OpenGL Grass rendering

Question

I have been researching different techniques for rendering grass. I've decided to go with a Geometry shader generated grass mainly so I can generate triangle fans on the fly when I render them as GL_POINTS but I'm not seeing the performance I'd like to see. I'm getting maybe 20-50 fps with 100,000 blades of grass, and I have a decent GPU. I'm wondering if my approach is wrong, or if I'm reaching the limitations of my GPU or If I am doing something incorrectly or maybe if their is a faster way (My aim is individual blades where I can manipulate the vertices ideally). The texture I am using 256x256

My rendering steps are:

Creation of the VAO and VBO and storing locations and binding once:

   float[] GrassLocations= new float[100000];
   int vaoID = createVAO();
 . //bind VBO to VAO
   storeDataInAttributeList(0, 3, GrassLocations,0,0);

I then render:

    GL30.glBindVertexArray(VAO);
    GL20.glEnableVertexAttribArray(0);
    GL13.glActiveTexture(GL13.GL_TEXTURE0);
    GL11.glBindTexture(GL11.GL_TEXTURE_2D, texture);
    GL11.glDrawArrays(GL11.GL_POINTS, 0, 100000);
    GL20.glDisableVertexAttribArray(0);
    GL30.glBindVertexArray(0);

then My Vertex Shader:

#version 400


layout (location = 0) in vec3 VertexLocation;


uniform float time;

out vec3 offsets;
out vec3 Position;
out vec3 Normal;
out vec2 TexCoord;
out float visibility;

uniform mat4 transformationMatrix;

uniform mat4 viewMatrix;
uniform mat4 MVPmatrix;
uniform mat4 modelViewMatrix;

const float density = .007;
const float gradient = 1.5;
out float Time;

void main()
{
Time = time;

vec4 worldPosition = transformationMatrix * vec4(VertexLocation,1.0);
vec4 positionRelativeToCam = modelViewMatrix* vec4(VertexLocation,1.0); 

Normal = vec3(0,1,0);
Position = vec3(  worldPosition );

gl_Position = MVPmatrix* vec4(VertexLocation,1.0);

float distance = length(positionRelativeToCam.xyz);
visibility = exp(-pow((distance * density), gradient));
visibility = clamp(visibility,0.0,1.0);
offsets = offset;


}

I did gut the vertex shader and left only GL_POSITION and still not the issue. My Geometry Shader:

#version 400

layout( points ) in;
layout( triangle_strip, max_vertices = 10 ) out;

float Size2=1;   // Half the width of the quad



in vec3 Position[];
in vec3 Normal[];
in vec3 offsets[];

out vec3 position;
out vec3 normal;
in float Time[];

out vec2 TexCoord;
out vec3 color;
const  float width = 5;
void main()
{
position = Position[0];
normal = Normal[0];

color = offsets[0];

gl_Position =  (vec4(-Size2*width,-Size2,0.0,0.0) + gl_in[0].gl_Position);
TexCoord = vec2(0.0,0.0);
EmitVertex();

gl_Position =  (vec4(Size2*width,-Size2,0.0,0.0) + gl_in[0].gl_Position);
TexCoord = vec2(1.0,0.0);
EmitVertex();

gl_Position = (vec4(-Size2*width+(Time[0].x),10,0.0,0.0) + 
gl_in[0].gl_Position);
TexCoord = vec2(0.0,.25);
EmitVertex();

gl_Position = (vec4(Size2*width+(Time[0].x),10,0.0,0.0) + 
gl_in[0].gl_Position);
TexCoord = vec2(1.0,.25);
EmitVertex();
///////////////////////////////////////////////////
gl_Position =  (vec4(-Size2*width+(Time[0].x)*2,15,0.0,0.0) +   
gl_in[0].gl_Position);
TexCoord = vec2(0.0,.50);
EmitVertex();
gl_Position =  (vec4(Size2*width+(Time[0].x)*2,15,0.0,0.0) +   
gl_in[0].gl_Position);
TexCoord = vec2(1.0,.50);
EmitVertex();   
///////////////////////////////////////////////////
gl_Position =  (vec4(-Size2*width+(Time[0].x)*3,25,0.0,0.0) + 
gl_in[0].gl_Position);
TexCoord = vec2(0.0,.75);
EmitVertex();

 gl_Position =  (vec4(Size2*width+(Time[0].x)*3,25,0.0,0.0) + 
gl_in[0].gl_Position);
TexCoord = vec2(1.0,.75);
EmitVertex();  
 ///////////////////////////////////////////////////
gl_Position =  (vec4(-Size2*width,Size2*7,Time[0].x,0.0) +   
gl_in[0].gl_Position);
TexCoord = vec2(0.0,1.0);
EmitVertex();

gl_Position = (vec4(Size2*width,Size2*7,Time[0].x,0.0) +     
gl_in[0].gl_Position);
TexCoord = vec2(1.0,1.0);
EmitVertex();    
}

and my fragment Shader: (This is in a deferred engine, I've tried it with forward rendering also and I don't think performance hit is here)

#version 400

in vec2 TexCoord;

layout (binding=0) uniform sampler2D SpriteTex;
in vec3 color;
in vec3 normal;
in vec3 position;
layout( location = 0 ) out vec4 FragColor;

void main() {
vec4 texColor =  texture(SpriteTex,TexCoord);
vec4 posColor = vec4(position.xyz,0);


gl_FragData[1] =  posColor;
gl_FragData[2] =  vec4(normal,1);


if(texColor.a<.5){
discard;
}


gl_FragData[0] = texColor;

}

I can't really tell but i guess it might be faster to go with a plane approach. — Felix K., Sep 01 '15 at 11:04
Oh yea, that is definately faster if I skip the Geometry stage and go with plain point sprites but I don't have the option of animating the vertices and creating realistic grass movement. — , Sep 01 '15 at 11:11
No not point sprites, i said plane not point sprite. Use instancing with planes as mesh. — Felix K., Sep 01 '15 at 11:19
Thanks Felix, I'm looking at how it is done now . I will try and post the result today. — , Sep 01 '15 at 12:11
Geometry shaders are quite slow, maybe this [paper](https://www.cg.tuwien.ac.at/research/publications/2013/JAHRMANN-2013-IGR/JAHRMANN-2013-IGR-paper.pdf) helps. They use a quad + tessellation shaders instead. — BDL, Sep 01 '15 at 14:37

score 4 · Answer 1 · answered Sep 01 '15 at 13:42

What you want is a technique called Instancing. The tutorial I've linked is fantastic for figuring out how to do instancing.

I would probably advise that you avoid the geometry shader (since the geometry shader doesn't usually scale well when its purpose is to expand the quantity of vertices), and instead just define a buffer containing all the vertices necessary to draw a single blade (or patch) of grass, then use instancing to redraw that object thousands of times.

OpenGL Grass rendering

1 Answers1