leocad/docs/asset-loading.md

Asset loading

This guide describes the current implementation. Update it with changes to loading, ownership, readiness, scheduling, or output policy. Names below refer to existing APIs, rather than proposed interfaces.

This is the implementation record replacing the streaming-assets plan. It documents the current contracts and accepted decisions, rather than retaining completed stages or superseded proposals as future requirements.

What users see

Opening a model still parses its file on the UI thread, but does not wait for every referenced mesh or texture. Unavailable geometry initially draws as cubes; pieces appear incrementally once their meshes and textures are ready. Already-loaded shared assets can appear immediately. A disk cache hit still requires worker loading and any texture dependencies; it is not a UI-thread fast path and receives no special queue priority.

The status-bar spinner is visible while the active project has pending pieces or generated meshes. Its tooltip reports loading counts, or regeneration when only synth work remains. Counts describe unique required PieceInfo identities plus generated piece instances, not the number of placed bricks or global jobs. Requirements include all project models and their nested dependencies, not just the visible step. The spinner hides immediately when work settles; there is no "all pieces loaded" message. A separate failure indicator retains error details until the project's failures disappear or the project changes.

Views redraw and update bounds as results arrive, but keep the user's camera. Interactive and output camera fits go through lcView::ZoomExtents(), with fallback geometry included for interactive views and only available geometry included for outputs. The view delegates fitting and saved camera undo history to lcModel::ZoomExtents(). lcView::ZoomExtentsAtStep() restores the model's current step after fitting. The adjustment flag passes through view, model, and camera. The camera adjusts clipping after updating its fitted position, using the fit's transformed bounding points without collecting bounds again. The view fit entry points take an explicit AdjustFarPlane flag. Initial view and preview fits and automatic image output fits enable it, using the bounds available at that time; ordinary fits disable it. The distance expands only when those bounds extend beyond both the default 50,000 and the current distance. Later asset updates, redraws, navigation, and fits do not adjust clipping again. Increases include a 10% margin and round up to multiples of 10,000; they never shrink the distance or change saved model cameras. Explicit command-line clipping planes remain authoritative for image output. Restored tab layouts defer clipping adjustment until the ordinary camera's saved position and settings have been applied, so a saved distance cannot undo the load-time expansion. Named model cameras retain their saved clipping settings. During synth edits, the previous generated mesh stays visible until a replacement settles. Images and geometry exports wait for their requirements and omit failed geometry instead of saving loading cubes. See Caller policies for the deliberately stricter preview rules and thumbnail fallbacks.

Accepted decisions

These decisions are the baseline, not open TODO items. Do not propose reversing them merely because another design is possible or an older requirement differs. Revisit one only at the user's request or when a concrete regression, changed requirement, or relevant measurement invalidates its rationale. Explain that new evidence and update this record with any agreed change.

Decision	Rationale and alternatives set aside
Keep identities, sources, references, and shared buffers in the library; pending work belongs to its loader.	One registry and one completion path avoid competing lifetime/readiness authorities. Do not create a second asset registry in the loader.
Use the existing Qt worker pool and shared offscreen GL context, with UI-thread publication.	Reuses existing infrastructure and keeps model mutation, color registration, upload, and rendering sequential. Do not publish assets from workers or introduce a separate renderer/context for loading.
Use promotion-only priorities and existing reference counts.	Models open at their last step; precise consumer demotion added bookkeeping for little demonstrated benefit. Per-consumer pointer maps, step-based priorities, outgoing-project demotion, and scoped priority removal were deliberately set aside. Accept briefly elevated work after a requesting view closes.
Treat missing geometry as Failed; keep structural type and library membership separate.	A separate Placeholder/Missing type or readiness value duplicates failure state. Cubes are shared display fallbacks, not stored asset meshes. Library membership is established explicitly by the PieceInfo constructor.
Require all includes and textures for successful asset loading; never cache an erroneous mesh as successful.	Partial private builds must not masquerade as ready assets. Permissive output is a caller policy that omits failed assets, not relaxed loader success criteria.
Allow model outputs to save available geometry after waiting.	Geometry exports, HTML, images, instructions, and printing do not reject the whole operation because an asset failed. Actual rendering/file failures remain errors. Strict previews and thumbnail fallback rules remain separate.
Keep missing HTML parts-list entries with empty thumbnails and quantities.	Missing geometry must not erase the inventory entry or substitute a loading cube. Instruction parts lists currently omit unavailable thumbnails.
Fit outputs to available geometry; interactive fitting includes fallbacks.	Failed cubes must not distort exported framing. Include root direct geometry and available children, preserve explicit cameras, and never automatically refit interactive views as assets arrive.
Build synth meshes per instance on workers; retain the previous generated mesh during edits and cancel obsolete work cooperatively.	Control points belong to placements, not shared library assets. Keep interactive edits stable while ensuring output never mistakes an older mesh for the completed replacement.
Keep the uncached mesh draw path and debounced shared-buffer repacking.	Measured repacking cost was small and repeated scenes benefited from shared buffers. Per-mesh buffers, visibility-triggered uploads, and mesh-buffer upload as a readiness requirement were set aside without evidence of a benefit. Texture upload remains part of readiness.

