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5.03.2012 GPU Gems: Chapter 7. Rendering Countless Blades of Waving Grass NVIDIA Developer Zone

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GPU Gem: Chape 7. Rendeing Conle Blade of Waing Ga

Last updated: Apr 05 2011

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Chapter 7. Rendering Countless Blades of Waving Grass

Kurt Peler

Piranha Btes

7.1 Introduction

To sim ula te an idy llic na tu re scene in a r ealistic fashion, besides detailed trees and bushes, as well as a

com plex water a nd sky dome sim ula tion, we need a hig h-quality gr ass effect. We must be able to cov er

large a reas of the terr ain w ith it, w ithout m onopolizing t he GPU. The grass should look na tur ally grown

and should wav e realistically in the win d.

In th e past, a h igh-qualit y gr ass simu lation would hav e been considered too complex for r eal-time

applications. The Codecreatu res Benchm ar k (published by Codecult in 2 002) disprov ed this pessim istic

assert ion. See Figu re 7 -1. In this cha pter, w e describe a flexible, widely applicable grass sim ula tion

based on th e gra ss effect shown in th e benchm ark . Additionally , a special v ersion of our Codecreatu res

Benchm ark application is included in th is book's accompany ing m aterial, w hich offers an int eractiv e

demo m ode.

Dev eloper Cen ter s Tech nologies Tools

Resou rces Com m un ity


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Figure 7-1 Screenshot of the Codecreat ur es Benchm ar k Application

7.2 Overview

First, we should realize that a detailed modeling of the indiv idual bla des of gra ss is not m eanin gful,

becau se the num ber of poly gons that w ould be required for lar ger m eadows would be m uch too hig h. A

scene with count less blades of poly gonal gr ass would not be display able in real t im e with the gra phics

hardware av ailable today.

So we ha v e to build a simple and useful alter nat iv e that m eets the following conditions:

Many blades of gra ss mu st be represented by few poly gons.

Grass mu st appear dense from different lin es of sight.

In th e next section, we build gr ass objects that meet t hese conditions.

Additionally , we m ust be able to anim ate th e grass realistically . In Section 7 .4, we discuss thr ee

different animation methods.

7.3 Preparation of the Grass Objects

As we just ment ioned, m any blades of grass mu st be represented by few poly gons. We start by solv ing

this problem. Independent of the cam era position an d direction, the appearan ce should be like that of an

open coun tr y side. Fortun ately , the solut ion is not too difficult . In Section 7 .3 .1 , we start by combining

sev eral blades of grass and display ing t hem in one textur e. But th is is not enoug h: som e poly gons that

use this texture m ust be combined in such a way that the indiv idual poly gons are not noticeable (see

Section 7 .3.2 ). When th e v iewer m ov es around, we a dd or r emove gr ass objects in the distance by

blending t hem in or out. This ensures that the complete gra ss effect w ill hav e robust v isual qua lity .

7.3.1 Grass Texture


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Now let us see how t o build a text ur e for th e task we ha v e to solv e. The requir ed textur e has to cluster

sev eral blades of grass; otherw ise, it will h av e large tran sparent areas. We obtain th is simply by

draw ing solid grass stem s in a tran spar ent alpha cha nnel. In the color chann el, we should use different

shades of green an d y ellow to get a better differentiat ion of sing le blades. We m ay wa nt t o simu late

blades of gra ss in good and bad conditions, to represent differences in a ge or r ipeness, and ev en t o

disting uish front and back faces of the blades.

A concr ete exam ple of a gra ss textu re is shown in Figu re 7 -2.

Figure 7-2 Schematic of a Gra ss Texture

7.3.2 Grass Objects

This section explains how to combine som e poly gons, mapped wit h th e grass textu re built in the

previous section, in a way that the simulat ed grass appears dense, and without h ighlight ing indiv idual

poly gons. The technique also guar antees that the indiv idual poly gons are not v isible.

Because the user can nav igate freely thr ough the scene, a construction similar to the one shown in

Figure 7 -3 would be insufficient to produce a conv incing effect. A linear a rra ngement of the gr ass

poly gons would imm ediately ma ke the stru cture recognizable if someone were to v iew th e scene

perpendicular relativ e to the direction of the poly gons. Additionally , th e gra ss would look v ery thin in

this case. An arr angem ent like this one should be considered only with aut oma tic cam era na v igation or

unreachable, far-distant meadows.

