oglplus/018_particle_system.cpp

Shows how to draw a simple particle system using the geometry shader

Copyright 2008-2014 Matus Chochlik. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

#include <oglplus/gl.hpp>
#include <oglplus/all.hpp>

#include <vector>
#include <cmath>

#include "example.hpp"

namespace oglplus {

class ParticlesExample : public Example
{
private:
    // wrapper around the current OpenGL context
    Context gl;

    // Program
    Program prog;
    static Program make(void)
    {
        VertexShader vs;
        vs.Source(
            "#version 330\n"
            "uniform mat4 ModelMatrix, CameraMatrix;"
            "in vec4 Position;"
            "in float Age;"
            "out float vertAge;"
            "void main(void)"
            "{"
            "   gl_Position = "
            "       CameraMatrix *"
            "       ModelMatrix *"
            "       Position;"
            "   vertAge = Age;"
            "}"
        );
        vs.Compile();

        GeometryShader gs;
        gs.Source(
            "#version 330\n"
            "layout(points) in;"
            "layout(triangle_strip, max_vertices = 4) out;"
            "uniform mat4 ProjectionMatrix;"
            "in float vertAge[];"
            "out float geomAge;"
            "void main(void)"
            "{"
            "   float s = 0.5;"
            "   float yo[2] = float[2](-1.0, 1.0);"
            "   float xo[2] = float[2](-1.0, 1.0);"
            "   for(int j=0;j!=2;++j)"
            "   for(int i=0;i!=2;++i)"
            "   {"
            "       float xoffs = xo[i]*(1.0+vertAge[0])*s;"
            "       float yoffs = yo[j]*(1.0+vertAge[0])*s;"
            "       gl_Position = ProjectionMatrix * vec4("
            "           gl_in[0].gl_Position.x-xoffs,"
            "           gl_in[0].gl_Position.y-yoffs,"
            "           gl_in[0].gl_Position.z,"
            "           1.0"
            "       );"
            "       geomAge = vertAge[0];"
            "       EmitVertex();"
            "   }"
            "   EndPrimitive();"
            "}"
        );
        gs.Compile();

        FragmentShader fs;
        fs.Source(
            "#version 330\n"
            "in float geomAge;"
            "out vec4 fragColor;"
            "void main(void)"
            "{"
            "   vec3 Color1 = vec3(1.0, 0.5, 0.5);"
            "   vec3 Color2 = vec3(0.3, 0.1, 0.1);"
            "   fragColor = vec4("
            "       mix(Color1, Color2, geomAge),"
            "       1.0 - geomAge"
            "   );"
            "}"
        );
        fs.Compile();

        Program prog;
        prog.AttachShader(vs);
        prog.AttachShader(gs);
        prog.AttachShader(fs);
        prog.Link();
        prog.Use();

        return prog;
    }

    // Uniforms
    Uniform<Mat4f> projection_matrix, camera_matrix;

    const GLuint particle_count;
    std::vector<Vec3f> positions;
    std::vector<Vec3f> directions;
    std::vector<float> ages;

    // A vertex array object for the particles
    VertexArray particles;

    // VBOs for the particle positions and ages
    Buffer pos_buf, age_buf;

    double prev_time, prev_spawn;
public:
    ParticlesExample(void)
     : prog(make())
     , projection_matrix(prog, "ProjectionMatrix")
     , camera_matrix(prog, "CameraMatrix")
     , particle_count(100)
     , prev_time(0.0)
     , prev_spawn(0.0)
    {

        // bind the VAO for the particles
        particles.Bind();

        // bind the VBO for the particle positions
        pos_buf.Bind(Buffer::Target::Array);
        {
            Buffer::Data(Buffer::Target::Array, positions);
            VertexArrayAttrib attr(prog, "Position");
            attr.Setup<GLfloat>(3);
            attr.Enable();
        }
        positions.reserve(particle_count);
        directions.reserve(particle_count);

