OpenGL: Managing and editing large amounts of line strips

Question

I am currently working on a small 2D game with LWJGL. I use line strips to draw randomly generated grass. Each of the blades consists of 6 points and has a random green color.

The problem I face:

To fill the ground with a gapless layer of grass, I need approx. 400 line strips... In addition, every line strip has to be shifted, when the player moves around and should (optionally) wave in the wind. Therefore I need to change the data of 400 vbos every frame.

Is there any way to accelerate these operations?

My Code:

//upload the vertex data
void uploadGrass(int offset){
    FloatBuffer grassBuffer=BufferUtils.createFloatBuffer(5*5);
    for(int i=0;i<Storage.grasslist.size();i++){
        if(grassvbo[i]==0){
            grassvbo[i]=GL15.glGenBuffers();
        }
        grassBuffer.clear();
        for(int j=1;j<6;j++){
            grassBuffer.put(Utils.GL_x((int) Storage.grasslist.get(i)[j][0]-offset));
            grassBuffer.put(Utils.GL_y((int)  Storage.grasslist.get(i)[j][1]));

//index 0 of every blade contains RGB values for the color.
            grassBuffer.put((float) Storage.grasslist.get(i)[0][0]);
            grassBuffer.put((float) Storage.grasslist.get(i)[0][1]);
            grassBuffer.put((float) Storage.grasslist.get(i)[0][2]);
        }
        grassBuffer.flip();
        GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER,grassvbo[i]);
        GL15.glBufferData(GL15.GL_ARRAY_BUFFER, grassBuffer, GL15.GL_STATIC_DRAW);
        GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER, 0);
    }
}

//draw line strips
void drawGrass(){
    GL20.glUseProgram(pId2); //color shader
    for(int i=0;i<grassvbo.length;i++){ //go through all the vbos
        GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER,grassvbo[i]);
        GL20.glVertexAttribPointer(0, 2, GL11.GL_FLOAT, false, 5*4, 0);
        GL20.glVertexAttribPointer(1, 2, GL11.GL_FLOAT, false, 5*4, 2*4);
        GL20.glEnableVertexAttribArray(0);
        GL20.glEnableVertexAttribArray(1);
        GL11.glDrawArrays(GL11.GL_LINE_STRIP, 0, 5);
    }
    GL20.glUseProgram(0);
    GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER,0);
    GL20.glDisableVertexAttribArray(0);
}

until now it looks like that ;) (still needs antialiasing and alpha blending): http://i.imgur.com/x3qXlQ5.png

Answer 1

Chapter 12 of the OpenGL SuperBible has a section on "Drawing a lot of Geometry Efficiently", in which they have a demo of millions of blades of grass being animated. This is done by using a single vertex description, the glDrawElementsInstanced method, and a shader to modify each 'instance' stamped out in whatever manner you like (e.g. perturb vertices, scale & rotate, change texture lookup, etc.)

This is very similar to your 'go through all the vbos' loop, except that you would only upload vertices for a single blade of grass, and OpenGL will take care of passing a unique gl_InstanceID to your shader. You can then encode the changes each frame either procedurally, or in a 'texture' that you upload as often as needed. The book has sample code (and it may be available from the web site as well).

Edit: Confirmed that the sample code is in the downloads from the site - look at the src\grass\grass.cpp to see a sample using textures to control grass length, orientation, color, and bend.