Reflect the environment on a shape

Goal: display a chrome-like torus that reflects the sky around it.

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 (reflect-the-environment). Open in a new tab.

How it works

For each pixel of the shape, the fragment shader computes the direction from the camera to the surface, reflects it around the normal, and reads the color of the skybox in that direction. The shader therefore needs a second texture, the cube map of the skybox. A mesh_drawable can carry extra textures in its supplementary_texture map: each entry name -> texture is bound at draw time to the GLSL sampler of the same name.

For a point of the torus, V goes from the surface toward camera_position and N is the normal; reflect(-V, N) gives the mirror direction, and the pixel takes the color of the cube map in this direction. Tap to enlarge.

This page starts from the template with a skybox already set up as in Add a skybox (the member skybox, drawn first in display_frame()).

1. Write the fragment shader

Copy shaders/mesh/mesh.frag.glsl to shaders/environment_map/environment_map.frag.glsl (create the folder shaders/environment_map/) and add the reflection (the vertex shader stays the default one):

// shaders/environment_map/environment_map.frag.glsl (only the changed lines)
uniform samplerCube image_skybox;   // the cube map, sent as a supplementary texture
uniform mat3 skybox_rotation;       // same rotation as the skybox

void main()
{
    // ... default code computing camera_position, N (normal) and L ...

    // V, unit vector toward the camera: declared inside the "if" of the specular
    // component in the default shader, move this line before the "if"
    vec3 V = normalize(camera_position - fragment.position);

    // ... default code computing the diffuse and specular components, color_image_texture ...

    vec3 R_skybox = reflect(-V, N);
    vec4 color_environment_map = texture(image_skybox, skybox_rotation * R_skybox);

    vec3 color_object = fragment.color * material.color * color_image_texture.rgb * color_environment_map.rgb;
    // ... rest of the default shader unchanged
}

2. Load the shader and attach the cube map (once)

// scene.hpp - add in scene_structure
mesh_drawable torus;
uniform_generic_structure uniforms_reflection;

// scene.cpp - add at the end of initialize(), after the creation of the skybox
torus.initialize_data_on_gpu(mesh_primitive_torus());
torus.model.translation = {0, 0, 1.0f};   // above the sphere and the cylinder of the template
torus.shader.load(
    project::path + "shaders/mesh/mesh.vert.glsl",
    project::path + "shaders/environment_map/environment_map.frag.glsl");

torus.supplementary_texture["image_skybox"] = skybox.texture;     // name of the GLSL sampler
uniforms_reflection.uniform_mat3["skybox_rotation"] = skybox.skybox_rotation;

3. Draw (every frame)

The skybox is already drawn first (see Add a skybox); add the torus after it:

// scene.cpp - add in display_frame(), before timer.update()
draw(torus, environment, 1, true, uniforms_reflection);   // extra uniforms for this shape only

The arguments after environment are the number of instances (1), expected_uniforms (true: warn if the shader does not use a uniform) and additional_uniforms; this fifth argument sends the extra uniforms to the torus only: the other shapes, which use the default shader, do not receive skybox_rotation.

Common pitfalls

The torus is black: the cube map is not bound (the key of supplementary_texture differs from the name of the sampler in GLSL), or the rotation matrix was never sent (an unset uniform matrix is zero, and so is the direction read in the cube map).
The reflection does not match the sky: the shader does not apply the same rotation as the skybox. Send skybox.skybox_rotation, after it has been set.
The reflection is mirrored or does not move with the camera: V must go from the surface toward the camera, and both N and V must be in world space (the default shader computes them so).
Warning about an unused uniform on every shape: the rotation was put in environment.uniform_generic, which is sent to all shapes. Use the per-draw uniforms as above, or set environment.default_expected_uniform = false; (it silences these warnings for all environments).

Variations

Metallic tint: keep material.color to tint the reflection, and lower the diffuse coefficient: torus.material.phong.diffuse = 0.2f;.
Combine two ordinary textures (brick image darkened by cracks): load the second one directly into the map, wall.supplementary_texture["image_texture_2"].load_and_initialize_texture_2d_on_gpu(project::path + "assets/crack.png");, declare uniform sampler2D image_texture_2; in the fragment shader and multiply the two colors read from the textures.
Refraction (glass): in the shader, replace the call reflect(-V, N) by refract(-V, N, 1.0/1.5) (ratio of the refraction indices of air and glass).

Source: EPITA Ani3D repository epita_ani3d_lab_code-2027, examples 05_environment_map/02_environment_map (scene.cpp, environment_map.frag.glsl) and 03_shaders/02_multi_texture (scene.cpp, mesh_multi_texture.frag.glsl). Adaptation: the example sends the rotation through environment.uniform_generic and disables the uniform warnings globally; this page uses the per-draw uniforms of draw.