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// Creating an SFML window and OpenGL basic setup.
#include <GL/glew.h>
#include <GL/gl.h>
#include <SFML/Graphics.h>
#include <iostream>

int main() {
    // First we tell SFML how to setup our OpenGL context.
    sf::ContextSettings context{ 24,   // depth buffer bits
                                  8,   // stencil buffer bits
                                  4,   // MSAA samples
                                  3,   // major opengl version
                                  3 }; // minor opengl version
    // Now we create the window, enable VSync
    // and set the window active for OpenGL.
    sf::Window window{ sf::VideoMode{ 1024, 768 },
                       "opengl window",
                       sf::Style::Default,
               context };
    window.setVerticalSyncEnabled(true);
    window.setActive(true);
    // After that we initialise GLEW and check if an error occurred.
    GLenum error;
    glewExperimental = GL_TRUE;
    if ((err = glewInit()) != GLEW_OK)
        std::cout << glewGetErrorString(err) << std::endl;
    // Here we set the color glClear will clear the buffers with.
    glClearColor(0.0f,    // red
                 0.0f,    // green
                 0.0f,    // blue
                 1.0f);   // alpha
    // Now we can start the event loop, poll for events and draw objects.
    sf::Event event{ };
    while (window.isOpen()) {
        while (window.pollEvent(event)) {
            if (event.type == sf::Event::Closed)
                window.close;
        }
        // Tell OpenGL to clear the color buffer
        // and the depth buffer, this will clear our window.
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        // Flip front- and backbuffer.
        window.display();
    }
    return 0;
}

GLuint createShaderProgram(const std::string& vertexShaderPath,
                           const std::string& fragmentShaderPath) {
    // Load the vertex shader source.
    std::stringstream ss{ };
    std::string vertexShaderSource{ };
    std::string fragmentShaderSource{ };
    std::ifstream file{ vertexShaderPath };
    if (file.is_open()) {
        ss << file.rdbuf();
        vertexShaderSource = ss.str();
        file.close();
    }
    // Clear the stringstream and load the fragment shader source.
    ss.str(std::string{ });
    file.open(fragmentShaderPath);
    if (file.is_open()) {
        ss << file.rdbuf();
        fragmentShaderSource = ss.str();
        file.close();
    }
    // Create the program.
    GLuint program = glCreateProgram();
    // Create the shaders.
    GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
    GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    // Now we can load the shader source into the shader objects and compile them.
    // Because glShaderSource() wants a const char* const*,
    // we must first create a const char* and then pass the reference.
    const char* cVertexSource = vertexShaderSource.c_str();
    glShaderSource(vertexShader,     // shader
                   1,                // number of strings
                   &cVertexSource,   // strings
                   nullptr);         // length of strings (nullptr for 1)
    glCompileShader(vertexShader);
    // Now we have to do the same for the fragment shader.
    const char* cFragmentSource = fragmentShaderSource.c_str();
    glShaderSource(fragmentShader, 1, &cFragmentSource, nullptr);
    glCompileShader(fragmentShader);
    // After attaching the source and compiling the shaders,
    // we attach them to the program;
    glAttachShader(program, vertexShader);
    glAttachShader(program, fragmentShader);
    glLinkProgram(program);
    // After linking the shaders we should detach and delete
    // them to prevent memory leak.
    glDetachShader(program, vertexShader);
    glDetachShader(program, fragmentShader);
    glDeleteShader(vertexShader);
    glDeleteShader(fragmentShader);
    // With everything done we can return the completed program.
    return program;
}

GLint logSize = 0;
std::vector<GLchar> logText{ };
glGetShaderiv(vertexShader,         // shader
              GL_INFO_LOG_LENGTH,   // requested parameter
              &logSize);            // return object
if (logSize > 0) {
    logText.resize(logSize);
    glGetShaderInfoLog(vertexShader,      // shader
                       logSize,           // buffer length
                       &logSize,          // returned length
                       logText.data());   // buffer
    std::cout << logText.data() << std::endl;
}

// Now we can create a shader program with a vertex and a fragment shader.
// ...
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);

GLuint program = createShaderProgram("vertex.glsl", "fragment.glsl");

sf::Event event{ };
// ...
// We also have to delete the program at the end of the application.
// ...
    }
    glDeleteProgram(program);   
    return 0;
}
// ...

