opengl.cpp

#include <MainState.h>
#include "ShaderProgram.h"

/* Created by following the example from the excellent OpenGL Superbible 7 book */

GLuint MainState::compile_shaders()
{
    ShaderProgram program;

    // Source code for vertex shader
    const std::string vertex_shader_source = R"(
        #version 330 core
        layout(location = 0) in vec4 offset;
        void main(void)
        {
            const vec4 vertices[3] =
               vec4[3](vec4(0.25, -0.25, 0.5, 1.0),
               vec4(-0.25, -0.25, 0.5, 1.0),
               vec4(0.25, 0.25, 0.5, 1.0));
            gl_Position = vertices[gl_VertexID] + offset;
        }
)";

    // Source code for fragment shader
    const std::string fragment_shader_source = R"( 
        #version 330 core
        out vec4 color;
        void main(void)
        {
           color = vec4(0.0, 0.8, 1.0, 1.0);
        }
)";
    program.add_shader_from_source(vertex_shader_source, GL_VERTEX_SHADER);
    program.add_shader_from_source(vertex_shader_source, GL_FRAGMENT_SHADER);

    // Create program, attach shaders, and link the shaders
    program.create_program();
    program.link_program();

    return program.id();
}

void MainState::initialize()
{
    // Create our shader program
    rendering_program = compile_shaders();

    // Create a dummy VAO
    glGenVertexArrays(1, &vertex_array_object);
    glBindVertexArray(vertex_array_object);
}

void MainState::finalize()
{
    // Delete our dummy VAO and our shader program 
    glDeleteVertexArrays(1, &vertex_array_object);
    glDeleteProgram(rendering_program);
}

void MainState::handle_events(SDL_Event event) {}
void MainState::update() {}

void MainState::draw()
{
    // Get the current time
    double currentTime = SDL_GetTicks() / 1000.00;

    // Create a color that varies gradually and cyclically with time
    const GLfloat color[] = { (float)sin(currentTime) * 0.5f + 0.5f,
        (float)cos(currentTime) * 0.5f + 0.5f,
        0.0f, 1.0f };

    // Clear the screen to the color above
    glClearBufferfv(GL_COLOR, 0, color);

    // Use the shader program we created earlier
    glUseProgram(rendering_program);

    // Create an array of four floats that move a point in a circle in x,y over time
    GLfloat attrib[] = { 
        (float)sin(currentTime) * 0.5f,
        (float)cos(currentTime) * 0.6f,
        0.0f, 0.0f };    

    // Update the 'offset' vertex attribute with the value above
    glVertexAttrib4fv(0, attrib);

    // Process three vertices as a triangle
    glDrawArrays(GL_TRIANGLES, 0, 3);
}