Responsibilities and entry points

Owner	Responsibility	Source
lcPiecesLibrary	Piece identities and references, all source discovery and lookup, texture file indexes, archives, primitive and mesh caches, settings, shared mesh buffers, and public forwarding APIs. Owns the texture manager and supplies its archive reader.	lc_library.h, lc_library.cpp
lcTextureManager	Texture identities and references, snapshots, and private CPU decoding of resolved sources.	lc_texturemanager.h, lc_texturemanager.cpp
lcAssetLoader	Pending requests, worker scheduling, private results, texture upload, publication, cancellation, readiness waits, and deferred settlement notifications.	lc_assetloader.h, lc_assetloader.cpp
PieceInfo / lcTexture	Published geometry or texture data, readiness state, and asset metadata.	pieceinf.h, lc_texture.h
Projects, models, and views	Project-local names, required-asset traversal, generated-piece ownership, bounds, fallback display, cameras, and redraws.	project.cpp, lc_model.cpp, piece.cpp

The library owns the loader. Call library entry points from the UI thread for asset acquisition, release, scheduling, waiting, and invalidation. Workers use source snapshots and the library's build services; they do not consult the active project or publish into live models.

The texture manager is owned by the library and owns the only texture registry. FindTextureDeferred() resolves embedded data, project-directory files, and indexed library files in the library, then asks the manager to acquire an identity for that resolved source. Resolution preserves the prefixed search order and canonicalizes disk paths. The project directory is passed separately to acquisition so the manager preserves temporary-texture metadata without searching files itself. Library texture APIs forward reference changes and decoding to the manager. FindTexture() still gathers the explicit project's context and waits through the loader. Texture requests, priority promotion, cancellation, upload/retries, and part dependency settlement remain in lcAssetLoader; its in-flight texture references are acquired through the manager. lcFileSource lives in lc_filesource.h; texture build results live in lc_texturemanager.h. lcTexture::mSource stores this value; copies for workers do not snapshot file contents. Texture names, project directories, and temporary flags remain separate. The loader copies lcTexture::mSource into each texture request. The library's BuildTextureData() and AddTextureReference() are internal loader services, accessible through the existing lcAssetLoader friendship.

The library calls OpenFile() on the reading thread and passes the opened lcFile& to the texture manager for decoding. BuildTextureData() continues to run on a worker, so file opening and archive extraction stay on that worker. The manager stores no file-reader callback or library instance. Success returns an owned lcFile positioned at zero: disk sources retain an open lcDiskFile, archive entries and decoded embedded data use lcMemFile. Opening never searches for another source or parses its contents. Failures carry an error; the caller retains the result while consuming the file. Archive access is unlocked before image decoding, and returned memory files are independent of their archive or embedded source. Each lcZipFile serializes its own extraction; the library adds no archive-wide mutex. Archive replacement and closure remain protected by draining/pausing workers first. Texture identities use the manager's own mutex; source indexes and source resolution belong to the library. Library unload drains loader work, destroys meshes that release texture references, clears the texture manager, then closes archives. Workers must be drained before the manager is cleared or archive indexes are replaced.

