I am struggling to understand how to change my algorithm to handle raycasting (utilized for MousePicking) using a Perspective projection and an Orthographic projection.
Currently I have a scene with 3D objects that have AxisAligned bounding boxes attached to them.
While rendering the scene using a perspective projection (created with glm::perspective) I can successfully use raycasting and my mouse to "pick" different objects in my scene. Here is a demonstration.
If I render the same scene, but using an Orthographic projection, and positioning the camera above the facing down (looking down the Y axis, Imagine like a level editor fora game) I am unable to correctly raycasting from the where the user clicks on the screen so I can get MousePicking working while rendering using an Orthographic projection. Here is a demonstration of it not working.
My algorithm at a high level:
auto const coords = mouse.coords();
glm::vec2 const mouse_pos{coords.x, coords.y};
glm::vec3 ray_dir, ray_start;
if (perspective) { // This "works"
auto const ar = aspect_rate;
auto const fov = field_of_view;
glm::mat4 const proj_matrix = glm::perspective(fov, ar, f.near, f.far);
auto const& target_pos = camera.target.get_position();
glm::mat4 const view_matrix = glm::lookAt(target_pos, target_pos, glm::vec3{0, -1, 0});
ray_dir = Raycast::calculate_ray_into_screen(mouse_pos, proj_matrix, view_matrix, view_rect);
ray_start = camera.world_position();
}
else if (orthographic) { // This "doesn't work"
glm::vec3 const POS = glm::vec3{50};
glm::vec3 const FORWARD = glm::vec3{0, -1, 0};
glm::vec3 const UP = glm::vec3{0, 0, -1};
// 1024, 768 with NEAR 0.001 and FAR 10000
//glm::mat4 proj_matrix = glm::ortho(0, 1024, 0, 768, 0.0001, 10000);
glm::mat4 proj_matrix = glm::ortho(0, 1024, 0, 768, 0.0001, 100);
// Look down at the scene from above
glm::mat4 view_matrix = glm::lookAt(POS, POS + FORWARD, UP);
// convert the mouse screen coordinates into world coordinates for the cube/ray test
auto const p0 = screen_to_world(mouse_pos, view_rect, proj_matrix, view_matrix, 0.0f);
auto const p1 = screen_to_world(mouse_pos, view_rect, proj_matrix, view_matrix, 1.0f);
ray_start = p0;
ray_dir = glm::normalize(p1 - p0);
}
bool const intersects = ray_intersects_cube(logger, ray_dir, ray_start,
eid, tr, cube, distances);
In perspective mode, we cast a ray into the scene and see if it intersects with the cube surrounding the object.
In orthographic mode, I'm casting two rays from the screen (one at z=0, the other at z=1) and creating a ray between those two points. I set the ray start point to where the mouse pointer is (with z=0) and use the ray direction just calculated as inputs into the same ray_cube_intersection algorithm.
My question is this
Since the MousePicking works using the Perspective projection, but not using an Orthographic projection:
- Is it reasonable to assume the same ray_cube intersection algorithm can be used with a perspective/orthographic projection?
- Is my thinking about setting the ray_start and ray_dir variables in the orthographic case correct?
Here is the source for the ray/cube collision algorithm in use.
glm::vec3
Raycast::calculate_ray_into_screen(glm::vec2 const& point, glm::mat4 const& proj,
glm::mat4 const& view, Rectangle const& view_rect)
{
// When doing mouse picking, we want our ray to be pointed "into" the screen
float constexpr Z = -1.0f;
return screen_to_world(point, view_rect, proj, view, Z);
}
bool
ray_cube_intersect(Ray const& r, Transform const& transform, Cube const& cube,
float& distance)
{
auto const& cubepos = transform.translation;
glm::vec3 const minpos = cube.min * transform.scale;
glm::vec3 const maxpos = cube.max * transform.scale;
std::array<glm::vec3, 2> const bounds{{minpos + cubepos, maxpos + cubepos}};
float txmin = (bounds[ r.sign[0]].x - r.orig.x) * r.invdir.x;
float txmax = (bounds[1 - r.sign[0]].x - r.orig.x) * r.invdir.x;
float tymin = (bounds[ r.sign[1]].y - r.orig.y) * r.invdir.y;
float tymax = (bounds[1 - r.sign[1]].y - r.orig.y) * r.invdir.y;
if ((txmin > tymax) || (tymin > txmax)) {
return false;
}
if (tymin > txmin) {
txmin = tymin;
}
if (tymax < txmax) {
txmax = tymax;
}
float tzmin = (bounds[ r.sign[2]].z - r.orig.z) * r.invdir.z;
float tzmax = (bounds[1 - r.sign[2]].z - r.orig.z) * r.invdir.z;
if ((txmin > tzmax) || (tzmin > txmax)) {
return false;
}
distance = tzmin;
return true;
}
edit: The math space conversions functions I'm using:
namespace boomhs::math::space_conversions
{
inline glm::vec4
clip_to_eye(glm::vec4 const& clip, glm::mat4 const& proj_matrix, float const z)
{
auto const inv_proj = glm::inverse(proj_matrix);
glm::vec4 const eye_coords = inv_proj * clip;
return glm::vec4{eye_coords.x, eye_coords.y, z, 0.0f};
}
inline glm::vec3
eye_to_world(glm::vec4 const& eye, glm::mat4 const& view_matrix)
{
glm::mat4 const inv_view = glm::inverse(view_matrix);
glm::vec4 const ray = inv_view * eye;
glm::vec3 const ray_world = glm::vec3{ray.x, ray.y, ray.z};
return glm::normalize(ray_world);
}
inline constexpr glm::vec2
screen_to_ndc(glm::vec2 const& scoords, Rectangle const& view_rect)
{
float const x = ((2.0f * scoords.x) / view_rect.right()) - 1.0f;
float const y = ((2.0f * scoords.y) / view_rect.bottom()) - 1.0f;
auto const assert_fn = [](float const v) {
assert(v <= 1.0f);
assert(v >= -1.0f);
};
assert_fn(x);
assert_fn(y);
return glm::vec2{x, -y};
}
inline glm::vec4
ndc_to_clip(glm::vec2 const& ndc, float const z)
{
return glm::vec4{ndc.x, ndc.y, z, 1.0f};
}
inline glm::vec3
screen_to_world(glm::vec2 const& scoords, Rectangle const& view_rect, glm::mat4 const& proj_matrix,
glm::mat4 const& view_matrix, float const z)
{
glm::vec2 const ndc = screen_to_ndc(scoords, view_rect);
glm::vec4 const clip = ndc_to_clip(ndc, z);
glm::vec4 const eye = clip_to_eye(clip, proj_matrix, z);
glm::vec3 const world = eye_to_world(eye, view_matrix);
return world;
}
} // namespace boomhs::math::space_conversions
