Draw curves, segments and trajectories

Goal: display a wavy parametric curve, the edges of a box, and the trail left by a moving sphere.

Starting point: the default project template (src/scene.hpp, src/scene.cpp, see Your first scene). It already declares the shapes sphere and cylinder, a timer, and the members camera_control, environment and gui: reuse them, do not declare them again. The first comment of each code block says in which file and where its lines go: add means insert them, replace means replace the whole function by the one shown.

Interactive CGP demo (draw-curves-and-segments). Open in a new tab.

curve_drawable in short

A curve_drawable stores a list of 3D positions on the GPU and draws them as lines with a single color, without lighting. It has its own model transform and color, like a mesh_drawable. Its display_type chooses how the points are linked:

`display_type`	Draws	OpenGL mode
`curve_drawable_display_type::Curve` (default)	one polyline through all the points	`GL_LINE_STRIP`
`curve_drawable_display_type::Segments`	one segment per pair of points (0-1, 2-3, ...)	`GL_LINES`

1. Declare the drawables

// scene.hpp - add in scene_structure
curve_drawable curve;
curve_drawable box;

2. Fill the positions and upload them (once)

// scene.cpp - add at the end of initialize()
numarray<vec3> positions;
int const N = 150;
for (int k = 0; k < N; ++k) {
    float const u = k / (N - 1.0f);                  // u in [0,1]
    positions.push_back({ 0.0f, 4.0f * (u - 0.5f), 0.1f * std::cos(16 * Pi * u) });
}
curve.initialize_data_on_gpu(positions);
curve.color = { 0, 0.6f, 0 };
curve.model.translation = { 1.5f, 0, 0 };

// The 12 edges of a cube, given as pairs of extremities
numarray<vec3> edges = {
    {-1,-1,-1},{1,-1,-1},  {1,-1,-1},{1,1,-1},  {1,1,-1},{-1,1,-1},  {-1,1,-1},{-1,-1,-1},
    {-1,-1,1},{1,-1,1},    {1,-1,1},{1,1,1},    {1,1,1},{-1,1,1},    {-1,1,1},{-1,-1,1},
    {-1,-1,-1},{-1,-1,1},  {1,-1,-1},{1,-1,1},  {1,1,-1},{1,1,1},    {-1,1,-1},{-1,1,1} };
box.display_type = curve_drawable_display_type::Segments;   // set before the upload
box.initialize_data_on_gpu(edges);

3. Draw (every frame)

// scene.cpp - add in display_frame(), before timer.update()
draw(curve, environment);
draw(box, environment);

Common pitfalls

The box is drawn as a zigzag: display_type was left to its default (Curve), which links every point to the next one. Use Segments for independent edges, vectors or grids.
The lines stay thin whatever I try: the line width cannot be changed in the OpenGL core profile used by CGP (glLineWidth above 1 is ignored on most systems, macOS included). For thick lines, draw thin cylinders (mesh_primitive_cylinder) instead.
The curve is red: color was not set, or it was set before initialize_data_on_gpu, which resets it to red (and model to the identity). Set both after the upload.
Changing the positions array has no effect: the GPU keeps its own copy. Send the new values with curve.vbo_position.update(positions); (see below).

Variations

Animate the curve: keep the numarray<vec3> in the scene, modify it in display_frame(), then call curve.vbo_position.update(positions);. The number of points must not exceed the one given to initialize_data_on_gpu; to draw only the first 50 points, call draw(curve, environment, 50);.
Circle: circle.initialize_data_on_gpu(curve_primitive_circle(0.5f, {0,0,0}, {0,0,1}, 40)); (radius, center, normal, number of samples). Scale it with circle.model.scaling.
Trail of a moving object: declare trajectory_drawable trail{100}; (keeps the 100 last positions), call trail.add(sphere.model.translation); at every frame, then draw(trail, environment);. Its color is trail.visual.color, to set after the first add (which uploads the buffer and resets the color to red), for instance right after each add.
Curve whose number of points grows (strokes, recorded paths): see Sketch curves with the mouse.

Source: EPITA Ani3D repository epita_ani3d_lab_code-2027, scenes scenes_ani3d/00_introduction (parametric curve) and scenes_ani3d/02_sphere_collision (edges of the cube), scenes_ani3d/05_stable_fluids (velocity vectors updated with vbo_position.update) and scenes_ani3d/06_sph (curve_primitive_circle).