Figure 7-3 Linear Arrangement


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To ensure good visual qualit y independent of the curr ent line of sight, w e hav e to cross the grass

poly gons. Using configurations that look like stars prov es very worthwh ile. Figur e 7 -4 presents two

possible v ar iant s of "gra ss objects," consisting of thr ee intersecting quads. We hav e to render th e

poly gons with disabled back-face cullin g to achiev e v isibility on both sides. To attain proper

illum ina tion, we should orient t he norm al v ectors of all v ertices par allel to the poly gons' v ertica l edges.

This gu ar ant ees correct lig ht ing for all g ra ss objects situ ated on slopes, with no differences due to the

brightness of the terrain.

Figure 7-4 Grass Objects

If we set th ese gra ss objects quit e close together in a lar ge ar ea, as shown in Figu re 7 -5, sort them bac k-

to-front a t r untim e, use alpha blending, an d enable z-testing/wr iting in the draw call, th en the

impression of a natu rally and thickly grown m eadow a ppears.

Figure 7-5An Expanse of Grass

7.4 Animation

To continu e with the next step, we wan t to realistically anim ate th e grass of a complete m eadow, built

with "gra ss objects" like th ose presented in Figu re 7 -5. This section describes three different v ar iant s of

anim ation. Each h as its pros and cons. Section 7 .4 .1 presents the gener al idea of our a nim ation


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met hods. In Section 7.4 .2 , clusters of sev eral gr ass objects stan ding close together ar e anim ated in the

same way . In Section 7 .4.3 , each v ertex gets its own tr anslation v ector. Finally, in Section 7 .4.4 , we

try to find the golden m ean: a different a nim ation for each g rass object.

7.4.1 The General Idea

In order t o achieve a high ly realistic anima tion, w e are going to use a calculation based on

trig onometr ic functions, especially sine and cosine. This calculat ion should take into accoun t the

position that h as to be moved (whether it is a v ertex or the center of an object or clu ster) and the

curr ent tim e. Also, th e direction a nd strength of the prev ailing w ind will be factors. Each of our

techn iques moves only the upper v ertices of the gra ss objects. In a v ertex shader it is easy to

differentiat e between these v ertices and the lower ones by ex am ining t he textu re coordina tes. All upper

vertices should hav e the same v coordina te for t he gr ass textur e: such as zero, or a v alu e close to it. Th e

framework in t he v ertex shader code, as shown in Listing 7 -1 , is the same in all th ree techniques; only

the pur e anim ation part differs. The anim ation code can be foun d in the following sections.

Example 7-1. Framework in the Vertex Shader

//

// E C / HLSL

// S 7.4.2, 7.4.3, 7.4.4

//

VS_INPUT {

3vPosition : POSITION;

3vNormal : NORMAL;

2TexCoords : TEXCOORD0;

// T S 7.4.4

3vObjectPosition : TEXCOORD1;

;

VS_OUTPUT {

4 vPosition : POSITION;

4 vDiffuse : COLOR;

2TexCoords : TEXCOORD0;

;

VS_TEMP {

3vPosition; 3vNormal;

;

44 mWorldViewProjMatrix;


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4 vLight;

fObjectHeight;

VS_OUTPUT main( VS_INPUT v)

{

VS_OUTPUT out;

VS_TEMP temp;

// A

(v.TexCoords.y = 0.9)

// A N I M A T I O N ( )

// I 7.4.2, 7.4.3, 7.4.4

. . . //


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inter ru pt the renderin g of a complete meadow quit e often and use a separat e dra w call for each clu ster.

Pros

Complex animation calculations are made through CPU-based algorithms.

Ther e are n o distortions, because of the constant distan ce of the upper v ertices of a polyg on.

Cons

Many draw calls are required to display a complete meadow.

Clusters ma y be appar ent due to sy nchronized anim ation of all v ertices of a complete object

cluster.

Algorithm

1 . On the CPU, calculate the cur rent tr anslation v ector for th e next cluster using the position of thecluster's center .

