OrcaSlicer/docs/HLSD/section-view.md

Section view — High Level Design

Purpose and scope

Section view hides whatever lies between the camera and a plane, so the user can look inside objects in Prepare and in the assembly view, and inside the toolpaths in Preview. It is a view setting: it changes nothing in the model, the slice or the project file, and it does not reach plate thumbnails.

The user controls it from the section button of the canvas toolbar in the bottom left corner of the 3D view. The button opens a panel above it with a slider for the depth of the cut, a "Set viewing angle" button that turns the plane to face the camera at the same depth, and a button that resets the depth to zero. The panel is an ordinary overlay window, not a popup, so the scene keeps taking clicks and drags while it is open; the button or Esc closes it again. Esc closes the panel before it closes a gizmo or clears the selection. The button is highlighted while a section cuts the scene and has shortcuts of its own: the mouse wheel over it moves the plane, a right click switches the section off and back on, and a middle click sets the viewing angle. Alt + mouse wheel moves the plane anywhere in the 3D view, with or without a gizmo open.

State

Prepare and Preview share one section view, so a cut made in either tab is the same cut in the other. The assembly view, whose objects sit apart from their places on the plate, and the Design tab keep their own. The section itself is two values.

• Ratio, from 0 to 1. At 0 the section is off. As the ratio grows, the plane sweeps the sphere around the objects, from its side facing the camera to the opposite side, so at 1 everything is cut away.
• Normal, taken from the camera direction the first time the section is switched on, and again whenever the user sets the viewing angle. The plane keeps that orientation while the camera orbits and while the section is off, so the cut face can be seen from any side and bringing the depth back to 0 does not lose the angle.

The ratio in use when the section is switched off is kept, and the right click on the button brings the section back at that ratio, which restores the same cut. Whether the panel is open is shared along with the section.

The sphere is recomputed every frame from the volumes of the canvas the section view belongs to: the objects on the current plate, or every object when that plate is empty. Preview holds no objects of its own, so it places the plane across the volumes of Prepare, which makes it cut the toolpaths exactly where Prepare cuts the objects. Only G-code opened on its own, without objects, is measured by its toolpaths. In the assembly view the sphere is around the whole assembly. The ratio therefore keeps its meaning when objects move or the user switches plates. It is not a fixed position in world space.

Only the tab on screen can change the section, and switching tabs redraws the whole scene and closes the open gizmo, so neither tab ever shows a stale cut.

Where the plane applies

GLCanvas3D::_get_section_view_plane() turns the state into a plane in the convention of ClippingPlane::is_point_clipped(). Everything that draws or picks the scene reads that plane.

• Volumes. The plane goes to the clipping_plane uniform of the volume shaders. The same uniform serves the gouraud, phong and X-ray passes and the colour picking pass.
• Cut faces. Clipping only discards fragments, which would leave the cut volumes hollow. _render_section_view_caps() draws their cut faces with one MeshClipper per model part the plane passes through. A clipper recomputes its face only when the plane or the volume moves. Modifiers, the wipe tower and SLA auxiliaries get no face.
• Toolpaths. libvgcode takes the plane through Viewer::set_clipping_plane(). It draws each extrusion as only the faces of a diamond-section prism that turn towards the camera, so discarding the fragments on the clipped side would leave open shells. Instead, the segment shader follows the view ray from a fragment that is cut away to the plane. When the extrusion's diamond section still holds that point, the fragment is shaded as the cut face, lit as the plane faces; otherwise it is discarded. The cut face keeps the depth of the fragment it replaces, which is safe: along that ray everything else still shown lies behind the plane. The shader writes no gl_FragDepth, so early depth testing survives. Option markers are cut away whole, by their centres. The shadow casters share the segment shader, so what is cut away casts no shadow either. Preview shells are drawn by another shader and are not clipped.
• Picking. get_raycaster_clipping_plane() returns the same plane, so hover, selection and the perspective pan anchor ignore what the user cannot see.

Gizmos

A gizmo that clips its object itself owns the gizmo data pool's ObjectClipper, and its plane replaces the canvas section while the gizmo is open. GLGizmosManager::get_clipping_plane() reports that plane, or nothing when no open gizmo has a clipper. There are two cases.

• Painting tools and brim ears show the canvas section on the object they edit. GLGizmosManager::update_section_view() copies the ratio and normal into their clipper whenever the pool is updated or the section changes. The clipper then places the plane across the edited instance, which is the only object shown. The painting tools keep clipping their own triangles, raycasts and cut face through it. Brim ears always cut horizontally from the top, because the ears sit on the plate. At ratio 0 the clipper holds no plane at all, so the raycasts are not clipped.
• Cut and mesh boolean use the clipper for their own purposes, so the canvas section is suspended while they are open.

Every other gizmo, including move, rotate and scale, leaves the canvas section in place.

Alt + mouse wheel

The canvas handles Alt + wheel after the gizmos had their turn, so it works the same in every tab and with any gizmo open. On Windows, releasing Alt when no key was pressed since it went down opens the window menu, and a wheel turn does not count as a key. Under the custom title bar that menu is invisible, yet it takes the keyboard and the next click, which looks like a frozen 3D view. After Alt + wheel the canvas therefore consumes the Alt release instead of passing it on.

Redraw

The button and the panel are part of the ImGui overlay, which is built after the frame's scene is drawn. A change to the section therefore marks the scene dirty and asks for one more frame. The cached scene is never reused across a change.