lcTextureManager::DecodeTextureData(lcFile&) decodes disk files through their existing QFile and memory files through QImage::loadFromData(), without an additional device adapter or disk-to-memory copy. Disk cursor restoration runs on success and failure; memory decoding leaves the cursor unchanged and validates the remaining buffer length. Qt handles disk reads, seeking, EOF, and I/O errors. Files/cursors are private to each request, not shared between workers. Texture readiness waits use the same loader worker path; unused direct texture-loading overloads and their synchronous library/manager forwarding chain have been removed.

Decoded worker results hold a QImage, normalized to RGB888 or RGBA8888 and scaled with Qt's smooth transformation to the existing power-of-two dimensions. Textures retain the decoded QImage through upload without a second pixel buffer. Uploads honor Qt's scanline alignment, packing unusual row strides only when needed. CPU mipmap generation copies pixel rows without padding into its existing working buffer; its filtering, OpenGL formats, and shaders remain unchanged. Each transfer sets its required unpack alignment, and texture upload restores the caller's unpack alignment on every exit. Pack alignment is left unchanged. SetImage() and SetImages() only prepare and store pixels. Generated texture callers upload explicitly, while the loader controls upload attempts and retries. SetImages() retains cube-map faces and supplied mip levels.

lcTexture::SetFailed() assigns readiness and failure diagnostics together, clearing stale details when none are supplied. Decoding and upload failures use the same setter. It retains decoded pixels for bounded upload retries; terminal request handling explicitly discards them when required.

FindPiece() returns a borrowed identity, without acquiring a reference. Lookup checks the supplied project's name index, then the shared library catalog, then optionally an external file beside the project. Creating a missing identity records Failed. CreateModelPiece() differs: it returns one acquired reference for the model to release. Library membership is independent of readiness and Part/Model/Project type. Project-local names cannot replace another project's names. Worker include resolution uses library sources, while model geometry and texture search directories are snapshotted before dispatch.

Project-local identities and source paths

Project::mPieceIndex maps normalized local filenames to borrowed PieceInfo pointers. "Project-local names" means lookup visibility, not object ownership: the library tracks local identities separately in mProjectPieces, and reference counts govern their lifetime. Models know their owning project from construction; temporary models can have no project. Do not use the active project implicitly when resolving a candidate project, external file, or preview.

This scope lets different open/candidate/preview projects use the same submodel name without replacing one another's identities. Containers remain structurally Model or Project even when their direct mesh fails. The shared catalog remains separate; worker primitive/part includes must not accidentally resolve through a project's name index.

Source directories are also distinct from the project's current save filename. Models preserve asset search directories when moving between projects or saving to a new folder, so embedded geometry can still find its source textures. Save As uses QSaveFile; source state and the old filename are restored if writing or commit fails. After a successful folder change, local external references are remapped in place against the destination folder and direct model meshes are invalidated/requeued. Missing replacements become failed identities and retain their references. Saving in the same folder skips this remapping.

Merge validates name/identity conflicts before mutating either project, renames conflicting submodels, transfers local registrations, and preserves source directories. Renames update live instance IDs and undo/redo snapshots. Deletion and project teardown unregister borrowed index entries; they must not leave lookup pointers to deleted objects. These paths are in project.cpp and lc_model.cpp.

Active project file monitoring

Only the active project's own file is watched. Preview projects, loading candidates, and referenced model projects do not register filesystem watches. lcApplication::SetProject() activates the new project's watcher after changing the active project. Loaded bytes and committed saves establish its content hash baseline, so unchanged rewrites, metadata updates, and own saves do not prompt.

File and containing-directory events are debounced before checking the active file's contents, existence, and readability. Directory watches reattach after replacement and detect deletion/recreation, including missing parent directories. Transient read failures receive bounded retries. Events for unrelated files in the directory only cause the current file to be checked.

The reload handler verifies the active project and filename before showing a dialog and after the response. Candidate ownership is scoped so failed reloads release their resources. Referenced files retain their existing loading and readiness policies and are not monitored or reloaded in response to filesystem events.

