OrcaSlicer/docs/HLSD/gcode-preview-dragging.md

G-code preview while dragging

The sliced preview draws every toolpath segment of the plate as an instanced box. On a large plate that is tens of millions of segments, and the frame is GPU-bound: the cost is the number of instances drawn, not anything the CPU does per frame. Dragging the camera over such a plate cannot keep up. The preview_reduced_detail_mode preference (Graphics > G-code Preview, off by default) lets the preview draw less while the user drags and put the full toolpaths back when they let go.

Preference	Values	Effect
preview_reduced_detail_mode	off, solid, shell	what is drawn while dragging

libvgcode (src/libvgcode) builds and binds the reduced toolpath set, GCodeViewer maps the preferences onto it and draws the solid model, and GLCanvas3D decides when the user is dragging. The OpenGL ES path keeps a single set and ignores the preference.

Two sets, one walk

ViewerImpl::update_enabled_entities() walks the visible vertex range once and fills two segment index buffers side by side: the full set and the reduced set (segments and options). Building them together is what makes switching free: starting or ending a drag is a buffer binding, never a rebuild. A change of mode does rebuild. Nothing is built while the mode is off, and the reduced buffers are then uploaded empty so that the last set does not stay allocated.

Whatever the mode leaves out, the bottom and top layers of the visible range are kept whole: they are the faces the range cuts open, and the top is what the user is looking at.

Modes

• EndLayersOnly (solid in the preference) keeps only the two end layers. GCodeViewer then draws the sliced objects and the prime tower as opaque solids, see below.
• ShellOnly (shell) keeps every role but the sparse infill, internal solid infill, internal bridge infill and gap fill, which lie under the skins or between the walls. Walls, top and bottom surfaces, bridges, supports and the prime tower are drawn whole, so from outside the print looks as it does at rest. The roles come from the slicer, so the classification is exact wherever the slicer's is; what it cannot express is the inside of the prime tower and of a support, which share one role with their outside and are kept whole. A profile with no top or bottom shell layers, or no walls, leaves the infill on the surface, so GCodeViewer turns the hiding off for such a print when it loads it, from the print's default region settings; per-object overrides are not consulted.

The solid model

The preview already loads the sliced objects as shells for its translucent ghost. GCodeViewer::render_solid_model() draws those shells opaque, in their filament colors, with the gouraud shader, whose z range cuts them to the visible layer range: from the top of the range's bottom layer to the bottom of its top layer, since those two layers are drawn as toolpaths and a face of the model in either layer's plane would flicker against them. The translucent ghost is drawn first, as in every other mode, so the part above the cut stays visible while dragging. The cut itself is capped, one body at a time: a pass counts, per pixel and with the depth test off, the body's back faces less its front faces into the stencil buffer, which is zero for a closed body and one where the cut has opened it, and a quad in each cut plane, in the body's colour darkened, is drawn where it is not zero. Between the lines of a sparse layer on the cut, that quad is what shows, rather than the first layer or the bed far below moving with the camera. A view through both cut planes of a body with neither wall in the way is not capped. The two toolpath layers of the reduced set are drawn afterwards and cap the cut with what was really printed there. The shells hold only the objects, so while this mode is on the prime tower is added from its sliced mesh, positioned as the print placed it. It is added or removed on its own when the mode changes, without reloading the objects, keeps its opaque color so that it never appears among the translucent shells, and stays out of their bounding box. Supports have no mesh and are not shown, and load_shells() drops every non-model-part volume, so a negative volume is not cut out.

A plate whose shells are not loaded keeps drawing toolpaths, since the solid model would leave only the end layers.

Deciding that the user is dragging

GLCanvas3D::_update_preview_interaction() runs at the top of every preview frame, before the canvas decides whether to reuse its cached scene, so that the switch lands in that frame. Dragging is GLCanvas3D::is_user_interacting(), the same answer the scene cache reads: the camera, the navigator, a gizmo, the rectangle selection or either slider being held. A slider reports this from ImGui's active id rather than its dirty flag, which is raised and consumed inside one frame. A wheel step has no duration, so it holds the reduced set for a 150 ms settle time instead, and the frame that restores the toolpaths is scheduled for when that time runs out, since the render timer only wakes the idle loop. A drag cut short by focus or capture loss is ended explicitly, and a button release wakes the idle loop, because on some platforms nothing else would until the next input.

Reused scene frames

GLCanvas3D keeps its last scene pass for frames that only rebuild the overlay (SceneCache). Its key covers the canvas size, the camera and hover state, not what the toolpath sets draw, so a frame that reuses the scene must never be one on which the set is switched. _update_preview_interaction() therefore reports whether the bound set changed, and a frame on which it did redraws the scene. The canvas neither captures nor reuses the scene while the user drags, so no reduced frame outlives a drag, and the frame that ends a wheel's settle time is requested as a full frame.