// Declare which version of GLSL we use.
// Here we declare, that we want to use the OpenGL 3.3 version of GLSL.
#version 330 core
// At attribute location 0 we want an input variable of type vec3,
// that contains the position of the vertex.
// Setting the location is optional, if you don't set it you can ask for the
// location with glGetAttribLocation().
layout(location = 0) in vec3 position;
// Every shader starts in it's main function.
void main() {
    // gl_Position is a predefined variable that holds
    // the final vertex position.
    // It consists of a x, y, z and w coordinate.
    gl_Position = vec4(position, 1.0);
}

#version 330 core
// The fragment shader does not have a predefined variable for
// the vertex color, so we have to define a output vec4,
// that holds the final vertex color.
out vec4 outColor;

void main() {
    // We simply set the output color to red.
    // The parameters are red, green, blue and alpha.
    outColor = vec4(1.0, 0.0, 0.0, 1.0);
}

// The vertex data is defined in a counter-clockwise way,
// as this is the default front face.
std::vector<float> vertexData {
    -0.5f,  0.5f, 0.0f,
    -0.5f, -0.5f, 0.0f,
     0.5f, -0.5f, 0.0f,
     0.5f,  0.5f, 0.0f
};
// If you want to use a clockwise definition, you can simply call
glFrontFace(GL_CW);
// Next we need to define a Vertex Array Object (VAO).
// The VAO stores the current state while its active.
GLuint vao = 0;
glGenVertexArrays(1, &vao);
glBindVertexArray(vao);
// With the VAO active we can now create a Vertex Buffer Object (VBO).
// The VBO stores our vertex data.
GLuint vbo = 0;
glGenBuffers(1, &vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
// For reading and copying there are also GL_*_READ and GL_*_COPY,
// if your data changes more often use GL_DYNAMIC_* or GL_STREAM_*.
glBufferData(GL_ARRAY_BUFFER,     // target buffer
             sizeof(vertexData[0]) * vertexData.size(),   // size
             vertexData.data(),   // data
             GL_STATIC_DRAW);     // usage
// After filling the VBO link it to the location 0 in our vertex shader,
// which holds the vertex position.
// ...
// To ask for the attribute location, if you haven't set it:
GLint posLocation = glGetAttribLocation(program, "position");
// ..
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 3,       // location and size
                      GL_FLOAT,   // type of data
                      GL_FALSE,   // normalized (always false for floats)
                      0,          // stride (interleaved arrays)
                      nullptr);   // offset (interleaved arrays)
// Everything should now be saved in our VAO and we can unbind it and the VBO.
glBindVertexArray(0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
// Now we can draw the vertex data in our render loop.
// ...
glClear(GL_COLOR_BUFFER_BIT);
// Tell OpenGL we want to use our shader program.
glUseProgram(program);
// Binding the VAO loads the data we need.
glBindVertexArray(vao);
// We want to draw a quad starting at index 0 of the VBO using 4 indices.
glDrawArrays(GL_QUADS, 0, 4);
glBindVertexArray(0);
window.display();
// ...
// Ofcource we have to delete the allocated memory for the VAO and VBO at
// the end of our application.
// ...
glDeleteBuffers(1, &vbo);
glDeleteVertexArrays(1, &vao);
glDeleteProgram(program);
return 0;
// ...

std::vector<float> colorData {
    1.0f, 0.0f, 0.0f,
    0.0f, 1.0f, 0.0f,
    0.0f, 0.0f, 1.0f,
    1.0f, 1.0f, 0.0f
};

// ...
GLuint vbo[2];
glGenBuffers(2, vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
// ...
glDeleteBuffers(2, vbo);
/ ...
// With these changes made we now have to load our color data into the new VBO
// ...
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, nullptr);

glBindBuffer(GL_ARRAY_BUFFER, vbo[1]);
glBufferData(GL_ARRAY_BUFFER, sizeof(colorData[0]) * colorData.size(),
             colorData.data(), GL_STATIC_DRAW);
glEnableVertexAttribArray(1);
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 0, nullptr);

glBindVertexArray(0);  
// ...