Readiness

flowchart LR
    subgraph Part["PieceInfo request"]
        U[Unloaded] --> Q[Queued] --> L[Loading]
        L --> A[AwaitingTextures] --> R[Loaded]
        L --> R
        L --> F[Failed]
        A --> F
        Q --> C[Cancelled]
    end
    subgraph Texture["Texture request"]
        TU[Unrequested] --> TQ[Queued] --> TD[Decoding]
        TD --> TC[Decoded] --> TR[Ready]
        TD --> TF[Failed]
        TC --> TP[RetryPending] --> TC
        TC --> TF
    end

The diagram shows the normal paths. Parsing or synth generation and geometry conversion run consecutively in one worker job. Intermediate lcLibraryMeshData stays local to the library's BuildPieceData() or the synth definition's BuildMesh(); both return a private converted mesh for UI-thread finalization. Invalidation or unloading resets readiness; cancellation and stale-result disposal also have request-level terminal flags, independent of the published asset state. AwaitingCompletion tracks an outstanding worker job through completion processing.

A part becomes Loaded after its private mesh is committed and all declared textures are ready. Valid empty geometry may also settle successfully. A missing include or declared texture is a failure, even if some geometry was built. Texture names remain in mesh sections and cache data when lookup fails.

A texture is ready only when IsReady() reports Ready with a nonzero GL texture. CPU pixels, HasImageData(), and !NeedsUpload() do not establish readiness. The completion pump uploads through the global shared offscreen context without requiring the part to be drawn. Upload failures retry at most three times, with 50 ms and 200 ms delays; decoding failures settle immediately. Loader-decoded textures receive LC_TEXTURE_MIPMAPS.

Container readiness is not recursive readiness. A Project PieceInfo, or a Model without direct geometry, can be Loaded while its children are pending or failed. Models with directly embedded geometry have their own mesh request. Use lcModel::GetRequiredPieces() and GetRequiredSynthPieces() for recursive requirements, including nested and external models, with cycle protection.

Diagnostics and retries

Part error text lives in the library's sparse mFailedPartErrors map, queried through GetPieceLoadError(), rather than an empty string in every catalog part. Synth error text is likewise sparse in the loader and available through GetSynthMeshError(). Entries must be cleared when their identity is unloaded, deleted, invalidated, or successfully rebuilt; synth cancellation also clears its entry. Pointer-keyed errors must not survive destruction and later attach to a different object allocated at the same address.

Textures store a compact lcTextureLoadError enum. The loader's static TextureFailureMessage() translates it into user-facing text when reporting a dependency failure. Missing includes and texture names remain in diagnostics; an empty error string alone does not prove readiness. Preserve translatable messages and check state/result rather than parsing error text.

Only GPU upload attempts receive the bounded automatic retry described above. Missing files, invalid source geometry, and decode failures do not produce an endless render-time retry loop. A strict caller reports failure; a permissive caller omits unavailable geometry, while the loading indicator exposes errors.

A request from acquisition to notification

The loader implementation follows this lifecycle: request acquisition and queueing, worker scheduling and RunWorker(), UI-thread ProcessCompletions() dispatch to ProcessPieceCompletion(), ProcessSynthCompletion(), or ProcessTextureCompletion(), dependency settlement and publication, then deferred notifications. Readiness waits follow these paths; invalidation, pause/resume, and shutdown are grouped at the end of the file. Piece and synth handlers share mesh color finalization and texture dependency checks, while keeping their distinct ownership and publication rules explicit.

sequenceDiagram
    participant Caller as Caller / UI thread
    participant Library as Library
    participant Loader as Loader / UI thread
    participant Worker as Qt worker
    Caller->>Library: RequestPieceLoad(Info, LoadPriority)
    Library->>Library: Acquire consumer reference
    Library->>Loader: Queue/promote
    Loader->>Worker: Immutable source snapshot
    Worker->>Worker: Parse/generate and convert, or read cached mesh
    Worker-->>Loader: Private mesh
    Loader->>Loader: Set section colors, update flags, and sort sections
    Loader->>Loader: Resolve textures and queue decode if needed
    Loader->>Worker: Texture source snapshot
    Worker-->>Loader: Decoded CPU image
    Loader->>Loader: Upload texture and commit ready mesh
    Loader-->>Caller: Deferred settlement notification