2. Set the translation v ector as a constant for the v ertex shader.

3. Execute a draw call for the cluster.

4. In the v ertex shader, add the tra nslation v ector to the positions of the upper v ertices. See Listing 7 -

2 .

Example 7-2. Code for Animation per Cluster of Grass Objects

//

// A C G O (7.4.2)

//

3vClusterTranslation; // C CPU

VS_OUTPUT main( VS_INPUT v){

. . .

// A N I M A T I O N ( )

// H 7.4.2

temp.vPosition = v.vPosition + vClusterTranslation;

temp.vNormal = (v.vNormal * fObjectHeight +

vClusterTranslation);

...

7.4.3 Animation per Vertex


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One of the m ain pr oblems wit h t he m ethod discussed in Section 7 .4 .2 is poor performa nce because of the

high num ber of draw calls, which indiv idually render only a small n um ber of poly gons. It would be

better if we could render a large ar ea cov ered with gra ss by using a m uch lower num ber of draw calls.

Howev er, we h av e to relocate th e complete anim ation computa tion into the v ertex shader to be able to

mov e each v ertex separately , relativ e to its position. See Figure 7 -7 .

Figure 7-7Anim ation per Vertex

Because the tr anslations for each v ertex ar e computed indiv idually , th e length of the edge between the

upper v ertices of the gr ass poly gons is no longer c onstan t, as shown in Figur e 7 -8. Th erefore, v isible

distortions may appear because of the inconstant lengt h a nd th ickness of each bla de of gra ss, but

ty pically th ese artifacts will not be very noticeable.

Figure 7-8 Textu re Distort ion

Additionally , the overall effect may seem m ore unn atur al tha n in th e prev ious method. Because the

tran slation of all v ertices in a n earby region is v ery similar , an absence of local chaos and a v eryhomogeneous anim ation results. We are able to elimin ate th is disadv ant age by using a pseudo-random

function in t he v ertex shader to achiev e more v aried results.

Pros

Only a few draw calls, perha ps ev en just one, are necessary to display a complete meadow.

Vary ing th e v ertex position in th e v ertex shader allows for th e continu ity of a rippling wa v e of

wind.

The clusters are indistinguishable.

Cons


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Distortion appears, due to the v ar iable distan ce of the u pper v ertices of a poly gon.

Anim ation m ay appear homogeneous due to a lack of local chaos.

The complexity of the anim ation calculation is limit ed.

Algorithm

1 . Set constants, such as time stamp and th e basic strength a nd direction of the win d, for th e v ertex

shader.

2. Execute one draw ca ll for the com plete m eadow or large ar ea of grass.

3 . Use the ver tex shader to calcu late anim ation based on v ertex position. See Listing 7 -3.

Example 7-3. Code for Animation per Vertex

//

// A V (7.4.3)

//

fTimeStamp;

3vWindDirection;

fWindStrength;


{

. . .

// A N I M A T I O N ( )

// H 7.4.3

3vVertexTranslation = CalcTranslation(v.vPosition,

fTimeStamp,

vWindDirection,

fWindStrength);

temp.vPosition = v.vPosition + vVertexTranslation;


vVertexTranslation);

. . .

7.4.4 Animation per Grass Object

To incr ease the apparent v isual complexity of the anim ation based on th e meth ods presented in Sections

7 .4.2 and 7 .4.3 , we combine an u ndistorted grass texture and a low nu mber of draw calls with local

cha osand th ereby gain the adv ant ages of both m ethods. We are able to combin e these methods

becau se we do not compute th e anim ation for each v ertex based on it s position; ra ther, w e do it based on


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the cent er position of the gra ss objecteach consisting of three inter secting qua ds (see Section 7 .3 .2 ).

Because neighboring gr ass objects now h av e different an im ations, we can r epresent th e desired local

chaos, as shown in Figur e 7 -9. Additionally , th e constant anim ation for each gr ass object prev ents the

horizonta l text ur e distortions.

Figure 7-9 Anim ation per Grass Object

To ma ke this possible, each v ertex m ust know th e center position of its object, either relat iv e to its

position or absolut e in t he w orld. The gr ass object position v ector needed for t his informa tion m ust be in

the v ertex form at (t hat is, stored in textu re coordinates), because the v ertex shader has to read this

value.