#version 330 core

layout(location = 0) in vec3 position;
// The new location has to differ from any other input variable.
// It is the same index we need to pass to
// glEnableVertexAttribArray() and glVertexAttribPointer().
layout(location = 1) in vec3 color;

out vec3 fColor;

void main() {
    fColor = color;
    gl_Position = vec4(position, 1.0);
}

#version 330 core

in vec3 fColor;

out vec4 outColor;

void main() {
    outColor = vec4(fColor, 1.0);
}

// If you want to completely clear and refill a VBO use glBufferData(),
// just like we did before.
// ...
// There are two mains ways to update a subset of a VBO's data.
// To update a VBO with existing data
std::vector<float> newSubData {
    -0.25f, 0.5f, 0.0f
};
glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
glBufferSubData(GL_ARRAY_BUFFER,      // target buffer
                0,                    // offset
                sizeof(newSubData[0]) * newSubData.size(),   // size
                newSubData.data());   // data
// This would update the first three values in our vbo[0] buffer.
// If you want to update starting at a specific location just set the second
// parameter to that value and multiply by the types size.
// ...
// If you are streaming data, for example from a file,
// it is faster to directly pass the data to the buffer.
// Other access values are GL_READ_ONLY and GL_READ_WRITE.
glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
// You can static_cast<float*>() the void* to be more safe.
void* Ptr = glMapBuffer(GL_ARRAY_BUFFER,   // buffer to map
                        GL_WRITE_ONLY);    // access to buffer
memcpy(Ptr, newSubData.data(), sizeof(newSubData[0]) * newSubData.size());
// To copy to a specific location add a destination offset to memcpy().
glUnmapBuffer(GL_ARRAY_BUFFER);
// ...
// There is also a way to copy data from one buffer to another,
// If we have two VBO's vbo[0] and vbo[1], we can copy like so
// You can also read from GL_ARRAY_BUFFER.
glBindBuffer(GL_COPY_READ_BUFFER, vbo[0]);
// GL_COPY_READ_BUFFER and GL_COPY_WRITE_BUFFER are specifically for
// copying buffer data.
glBindBuffer(GL_COPY_WRITE_BUFFER, vbo[1]);
glCopyBufferSubData(GL_COPY_READ_BUFFER,    // read buffer
                    GL_COPY_WRITE_BUFFER,   // write buffer
                    0, 0,                   // read and write offset
                    sizeof(vbo[0]) * 3);    // copy size
// This will copy the first three elements from vbo[0] to vbo[1].

// Uniforms are variables like in and out, however,
// we can change them easily by passing new values with glUniform().
// Lets define a time variable in our fragment shader.
#version 330 core
// Unlike a in/out variable we can use a uniform in every shader,
// without the need to pass it to the next one, they are global.
// Don't use locations already used for attributes!
// Uniform layout locations require OpenGL 4.3!
layout(location = 10) uniform float time;

in vec3 fColor;

out vec4 outColor;

void main() {
    // Create a sine wave from 0 to 1 based on the time passed to the shader.
    float factor = (sin(time * 2) + 1) / 2;
    outColor = vec4(fColor.r * factor, fColor.g * factor, fColor.b * factor, 1.0);
}

// If we haven't set the layout location, we can ask for it.
GLint timeLocation = glGetUniformLocation(program, "time");
// ...
// Also we should define a Timer counting the current time.
sf::Clock clock{ };
// In out render loop we can now update the uniform every frame.
    // ...
    window.display();
    glUniform1f(10,   // location
                clock.getElapsedTime().asSeconds());   // data
}
// ...

// Lets create a quad from two rectangles.
// We can simply use the old vertex data from before.
// First, we have to create the IBO.
// The index is referring to the first declaration in the VBO.
std::vector<unsigned int> iboData {
    0, 1, 2,
    0, 2, 3
};
// That's it, as you can see we could reuse 0 - the top left
// and 2 - the bottom right.
// Now that we have our data, we have to fill it into a buffer.
// Note that this has to happen between the two glBindVertexArray() calls,
// so it gets saved into the VAO.
GLuint ibo = 0;
glGenBufferrs(1, &ibo);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(iboData[0]) * iboData.size(),
             iboData.data(), GL_STATIC_DRAW);
// Next in our render loop, we replace glDrawArrays() with:
glDrawElements(GL_TRIANGLES, iboData.size(), GL_UNSIGNED_INT, nullptr);
// Remember to delete the allocated memory for the IBO.