Texture-free meshes skip decoding/upload; cached meshes skip source parsing and conversion. Both paths use the same dependency and publication rules. Cache reads and mesh deserialization run in the library's worker build service; cache bytes stay local, and the result records whether it came from cache. Private mesh sections store color codes in ColorIndex until finalization converts them to indices. Fresh pieces, synths, and cached meshes all use FinalizeSections() on the UI thread to resolve color indices, refresh solid/translucent/default flags, and sort sections using the current palette. Workers build geometry without reading global colors or keeping palette snapshots. A corrupt mesh cache falls back to source parsing in the same worker job. Failed builds are not saved as successful mesh caches. Temporary disk textures are scoped by their resolved file identity, so same-name PNGs in different project directories remain distinct.

Mesh cache validity includes the cache format, source checksums, stud style, and stud-cylinder color setting. Style flags propagate through nested primitive includes. LoadPrimitive() returns lcResult<void> with include errors and clears partial data on failure, so failed primitive builds cannot seed a later successful mesh with incomplete cached geometry.

Workers post results and wake the loader's completion event. ProcessCompletions() processes at most 16 completions per pump, including texture retries. Publication and rendering remain sequential on the UI thread. DispatchNotifications() later delivers PartLoaded, PartLoadFailed, and GeneratedMeshSettled, with bounded batches and stale-target checks. Synchronous waits drive completion processing but defer these public settlement notifications to the event handler; they do not run an unrestricted nested UI event loop. AssetRequestsChanged also signals request-list or consumer changes and is not a success notification.

The main window coalesces updates, propagates affected bounds, and redraws views. Its loading indicator counts the active project's requirements, including generated meshes; unrelated work does not keep that indicator visible. Mesh publication schedules a debounced shared-buffer repack after loader work becomes quiescent. Until then, meshes draw through the existing uncached path. Shared buffer upload is not part of asset readiness.

References, cancellation, and generated geometry

RequestPieceLoad() acquires one reference even if loading fails. Always pair it with ReleasePieceInfo(). Its asynchronous return value means the call did not wait, not that the asset loaded. AddPieceReference() retains an identity without starting work. WaitForPieces() deduplicates its input without acquiring consumer references; callers retain every required identity and its owner through the wait.

A part request owns an additional in-flight reference. Removing one instance cannot destroy an asset still used by another instance or pending work. On the last consumer release, queued work is removed; running ordinary part work is allowed to finish and its result is discarded if no consumer survives. Successful or failed publication keeps the request's hold through deferred notification, then releases it. Library piece identities can remain in the catalog after their geometry unloads; unreferenced local leaf identities can be deleted. Model and external-project containers have their own teardown rules.

The library unregisters identities before deleting them. PieceInfo::Unload() only clears its resources: it releases its mesh and owned external project, detaches its borrowed model pointer, and resets readiness and bounds. It never unregisters or deletes itself; the destructor uses the same resource cleanup. Bulk library unload destroys external projects before remaining local identities and the shared catalog, so project teardown can safely release child references.

Texture requests likewise hold a reference, and staged/published mesh sections hold their texture dependencies. Cancellation removes unused queued texture work without releasing another mesh's texture. Failed assets remain terminal while retained; drawing does not retry them. Explicit invalidation or unloading can permit a later request.

Synth meshes are per lcPiece instance, separate from the shared library mesh. RequestSynthMesh() copies control points and retains the shared synth definition. Replacing or deleting the piece cancels its old request; workers check an atomic cancellation flag and stale results cannot replace newer meshes. Initial generation displays a cube. Control-point edits retain the previous generated mesh until replacement settles; that older mesh does not satisfy a wait for the new generation. Settlement holds the base PieceInfo through its deferred notification. See lc_synth.cpp and piece.cpp.

