oglplus/014_multi_cube_gs.cpp

Shows how to draw multiple instances of a cube using 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 <oglplus/shapes/cube.hpp>

#include <cmath>

#include "example.hpp"

namespace oglplus {

class CubeExample : public Example
{
private:
    // helper object building cube vertex attributes
    shapes::Cube make_cube;
    // helper object encapsulating cube drawing instructions
    shapes::DrawingInstructions cube_instr;
    // indices pointing to cube primitive elements
    shapes::Cube::IndexArray cube_indices;

    // wrapper around the current OpenGL context
    Context gl;

    // Vertex shader
    VertexShader vs;

    // Geometry shader
    GeometryShader gs;

    // Fragment shader
    FragmentShader fs;

    // Program
    Program prog;

    // Handle for uniforms in program
    Uniform<Mat4f> projection_matrix, camera_matrix;

    // A vertex array object for the rendered cube
    VertexArray cube;

    // VBOs for the cube's vertices
    Buffer verts;
public:
    CubeExample(void)
     : cube_instr(make_cube.Instructions())
     , cube_indices(make_cube.Indices())
     , projection_matrix(prog)
     , camera_matrix(prog)
    {
        // Set the vertex shader source and compile it
        vs.Source(
            "#version 330\n"
            "in vec4 Position;"
            "void main(void)"
            "{"
            "   gl_Position = Position;"
            "}"
        ).Compile();

        // Set the geometry shader source and compile it
        gs.Source(
            "#version 330\n"
            "layout(triangles) in;"
            "layout(triangle_strip, max_vertices = 108) out;"
            "uniform mat4 ProjectionMatrix, CameraMatrix;"
            "out vec3 vertColor;"
            "void main(void)"
            "{"
            "   for(int c=0; c!=36; ++c)"
            "   {"
            "       float angle = c * 10 * 2 * 3.14159 / 360.0;"
            "       float cx = cos(angle);"
            "       float sx = sin(angle);"
            "       mat4 ModelMatrix = mat4("
            "            cx, 0.0,  sx, 0.0,"
            "           0.0, 1.0, 0.0, 0.0,"
            "           -sx, 0.0,  cx, 0.0,"
            "           0.0, 0.0, 0.0, 1.0 "
            "       ) * mat4("
            "            1.0, 0.0, 0.0, 0.0,"
            "            0.0, 1.0, 0.0, 0.0,"
            "            0.0, 0.0, 1.0, 0.0,"
            "           12.0, 0.0, 0.0, 1.0 "
            "       );"
            "       for(int v=0; v!=gl_in.length(); ++v)"
            "       {"
            "           vec4 vert = gl_in[v].gl_Position;"
            "           gl_Position = "
            "               ProjectionMatrix *"
            "               CameraMatrix *"
            "               ModelMatrix *"
            "               vert;"
            "           vertColor = abs(normalize(ModelMatrix*vert)).xzy;"
            "           EmitVertex();"
            "       }"
            "       EndPrimitive();"
            "   }"
            "}"
        ).Compile();

        // set the fragment shader source and compile it
        fs.Source(
            "#version 330\n"
            "in vec3 vertColor;"
            "out vec4 fragColor;"
            "void main(void)"
            "{"
            "   fragColor = vec4(vertColor, 1.0);"
            "}"
        ).Compile();

        // attach the shaders to the program
        prog << vs << gs << fs;
        // link and use it
        prog.Link().Use();

        projection_matrix.BindTo("ProjectionMatrix");
        camera_matrix.BindTo("CameraMatrix");

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

        // bind the VBO for the cube vertices
        verts.Bind(Buffer::Target::Array);
        {
            std::vector<GLfloat> data;
            GLuint n_per_vertex = make_cube.Positions(data);
            // upload the data
            Buffer::Data(Buffer::Target::Array, data);
            // setup the vertex attribs array for the vertices
            VertexArrayAttrib attr(prog, "Position");
            attr.Setup<GLfloat>(n_per_vertex).Enable();
        }

        gl.ClearColor(0.9f, 0.9f, 0.9f, 0.0f);
        gl.ClearDepth(1.0f);
        gl.Enable(Capability::DepthTest);
    }

    void Reshape(GLuint width, GLuint height)
    {
        gl.Viewport(width, height);
        prog.Use();
        projection_matrix.Set(
            CamMatrixf::PerspectiveX(
                Degrees(70),
                double(width)/height,
                1, 50
            )
        );
    }

    void Render(double time)
    {
        gl.Clear().ColorBuffer().DepthBuffer();
        //
        // set the matrix for camera orbiting the origin
        camera_matrix.Set(
            CamMatrixf::Orbiting(
                Vec3f(),
                18.5,
                Degrees(time * 135),
                Degrees(SineWave(time / 20.0) * 30)
            )
        );

        // draw 1 instance of the cube
        // the geometry shader will spawn 36 copies
        // and take care of their placement
        cube_instr.Draw(cube_indices);
    }

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

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 CubeExample);
}

} // namespace oglplus