        // bind the VBO for the particle ages
        age_buf.Bind(Buffer::Target::Array);
        {
            Buffer::Data(Buffer::Target::Array, ages);
            VertexArrayAttrib attr(prog, "Age");
            attr.Setup<GLfloat>(1);
            attr.Enable();
        }
        ages.reserve(particle_count);

        Uniform<Mat4f>(prog, "ModelMatrix").Set(
            ModelMatrixf::Translation(0.0f, -10.0f, 0.0f)
        );
        //
        gl.ClearColor(0.9f, 0.9f, 0.9f, 0.0f);
        gl.ClearDepth(1.0f);
        gl.Enable(Capability::DepthTest);
        gl.Enable(Capability::Blend);
        gl.BlendFunc(BlendFn::SrcAlpha, BlendFn::OneMinusSrcAlpha);
    }

    void Reshape(GLuint width, GLuint height)
    {
        gl.Viewport(width, height);
        prog.Use();
        projection_matrix.Set(
            CamMatrixf::PerspectiveX(
                Degrees(60),
                double(width)/height,
                1, 40
            )
        );
    }

    // creates a directional vector for a new particle
    static Vec3f NewDirection(void)
    {
        float disp = 2.0f;
        float dx = (0.5f - (float(std::rand())/RAND_MAX))*disp;
        float dy = 5.0f + (0.5 - (float(std::rand())/RAND_MAX))*disp;;
        float dz = (0.5f - (float(std::rand())/RAND_MAX))*disp;
        return Vec3f(dx, dy, dz);
    }

    void Render(double time)
    {
        // update the particle positions, ages and directions
        GLuint i = 0;
        float time_diff = (time - prev_time);
        float age_mult = 0.2f;
        while(i != positions.size())
        {
            float drag = 0.1f * (time_diff);
            if((ages[i] += time_diff * age_mult) < 1.0f)
            {
                directions[i] *= (1.0f - drag);
                positions[i] += directions[i]*time_diff;
            }
            else
            {
                ages[i] = 0.0f;
                directions[i] = NewDirection();
                positions[i] = Vec3f();
            }
            ++i;
        }
        // if there are not enough particles yet
        if(i != particle_count)
        {
            float spawn_interval = 1.0f/(age_mult*particle_count);
            if(prev_spawn + spawn_interval < time)
            {
                directions.push_back(NewDirection());
                positions.push_back(Vec3f());
                ages.push_back(0.0f);
                prev_spawn = time;
            }
        }
        prev_time = time;

        assert(positions.size() == directions.size());
        assert(positions.size() == ages.size());

        // upload the particle positions
        pos_buf.Bind(Buffer::Target::Array);
        Buffer::Data(Buffer::Target::Array, positions);
        // upload the particle ages
        age_buf.Bind(Buffer::Target::Array);
        Buffer::Data(Buffer::Target::Array, ages);

        gl.Clear().ColorBuffer().DepthBuffer();
        //
        // set the matrix for camera orbiting the origin
        camera_matrix.Set(
            CamMatrixf::Orbiting(
                Vec3f(),
                18.0f,
                FullCircles(time * 0.5),
                Degrees(45)
            )
        );

        gl.DrawArrays(PrimitiveType::Points, 0, positions.size());
    }

    bool Continue(double time)
    {
        return time < 30.0;
    }

    double ScreenshotTime(void) const
    {
        return 5.0;
    }

    double HeatUpTime(void) const
    {
        return 0.0;
    }
};

void setupExample(ExampleParams& /*params*/){ }

std::unique_ptr<ExampleThread> makeExampleThread(
    Example& /*example*/,
    unsigned /*thread_id*/,
    const ExampleParams& /*params*/
){ return std::unique_ptr<ExampleThread>(); }

std::unique_ptr<Example> makeExample(const ExampleParams& /*params*/)
{
    return std::unique_ptr<Example>(new ParticlesExample);
}

} // namespace oglplus