confined to a fragment shader, itcan be easily integrated into any rendering system and used as a material in existing scenes.

KeywordsReal-time Rendering, Natural Phenomena, Natural Scene Rendering, GPU Programming

1. INTRODUCTIONInteractive rendering of vegetation in natural scenesplays an important role in virtual reality and computergames where grass is an essential part of most natu-ral scenes. Unfortunately, grass is also very complex:modeling each individual blade of grass would requirea huge amount of geometry, making it impossible torender in real time. Common acceleration techniquesrepresent grass using billboards [Pel04]. However,even these simplified billboards lead to massive over-draw in realistically modeled scenes. Furthermore,placing grass into a scene using geometry requires asignificant storage and modeling effort.In this paper, we present a new method to render ani-mated grass in real time that exhibits all important vi-sual characteristics of grass, namely parallax and oc-clusion effects when the viewpoint moves, as well asanimation due to wind. It is efficient to render andeasy to incorporate into existing rendering systems.The main target are video games with a first personviewpoint where grass is mostly seen at grazing an-gles. Grass is represented using implicitly defined tex-tured billboards perpendicular to the terrain geometrywhich are ray traced in a fragment shader and is there-fore suited for short and dense grass such as lawns

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or meadows. While the billboards could also be de-fined using conventional geometry, the advantage ofan implicit definition is that no extra geometry has tobe generated, only the terrain geometry and its asso-ciated tangent space is required as input. This makesit easy to apply the grass to different scenes and theoutput speed merely depends on the number of pix-els covered by grass rather than on the density of thegrass or the extent of the terrain. Standard lightingtechniques such as dynamic lighting and shadowing,including light maps or precomputed radiance transfercan be used without modification. Furthermore, weanimate grass using a texture based approach that in-corporates both low-frequency phenomena like gustsof wind and high-frequency phenomena like small tur-bulences, leading to a very realistic appearance of therendered grass.

Figure 1: A terrain textured with animated grass withmoderate grass density and height.

2. RELATED WORKIn order to display complex volumetric effects suchas fur and short hair, Kajiya and Kay [KK89] intro-duced volumetric textures, called “texels”. In this con-text, texels are representations of a three-dimensionalmaterial by a cubic reference volume that is mappedonto a surface repeatedly. A texel itself is a three-dimensional array approximating the visual proper-ties of a micro-surface. They were created to solvethe problem of spatial aliasing of ray-traced com-plex geometries. Rendering a texel involves front-to-back compositing along a ray, which is also usedin our method. An extension and application to nat-ural scenes of volumetric textures was presented byNeyret [Ney98]. The typical real-time implementa-tion of texels uses stacks of polygons, mapped withsemi-transparent textures [BH02] [Len00] [LPFH01][MN98] . However, slices that are parallel to a terraingeometry are not optimal for viewing positions typicalfor walkthroughs, with objectionable artifacts at graz-ing angles.The most common way to represent grass (also usedin many current games) are billboards mapped with atexture of several grass blades [Pel04]. The billboardvertices are usually animated analytically. However,a massive amount of polygons is required to denselycover a terrain, and analytical animation looks veryuniform.Perbet and Cani [PC01] combine different grass repre-sentations at different distances to render and animateprairies in real time. In the nearest level of detail, grassblades are modeled individually, which makes it diffi-cult to combine various types of grass and flowers.The proposed algorithm is closely related to real-timerelief mapping [POaLDC05] [OP05] [WWT+03]. Incontrast to those methods, which are based on ray trac-ing a height field within a shell on the surface of anobject, the presented method ray traces a regular grid.