// Lets save we have a texture called "my_tex.tga", we can load it with:
sf::Image image;
image.loadFromFile("my_tex.tga");
// We have to flip the texture around the y-Axis, because OpenGL's texture
// origin is the bottom left corner, not the top left.
image.flipVertically();
// After loading it we have to create a OpenGL texture.
GLuint texture = 0;
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
// Specify what happens when the coordinates are out of range.
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
// Specify the filtering if the object is very large.
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
// Load the image data to the texture.
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, image.getSize().x, image.getSize().y,
             0, GL_RGBA, GL_UNSIGNED_BYTE, image.getPixelsPtr());
// Unbind the texture to prevent modifications.
glBindTexture(GL_TEXTURE_2D, 0);
// Delete the texture at the end of the application.
// ...
glDeleteTextures(1, &texture);

// With the texture created, we now have to specify the UV,
// or in OpenGL terms ST coordinates.
std::vector<float> texCoords {
    // The texture coordinates have to match the triangles/quad
    // definition.
    0.0f, 1.0f,    // start at top-left
    0.0f, 0.0f,    // go round counter-clockwise
    1.0f, 0.0f,
    1.0f, 1.0f     // end at top-right
};
// Now we increase the VBO's size again just like we did for the colors.
// ...
GLuint vbo[3];
glGenBuffers(3, vbo);
// ...
glDeleteBuffers(3, vbo);
// ...
// Load the texture coordinates into the new buffer.
glBindBuffer(GL_ARRAY_BUFFER, vbo[2]);
glBufferData(GL_ARRAY_BUFFER, sizeof(texCoords[0]) * texCoords.size(),
             texCoords.data(), GL_STATIC_DRAW);
glEnableVertexAttribArray(2);
glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 0, nullptr);
// Because the VAO does not store the texture we have to bind it before drawing.
// ...
glBindVertexArray(vao);
glBindTexture(GL_TEXTURE_2D, texture);
glDrawElements(GL_TRIANGLES, iboData.size(), GL_UNSIGNED_INT, nullptr);
// ...

#version 330 core

layout(location = 0) in vec3 position;
layout(location = 1) in vec3 color;
layout(location = 2) in vec2 texCoords;

out vec3 fColor;
out vec2 fTexCoords;

void main() {
    fColor = color;
    fTexCoords = texCoords;
    gl_Position = vec4(position, 1.0);
}

#version 330 core
// sampler2D represents our 2D texture.
uniform sampler2D tex;
uniform float time;

in vec3 fColor;
in vec2 fTexCoords;

out vec4 outColor;

void main() {
    // texture() loads the current texture data at the specified texture coords,
    // then we can simply multiply them by our color.
    outColor = texture(tex, fTexCoords) * vec4(fColor, 1.0);
}

#version 330 core

layout(location = 0) in vec3 position;
layout(location = 1) in vec3 color;
layout(location = 2) in vec2 texCoords;
// Create 2 4x4 matricies, 1 for the projection matrix
// and 1 for the model matrix.
// Because we draw in a static scene, we don't need a view matrix.
uniform mat4 projection;
uniform mat4 model;

out vec3 fColor;
out vec2 fTexCoords;

void main() {
    fColor = color;
    fTexCoords = texCoords;
    // Multiplay the position by the model matrix and then by the
    // projection matrix.
    // Beware order of multiplication for matricies!
    gl_Position = projection * model * vec4(position, 1.0);
}

