Introduction to INF585 exercises

Download INF585 exercises

• - On Github: [ https://github.com/drohmer/inf585_code ]

• - Or via direct archive download:
• • - [ code_inf585.zip ]
  • - [ code_inf585.tar.gz ]

• Make sure you can compile and run the scene in scenes_inf585/00_introduction/.
• • Follow the detailed Installation and Compilation Instructions if needed.

• - Your C++ compiler should be compatible with C++14.
• - Glad is used to load function for OpenGL 3.3.
• - GLFW is used to create a window and handle mouse/key events.
• - ImGUI is used to handle GUI.

Introductory scene

• The first introductory scene is available in the directory scenes_inf585/00_introduction/
• • - Each exercise is described as a unique program (with its own main) defined in the scenes/ directory.
  • - A CMakeLists.txt is associated to each program for its compilation (+ a Makefile for Unix systems).
  • • (you need to adapt the path of the compilation instructions to the directory of the CMakeLists.txt for each exercise)

• Once executed, you should observe a basic scene as seen below with rotating cylinder, cube, and curve (some details may vary).
• • - Note that the source code of the scene is fully described in the files src/main.cpp and src/scene.cpp (and src/scene.hpp).
  • - The main.cpp file contains mostly common elements through all the exercises (general set up of the scene, animation loop, basic handling of the keyboard/mouse input). The specific content is defined in the files scene.cpp, and its header scene.hpp.

Information on the library (only general information, no exercise)

• - Structure for 3D vectors and matrices (and 2D, 4D).
• - Object structure ready to be displayed with OpenGL (Meshes, lines, etc) and their associated shaders.
• - Easy and secured storage for buffers of elements.
• - 3D scene manipulation: Mouse controlled camera

• - Minimalistic code written to be simple to understand (not necessarily the most generic nor efficient).
• - Objects are written to be lightweight and with minimal hidden states - You can directly interact with object attributes.
• - The code and functionalities remain close to basic C++/OpenGL - You can use direct OpenGL calls with the library. You should also keep in mind the way that OpenGL works to avoid unexpected display.

General structure of the code

• scenes_XX/exercise_name/ (ex.scenes_inf585/00_introduction/) Contains the code associated to each 3D scene.
• • - One exercise starts with an independant main file (+ possible other files) and associated CMakeLists.txt for its compilation.
  • - Most of the time, the main.cpp will only consists in the general call structure (initialization, events handling, animation loop). While the specific data and drawing calls of each exercise will be set in the files scene.cpp/hpp.
  • - Changing exercise consists in compiling the code from another directory.
  • - All your code will take place in these directories (unless you want to modify the library).

• cgp/ Relates to the CGP library.
• • - cgp/library/cgp/ the actual source code of the CGP library: set of structures and functions to ease generating your 3D scene.
  • - cgp/library/third_party/ External library used by CGP such as glad (OpenGL loader), imgui (GUI), lodepng (loader for png images), etc.
  • - cgp/examples/ provides a set of examples of use cases of the library

Code editor

• The library contains multiples files. Make sure you use a sufficiently advanced (or well parameterized) IDE to have
• • C++ code completion (in particular complete function names, display expected arguments and types, objects arguments, etc.)
  • efficiently switch between files and jump to the signature and code of any function and object.

• You may use for instance:
• • VS Code with a correct C++ plugins and setup for the compiler.
  • QtCreator that works for C++ out of the box (see instructions).

Use of the code library and program structure (the exercise starts here)

• > Observe the file main.cpp. You should recognize the general organization of the program, in particular the setup stage, and the animation loop stage. Note where the two main methods scene.initialize() and scene.display_frame() are called, and their implementation in the file scene.cpp.
  

• > Change the rotation (axis and angle) from some of the object within the scene in the function display_scene.
• • The function rotation_transform::from_axis_angle(axis_of_rotation, angle_of_rotation); generate a structure cgp::rotation_transform from a given 3D axis (cgp::vec3) and scalar angle (float) in radians.
  • The structure cgp::vec3 implements a model of 3D vector with (x,y,z) coordinates.
  • • You can apply most of the basic operations between vec3 and mat3 using mathematical operators (+,-,*,/).
  • The structure rotation_transform allowing to generate and manipulate a rotation. The rotation is stored internaly as a quaternion, and can be manipulated similarily to a matrix when applied on a vector.
  • The rotation is stored in the variable "model" of a mesh_drawable structure. model stores an affine transformation which is used as uniform variable when the shape is displayed.

• Note that there is two type of "mesh" structures in use

• • - cgp::mesh storing buffer on data (per vertex: position, normal, uv, color, and triangle connectivity) on CPU
  • • This structure allows to conveniently access to all the data defining a mesh from the C++ code. However these data are not on the GPU, so a mesh cannot be directly displayed.

• • - cgp::mesh_drawable storing VBOs associated to these buffer once sent on the GPU memory (in the sub-structure mesh_drawable_gpu_data) as well as its VAO. The structure also stores uniform parameters that are sent to the shader at every draw call. A default shader and texture id are also be stored with the structure.
  • • This structure only stores the index corresponding to elements on GPU. You cannot modify individual per-vertex elements easily from this structure.

• • Remark: a mesh_drawable can be automatically generated/initialized from a mesh structure in calling the construction mesh_drawable.initialize_data_on_gpu(meshName). However, you cannot create a mesh from a mesh_drawable.