3. RENDERING GRASSMotivationWe model grass as a collection of textured billboards,and arrange them in a regular grid.Typically, a grass texture is fully transparent betweenthe individual grass blades and fully opaque within theblades. However, partial opacity arises at the edges ofthe grass blades if the grass texture is a filtered versionof a higher resolution texture, or if it has been gener-ated using an anti-aliased renderer in the first place.Therefore, the colors and opacities of billboards over-lapping in screen space need to be correctly compos-ited. Just as in volume rendering, this can be done ei-ther in back-to-front or front-to-back fashion [LL94].Back-to-front compositing corresponds to standardtransparency alpha blending used when rendering the

billboards as geometry. However, back-to-front com-positing can be very inefficient because all slices haveto be traversed in order to get a correct result. Further-more, if the billboards intersect each other, a consistentback-to-front order does not exist. The popular alter-native of using alpha testing instead of alpha blendingleads to noticeable aliasing artifacts especially at theedges of the grass blades.Front-to-back compositing, on the other hand, is typ-ically used with ray tracing and allows for early raytermination when the accumulated opacity is suffi-ciently high. We exploit this fact for grass render-ing in the following way: Instead of rendering thetextured grass billboards using polygons, we definethem implicitly on a “carrier polygon” and ray tracethese “virtual billboards”’ in the fragment shader usingfront-to-back compositing (also known as the “over”-operator [PD84]). This allows exiting the fragmentshader when the opacity reaches a user-defined thresh-old. Furthermore, intersecting billboards are handledautomatically, always giving correct compositing re-sults. We have found that the illusion of grass can beperfectly maintained even when doing a small, fixednumber of iterations, which is more amenable to cur-rent graphics hardware.The setup of the ray tracing step is very similar to re-lief mapping [POaLDC05], where a height map, de-fined in a shell carried by polygons is ray traced in thefragment shader. As with relief mapping, the regulargrid of grass billboards therefore seem to reside insidethe carrier polygon (see Figure 2).

Figure 2: A quad patch (wireframe overlay) renderedwith fully opaque textures. The grid structure is gen-erated in the fragment shader.

The most significant advantage of rendering grass inthe fragment shader may be the ease of modeling and

integration into existing rendering systems. Grass canbe defined as a material and does not require any otherchange in the scene definition (whereas in polygonalrendering, each grass billboard has to be placed eitherby hand or automatically). Furthermore, we will showhow to animate the ray traced grass in high quality.

Grass Ray TracerThis section describes the grass ray tracer in more de-tail. A basic grass patch consists of the ground textureand a texture containing one subtexture for each vir-tual billboard (or slice) in the patch. We currently usethe same set of billboard textures for both axes of theregular grid. For current graphics hardware, we allowthe raytacing loop to exit before full opacity has beenreached. The remaining opacity can be filled usinga constant color or an additional, fully opaque grassslice (Figure 4). The fragment shader casts rays intoa shell defined by the carrier polygon at the top, anda virtual ground polygon at the bottom that is offsetby a user-defined distance along the negative tangent-space w axes at the vertices (i.e., the inverted normalvectors).

Figure 3: A ray is cast from the viewing point throughgrass slices.

Grass can be applied to any mesh in a scene that has atangent space (u,v,w) defined. A basic grass patch istiled onto the whole mesh. Note that this leads to sim-ilar restrictions as with relief mapping, where silhou-ettes are difficult to define. Additionally, analogousto relief mapping, the viewpoint cannot move into thegrass.The fragment shader takes as input the interpolatedtangent space vectors, the view vector ~v in tangentspace (interpolated from ~p−~s at each vertex ~p andviewpoint ~s), and the interpolated texture coordinates(which give the ray entry point ~e see Figure 3). Theuser also has to provide the parameters du,v for the dis-tance between the slices in tangent space and the depthof the ground plane h. The shader executes the follow-ing steps:

1. Calculate for both u and v a texture offset to selectthe initial grass slices.

Figure 4: A grass data set consisting of grass blades(left), a ground texture (right) and a fully opaque grassslice (bottom). Note that as in any texture packingmethod, a one-texel border needs to be observed be-tween grass slices.