// The new vertex data, counter-clockwise declaration.
std::vector<float> vertexData {  
    0.0f, 1.0f, 0.0f,   // top left
    0.0f, 0.0f, 0.0f,   // bottom left
    1.0f, 0.0f, 0.0f,   // bottom right
    1.0f, 1.0f, 0.0f    // top right
};
// Request the location of our matricies.
GLint projectionLocation = glGetUniformLocation(program, "projection");
GLint modelLocation = glGetUniformLocation(program, "model");
// Declaring the matricies.
// Orthogonal matrix for a 1024x768 window.
std::vector<float> projection {  
    0.001953f,       0.0f,  0.0f, 0.0f,
         0.0f, -0.002604f,  0.0f, 0.0f,
         0.0f,       0.0f, -1.0f, 0.0f,
        -1.0f,       1.0f,  0.0f, 1.0f
};
// Model matrix translating to x 50, y 50
// and scaling to x 200, y 200.
std::vector<float> model {  
    200.0f,   0.0f, 0.0f, 0.0f,
      0.0f, 200.0f, 0.0f, 0.0f,
      0.0f,   0.0f, 1.0f, 0.0f,
     50.0f,  50.0f, 0.0f, 1.0f
};
// Now we can send our calculated matricies to the program.
glUseProgram(program);
glUniformMatrix4fv(projectionLocation,   // location
                   1,                    // count
                   GL_FALSE,             // transpose the matrix
                   projection.data());   // data
glUniformMatrix4fv(modelLocation, 1, GL_FALSE, model.data());
glUseProgram(0);
// The glUniform*() calls have to be done, while the program is bound.

// There are many math librarys for OpenGL, which create
// matricies and vectors, the most used in C++ is glm (OpenGL Mathematics).
// Its a header only library.
// The same code using glm would look like:
glm::mat4 projection{ glm::ortho(0.0f, 1024.0f, 768.0f, 0.0f) };
glUniformMatrix4fv(projectionLocation, 1, GL_FALSE,
                   glm::value_ptr(projection));
// Initialise the model matrix to the identity matrix, otherwise every
// multiplication would be 0.
glm::mat4 model{ 1.0f };
model = glm::translate(model, glm::vec3{ 50.0f, 50.0f, 0.0f });
model = glm::scale(model, glm::vec3{ 200.0f, 200.0f, 0.0f });
glUniformMatrix4fv(modelLocation, 1, GL_FALSE,
                   glm::value_ptr(model));

#version 330 core

layout(location = 0) in vec3 position;
layout(location = 1) in vec3 color;
// Create an output interface block passed to the next shader stage.
// Interface blocks can be used to structure data passed between shaders.
out VS_OUT {
    vec3 color;
} vs_out;

void main() {
    vs_out.color = color
    gl_Position = vec4(position, 1.0);
}

#version 330 core
// The geometry shader takes in points.
layout(points) in;
// It outputs a triangle every 3 vertices emitted.
layout(triangle_strip, max_vertices = 3) out;
// VS_OUT becomes an input variable in the geometry shader.
// Every input to the geometry shader in treated as an array.
in VS_OUT {
    vec3 color;
} gs_in[];
// Output color for the fragment shader.
// You can also simply define color as 'out vec3 color',
// If you don't want to use interface blocks.
out GS_OUT {
    vec3 color;
} gs_out;

void main() {
    // Each emit calls the fragment shader, so we set a color for each vertex.
    gs_out.color = mix(gs_in[0].color, vec3(1.0, 0.0, 0.0), 0.5);
    // Move 0.5 units to the left and emit the new vertex.
    // gl_in[] is the current vertex from the vertex shader, here we only
    // use 0, because we are receiving points.
    gl_Position = gl_in[0].gl_Position + vec4(-0.5, 0.0, 0.0, 0.0);
    EmitVertex();
    gs_out.color = mix(gs_in[0].color, vec3(0.0, 1.0, 0.0), 0.5);
    // Move 0.5 units to the right and emit the new vertex.
    gl_Position = gl_in[0].gl_Position + vec4(0.5, 0.0, 0.0, 0.0);
    EmitVertex();
    gs_out.color = mix(gs_in[0].color, vec3(0.0, 0.0, 1.0), 0.5);
    // Move 0.5 units up and emit the new vertex.
    gl_Position = gl_in[0].gl_Position + vec4(0.0, 0.75, 0.0, 0.0);
    EmitVertex();
    EndPrimitive();
}

in GS_OUT {
    vec3 color;
} fs_in;

out vec4 outColor;

void main() {
    outColor = vec4(fs_in.color, 1.0);
}