Adding a sphere

• > Add the class variable to scene_structure class (in the file scene.hpp)

cgp::mesh_drawable sphere; 

• In this example, cgp:: is optional as the header file already indicated "using cgp::mesh_drawable;" in its beginning.

• > Initialize this variable in the file scene.cpp in the initialize() function as a sphere of radius 0.5

mesh sphere_mesh = mesh_primitive_sphere(0.5f);
sphere.initialize_data_on_gpu(sphere_mesh);

sphere.initialize_data_on_gpu(mesh_primitive_sphere(0.5f));

• > Display this sphere in the display_frame() function with the following code

draw(sphere, environment);

• > Compile, run, and observe the sphere in the 3D scene.

• Note: You can set geometric transformation using the uniform properties of the mesh_drawable structure (reminder: uniform are parameters passed to the shaders).
• • sphere.model.translation/rotation/scaling = ... (see example on the other displayed shapes)
• Note also that unless specified explicitely, the default shader associated to the mesh_drawable structure is used.
• The three trasformations: translation, rotation, and scaling are simply variables that you can write on. By default, their values are respectively, (0,0,0), the identity, and 1. The content of the variables are used (as uniform values in the shader) when the draw call is used.

• > Add the following line before calling draw on the sphere and observe that the color (as well as any other uniform parameter) can be changed through time in modifying its value at every frame.

sphere.material.color = vec3(1+std::cos(time), 1+std::sin(time), 2.0)/2.0f;

Checkbox interface

• > Add the following variable in the definition of the structure struct gui_parameters (in the file scene.hpp) to store a boolean state indicating when the wireframe should be displayed or not

bool display_wireframe = false;

• > In the function display_gui() (in scene.cpp) add a Checkbox (handled by ImGui library) and link it (through its adress) to the variable display_wireframe in adding this line of code

ImGui::Checkbox("Wireframe", &gui.display_wireframe);

• In running the code, the checkbox should appear. Every time you select/unselect it, the value of the variable display_wireframe change from true to false but doesn't change yet anything in the 3D display.

• > Add the following code in the display_frame function

  if (gui.display_wireframe)
    draw_wireframe(sphere, environment, { 1,1,0 });

• Check that you can now interactively display the wireframe representation of the sphere.

Deforming the vertices of a surface

• > Add the following elements as variables of the object scene_structure

  mesh shape;
  numarray<vec3> initial_position;
  mesh_drawable shape_visual;

• • shape is use to store (on CPU memory) the current state of the deformed mesh
  • initial_position is use to store (on CPU memory) the initial position of each vertex of the shape
  • shape_visual is use to display the deformed shape.

• > Initialise these variables to a uniformly sampled grid shape in the initialize function with the following code

int N = 100;
shape = mesh_primitive_grid({ 0,0,0 }, { 1,0,0 }, { 1,1,0 }, { 0,1,0 }, N, N);
initial_position = shape.position;
shape_visual.initialize_data_on_gpu(shape);
shape_visual.material.color = { 0.6f, 0.6f, 0.9f };

• > Call the drawing of this surface at the end of the display_frame() function

draw(shape_visual, environment);
if (gui.display_wireframe)
    draw_wireframe(shape_visual, environment, { 0,0,0 });

• > Add a new method to the scene class computing the deformation of the surface in
• • Adding the following content in the file scene.cpp
  • Adding the signature of this new method (void evolve_shape()) in the file scene.hpp
  • Call this method in the display() function.

void scene_structure::evolve_shape(float time)
{
    size_t const N = initial_position.size();
    for(size_t k=0; k<N; ++k)
    {
        vec3 const& p0 = initial_position[k];
        vec3& p        = shape.position[k];
        p.z = p0.z + 0.1f * std::cos(10 * p.x + 4 * time);
    }
}

• While the structure mesh is updated by the function, its visual representation (and the associated VBO) are not therefore no change is visible when the code is run. An explicit update of the data stored on the GPU is necessary.
• > To this end add the following code after your call to evolve_shape

shape_visual.vbo_position.update(shape.position);

• This function send again to the GPU the position from the buffer. Note that the update doesn't reallocate any VBO (it is therefore faster that generating a new object) and assume that the size of the buffer remains constant.

• Observe that the surface is now deformed, but its color remains uniform despite the undulation. Indeed, the shader still use the initial normals of the planar grid, and doesn't take into account the change of geometry in the shading.
• > Normals of the surface can be recomputed and updated to the GPU using the following code

// Recompute normals on the CPU (given the position and the connectivity currently in the mesh structure)
shape.normal_update();
// Send updated normals on the GPU
shape_visual.vbo_normal.update(shape.normal);

• Observe that the surface is now correctly updated through its deformation.

• > Change in the evolve_shape function the deformation to the following one using Perlin noise, make sure you understand the result.

float const dz = 0.3f * noise_perlin({ p0.x + 0.2f * time, p0.y, 0 }, 2) + 0.015f * noise_perlin({ 4 * p0.x, 4 * p0.y, time }, 2);
p = p0 + vec3(0, 0, dz);

Texture

• > Update the initialization of your shape variable with the following two lines code and observe that your surface should now be textured

// Reset the color of the shape to white (only the texture image will be seen)
shape_visual.material.color = {1,1,1};

// Load the image and associate the texture id to the structure
shape_visual.texture.load_and_initialize_texture_2d_on_gpu("assets/squirrel.jpg");