2. Adjust this offset depending on the sign of theview vector so the same slice is seen from bothsides.

3. Calculate the positions pu,v of the first planes tobe ray traced in both u and v directions accordingto du,v using a floor() operator.

4. Enter the raytacing loop.

The inner ray tracing loop consists of the followingsteps:

1. Calculate the intersections with the next slice in uand v direction. Since the slices are axis aligned,the ray-plane intersection

~x =~e+~v ·~np · (~p−~e)

~np ·~v, (1)

where~np is the normal vector and ~p is an arbitrarypoint on the plane, simplifies to

~x =~e+~v ·pu,v,w− eu,v,w

vu,v,w, (2)

depending on which axis is used.

2. Choose the closer intersection point and incre-ment (or decrement, depending on the sign of v)the corresponding slice by du,v.

3. Test intersection point against the virtual groundpolygon. If the intersection is outside the shell,intersect the ray with the ground polygon usingequation 2.

4. Composit the current color ~c with the color ofthe slice ~ci (with associated alpha values α andαi) using the standard “over” blending function,

which assumes that colors are premultiplied withtheir corresponding opacity values:

~c = ~c+(1−α) ·~ci

α = α +(1−α) ·αi (3)

After the raytracing loop, the remaining transparencyis filled with a texture lookup from the fully opaquegrass slice or the average color of the grass data set. Asingle grass patch rendered with the data set of Figure4 using 16 slices for both u and v axes can be seenin Figure 5. We have found that a very low number (4was used in the images shown) of ray casting iterationsis sufficient for high image quality. This helps to keepthe number of required texture reads low.

Figure 5: A quad patch rendered with the data set ofFigure 4. The grid structure is apparent at perpendicu-lar angles but vanishes at more grazing angles.

Especially for higher grass, if the grass patch is ex-pected to be viewed at perpendicular angles, the gridstructure becomes apparent. This can be mostlyavoided by adding a horizontal grass slice (in the mid-dle of the shell) which is ray cast just like the verticalslices (Figure 6).

Visibility InteractionsIf the grass is to interact with the rest of the scene,visibility with scene objects has to be resolved. Other-wise, the objects will be clipped against the top of thegrass (Figure 7). The correct solution would be to ren-der the opaque objects first and generate an offscreenbuffer with the corresponding depth information (forexample using the multiple render target functionalityfound in current graphics hardware). When renderingthe grass, the depth value at which to terminate a raycan be read from this buffer.However, this method requires a non-trivial modifica-tion of the rendering pipeline, and therefore we optedfor a simpler solution. Instead of testing the ray againstthe current depth buffer, we generate a depth value di-rectly in the fragment shader by calculating the depth

Figure 6: A quad patch with the same data set as inFigure 5 but with an additional horizontal plane at halfthe ground depth. The grid structure is not dominanteven at perpendicular angles.

Figure 7: A grass patch with (left) and without (right)correct visibility.

when a threshold opacity has been reached. Depend-ing on the hardware used, it may prove to be effi-cient to terminate the ray casting loop through an earlyout if the threshold opacity is reached. The fragmentshader then outputs a depth value determined from theslice distance. This does not require any modifica-tion of the rendering pipeline and gives correct occlu-sion for the fully opaque parts of grass blades. Thesemi-transparent parts of grass blades are not handledexactly, but the introduced errors are unnoticeable inpractice.

4. ANIMATING GRASSThe perceived realism of rendered grass dependsgreatly on whether it is animated or not. Previousmethods to animate grass relied on analytic functions(usually combinations of sines and random perturba-tions) applied to billboard vertices, which results infairly simple grass blade movement. A realistic simu-lation of grass movement has to take two componentsinto account. On the one hand, gusts of wind causerelatively large areas of grass to bend in the same di-