Pros

Only a few draw calls, perha ps ev en just one, are necessary to display a complete meadow.

Ther e are n o distortions, because of the constant distan ce of the upper v ertices of a polyg on.

Local v arian ce creates a more natu ral look.

Cons

Additional data is required in the v ertex form at, because each v ertex also contains th e center

position v alu e of its gra ss object.The complexity of the anim ation calcu lations is lim ited, in order to minim ize shader cost.

Algorithm

1 . Set constants, such as time stamp and th e basic strength a nd direction of the win d, for th e v ertex

shader.

2. Execute one draw ca ll for the com plete m eadow or large ar ea of grass.

3 . In the vertex shader, comput e anim ation based on th e center position of the gr ass object. See

Listing 7 -4.

Example 7-4. Code for Animation per Grass Object


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//

// A G O (7.4.4)

//

fTimeStamp;

3vWindDirection;

fWindStrength;


{

. . .

// A N I M A T I O N ( )

// H 7.4.4

3vObjectTranslation = CalcTranslation(v.vObjectPosition,

fTimeStamp,

vWindDirection,

fWindStrength);

temp.vPosition = v.vPosition + vObjectTranslation;


vObjectTranslation);

. . .

7.5 Conclusion

We hav e succeeded in building a realistic grass simu lation th at m eets the th ree most im portan t

requirements:

Extensiv e usability with out ov erly stressing perform ance

Natural appearance from all lines of sightAnimat ion based on prev ailing w ind conditions (with thr ee different v arian ts)

A special v ersion of the Codecreatu res Benchm ar k application, offering an in tera ctiv e dem o mode as

shown in Figure 7 -1 0, can be foun d on th e book's CD or Web site. In th e application, y ou ca n na v igat e

using a free cam era an d switch the render states. You are encouraged to exam ine this application an d

take a look behin d the scenes!


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Figure 7-10 Screenshot of the Codecreatu res Benchm ark

7.6 Further Reading

If you ar e interested in doing som e more resear ch on sim ula ting anim ated gra ss, here ar e some

resources that deal with this subject.

You ca n find art icles that describe vertex sha ders using a sine fun ction to do the procedural a nim ation

of the gra ss geometr y here:

NVIDIA Corporation. 2003. "Basic Profile Sample Shaders: Grass." In The Cg Toolkit User's Manual.

Av ailable online at hp://deelope.nidia.com/objec/cg_e_manal.hml

Isidoro, J., a nd D. Card. 20 02. "Anim ated Grass with Pixel and Vert ex Shaders." InDirect3D ShaderX,

edited by W. F. Engel. Wordwar e Publishing .

Other demos presentin g g rass effects are a v ailable on th e following t wo Web sites. These two demos also

use vertex shaders to calculate th e wav ing m otions of the grass geometr y :

NVIDIA Web site: hp://deelope.nidia.com/ ie.ap?IO=demo_ga

ATI Web site: hp://.ai.com/deelope/Sample/G a.hml

I would like to thank m colleagues at Piranha Btes and Codecult who contributed to the Codecreatures

Benchmark, espec iall Horst Dw orcak (Lead Artist), w ho had the idea to build and animate the grass

objects in the w a presented in the Benchmark; and Oliver Hoeller (Lead Programmer), who helped to

integrate the complete effect into the engine.

Book

GPU Gem: Pa I - Naal Effec

GPU Gems: Chapter 1 . Effectiv e Water Sim ula tion from Phy sical Models

GPU Gems: Chapter 2 . Rendering Water Ca ustics


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GPU Gem s: Cha pter 3 . Skin in t he "Dawn" Demo

GPU Gems: Chapter 4 . An im ation in th e "Dawn" Demo

GPU Gem s: Cha pter 5. Implem entin g Improv ed Perlin Noise

GPU Gem s: Cha pter 6 . Fire in t he "Vulcan " Dem o

GPU Gem s: Cha pter 7 . Renderin g Countless Blades of Wav ing Grass

GPU Gems: Cha pter 8. Sim ula ting Diffraction

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