Interactive fallback rendering, bounds, ray/box picking, and selection outlines use the same displayed geometry and keep the original instance identity. A model's unavailable direct geometry can display a local cube while its children remain independently visible/selectable. Failed containers retain their structure; deleting a model or external project detaches its pointers explicitly. Undo restores external references using project-folder lookup, not only the library catalog. Placeholder appearance must never change piece type or asset ownership.

Scheduling and waits

Priorities only promote: Low < Normal < Blocking. Ordinary thumbnail requests start Low; asynchronous model pieces and generated meshes start Normal, regardless of step. Synchronous waits promote their required requests and pending texture dependencies to Blocking. The same lcPieceLoadPriority enum is used at the loading entry point and throughout the scheduler. RequestPieceLoad() waits only for Blocking; Low and Normal return immediately. Internal queueing uses priority solely to rank work and does not wait. Shared textures retain the highest promotion received from any parent or direct wait. There is no consumer identity map, priority demotion, or step-based scheduling. Priority lasts until that request completes or is cancelled, not merely until the requesting view closes.

At most four QtConcurrent drain workers run (fewer if the machine reports fewer threads). Running work is nonpreemptive. Queued work gains one scheduling priority level per ten seconds of waiting, capped at Blocking, with oldest work winning ties. Completion processing uses promoted priorities. Acquiring a pending asset shares its request rather than submitting another build.

WaitForPieces() starts or promotes loading as needed, then waits for the supplied identities, not the entire library. It returns false for failed or cancelled requirements and does not expand model containers itself. Callers retain every required identity through the wait; it does not acquire consumer references. WaitForSynthMeshes() promotes and waits for queued generations of the specified instances; callers start generations with RequestSynthMesh(). The loader's WaitForTexture() starts or promotes decoding as needed and waits through upload and retries. lcModel::WaitForAssets() combines recursive part and synth waits and updates bounds, but deliberately ignores their failure results. EnsureAssetsReady() adds strict failure reporting. The loader's WaitForAllWork() drains all pending work and workers, including unrelated assets; it is not a scoped success check.

Choosing an API

Need	API and lifetime contract
Resolve a name	FindPiece() with the intended project and explicit CreateMissing / SearchProjectFolder arguments. The returned pointer is borrowed; retain it before keeping it beyond its owner's lifetime.
Retain and start ordinary asynchronous work	RequestPieceLoad() with lcPieceLoadPriority::Low for background work or Normal for model/view work. Pair every acquisition with ReleasePieceInfo(), including failures.
Retain and synchronously load one identity	RequestPieceLoad() with lcPieceLoadPriority::Blocking; check its return value and pair the acquisition with ReleasePieceInfo(), including failures.
Retain without starting work	AddPieceReference(), paired with ReleasePieceInfo(). This does not make geometry ready.
Wait for an explicit part set	WaitForPieces() (use { Info } for one identity), check the boolean for strict consumers. Retain every identity and its owner throughout the wait; this call acquires no consumer references.
Wait for one identity and its container geometry	EnsurePieceAssetsReady() settles the identity, then delegates recursive children, synths, and bounds to the model/project API. Retain the identity and its owner throughout.
Wait for a model, including nested synths	WaitForAssets() for permissive output; EnsureAssetsReady() for strict preview-style failure reporting. Keep the model and its pieces alive throughout.
Replace/wait for generated geometry	RequestSynthMesh() / WaitForSynthMeshes(). Supply live lcPiece instances; a shared part mesh or a retained previous generated mesh is not readiness for the replacement.
Acquire a texture directly	FindTexture() waits for readiness and returns an acquired texture or null. FindTextureDeferred() acquires without waiting. The loader handles GPU upload readiness internally. Retain the texture during a direct wait and pair acquisition with ReleaseTexture().
Synchronize an entire library operation	The library calls the loader's WaitForAllWork() internally for settings changes. Ordinary previews must use scoped waits.

Scoped waits may process unrelated completions while pumping publication, but their completion condition depends only on the requested assets. They cannot preempt a worker already parsing, converting, or decoding another asset. Waiting on a single container PieceInfo is insufficient for a submodel preview: collect its recursive pieces and per-instance synths, or use the model-level API.