rection. On the other hand, high frequency wind turbu-lences near the ground cause smaller, but more randommovements of grass blades.In this paper we propose a texture-based animationscheme for grass billboards. Instead of animating thebillboard vertices, we distort the texture lookups of thegrass billboard horizontally in u or v direction, depend-ing on the billboard orientation. This offset is lookedup in a separate noise map that covers the whole meshand not only an individual grass patch. The offset isscaled with the height above the ground plane of thegrass so that at the bottom of the grass billboards stayfixed. The noise map is translated each frame to definethe overall wind direction. Although this is a shearoperation, with sufficiently small pertubations the im-pression of moving grass can be maintained. Notethat this animation technique works both for standardpolygonal billboards as well as for our raytraced vir-tual billboards.The advantage of texture-based animation is that anyprocedural or hand-crafted texture can be used, whilethe animation over the whole mesh will always remainconsistent. The noise texture map used in this paper isa combination of two Perlin noise functions [Per85].A low-frequency noise function with higher ampli-tudes simulates gusts of wind, and a high-frequencynoise map with lower amplitudes introduces more er-ratic movements to the grass blades.

5. RESULTSThe proposed method was implemented on a 3.2 GHzPentium 4 and a GeForce 7900 GT, using DirectXHLSL Shader Model 3.0 and the OGRE [OGR] opensource graphics engine. To generate grass slices, thecommercial 3D software Maya and its PaintFX fea-tures were used (Figure 4). The accompanying videoshows a scene with 8× 8 grass patches, where eachpatch contains 16 slices in u and v direction (Figure1) and a second, denser and shorter grass data set with32× 32 slices in u and v (Figure 8). The grass is an-imated using the noise map described in the previoussection, and visibility with shown polygonal objects isresolved correctly.The performance of the algorithm depends on thenumber of pixels covered and on the ray-casting it-eration depth. The camera path shown in the video,rendered at a resolution of 1024× 768 and an itera-tion depth of 4, results in an average frame rate of 140frames per second.For comparison, we generated a simple scene withgrass billboards represented by hand-placed billboardpolygons using standard alpha blending. With 32× 2slices per grass patch and 8 × 8 patches, an aver-age frame rate of 90 frames per second can be ob-tained. Compared to the billboard implementation,

our method incorporates correct alpha blending, tex-ture based animation and does not require the geome-try to be modeled by hand. The much higher perfor-mance of our method can be explained by the reducedoverdraw and the fact that current hardware is fill-rateoptimized.A HLSL implementation of the grass shader and thetextures used in this paper can be found at [Hab].

Figure 8: A terrain textured with short, dense grass.

6. CONCLUSION AND FUTUREWORK

This paper has introduced a new method to render ani-mated grass in real time. Instead of rendering polygo-nal billboards, the technique uses front-to-back com-positing of implicitly defined grass slices in a frag-ment shader and therefore significantly reduces theoverhead associated with rendering dense vegetationscenes. One of the main advantages of the methodis its ease of integration: as all operations are refinedto a single fragment shader, grass can simply be in-corporated as a material into any existing scene andrenderer that supports hardware shading, thus avoidingthe tedious modeling effort and storage costs of geo-metric grass billboards. Furthermore, we have shown atexture-based animation technique that combines con-sistent global wind motion with small, high-frequencyperturbation, leading to a much more natural impres-sion than previous analytic methods.The performance of the shader is independent of thedensity of the grass, so a massive amount of slices canbe rendered. Standard filtering techniques and levelsof detail can be used with the proposed method. Weare currently investigating methods to display silhou-ettes of grass by adapting higher order surface approx-imations to the presented algorithm and ways to movethe camera into the grass consistently. Furthermore, itshould be easy to break up the regularity of the grasspatches using Wang tiling [CSHD03]. Finally, we are

investigating ways to incorporate advanced lightingtechniques like self shadowing to even further increaserealism.

7. ACKNOWLEDGEMENTSThis research was funded by the Austrian ScienceFund (FWF) under contract no. P17261-N04. The au-thors would like to thank Oliver Mattausch for veryhelpful discussions and comments.

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