Caller policies

Caller	Waiting and failure behavior
Ordinary LDraw/LDD opening and interactive views	Parse and queue, then return. Draw shared fallback cubes for unavailable geometry; recurse through available children. Update bounds and redraw as assets settle without automatically refitting the camera.
Inventory import	Wait for required geometry before computing persistent placement; failure prevents using cube bounds as final layout.
Insertion layout	Wait for required geometry before computing persistent placement; failed geometry uses fallback boxes.
Preview	Wait for its recursive requirements and generated meshes; failure rejects the preview. See lc_previewwidget.cpp.
Thumbnail manager	Acquire required assets asynchronously. Missing library geometry/textures produce an error thumbnail. Pending generated meshes delay rendering; synth failure can use a loaded library mesh. See lc_thumbnailmanager.cpp.
OBJ, COLLADA, 3DS, POV-Ray export	Wait for model requirements, then omit failed meshes and failed generated geometry. Empty output is allowed. Render/write failures still report errors. See project.cpp.
Saved images, screenshots, HTML, instructions, printing	Wait for settlement, then render available geometry without loading cubes. HTML parts lists retain missing entries and quantities with blank thumbnails; instruction parts lists omit unavailable thumbnails. Actual rendering/writing failures still fail the operation.
Model save, CSV, BrickLink inventory	Save references or inventory data without requiring successful geometry loading.

Insertion waits for the candidate's recursive requirements before computing its placement. Stacking and mouse placement also wait for the existing piece whose geometry determines the position, including its current synth generation. Mouse placement repeats picking after pending geometry settles. A failed candidate replaces the previous insertion preview with the selected identity's fallback box so a later click inserts the current selection. Free dragging existing pieces also settles the moving piece and hit geometry, including current synth generations, and repeats picking before placement. If loading fails, both dragging and insertion use the available or fallback geometry rather than rejecting placement.

Automatic output fitting uses lcGeometryBoundsMode::AvailableOnly; interactive fitting uses IncludeFallback. A whole-model wait can conservatively include later steps even when output requests an earlier step. Scene flags preventing fallback cubes do not themselves initiate a wait or guarantee success. See lc_view.cpp and lc_scene.h.

SetRequireCompleteAssets(true) suppresses unavailable fallback geometry and marks missing assets in the scene; SetRequireGeneratedMeshes(true) also excludes the fixed library mesh when the current synth generation is unavailable. A strict renderer checks HasMissingAssets() after drawing. Permissive output uses these exclusion rules without making missing assets an operation-wide error. Do not infer readiness from a successfully allocated framebuffer or a nonempty image: it may still contain only the available portion of a model.

Thumbnail identity is based on PieceInfo, not just its filename. Pending thumbnails retain recursive requirements and receive settlement notifications. Category resets release thumbnail IDs before repopulating the list. The submodel category excludes models that include the active model, preventing recursive insertion. lcThumbnailManager::RefreshPieces() refreshes affected identities and requirements after Save As remapping; preserve that refresh when changing sources without changing pointers. The preview dock's RefreshModel() checks its recursive requirements and respects its lock; it refreshes a displayed submodel affected by editing elsewhere, not an editable temporary preview model.

Wait and propagate final bounds before automatic output fitting, not only inside the later image-rendering call. CLI automatic fitting uses the requested step for one image and the last step for an image range, reusing that camera across the range. Named cameras and explicit positions are preserved. Headless HTML output likewise fits the last step and reuses its camera. Temporary steps, cameras, framebuffer state, and the caller's GL context must be restored on success and failure; texture upload preserves the previous context/surface.

Geometry collection versus inventory

Project::GetModelParts() is the settled, flattened geometry list used by 3D exporters. Entries carry an explicit mesh for generated or directly embedded geometry, including root geometry; exporters must prefer that mesh over the shared PieceInfo mesh. Failed entries and unfinished/failed synth replacements are filtered out. A retained editing mesh is not exported as the new generation. This list is separate from GetPartsList() inventory traversal: changing export collection must not remove missing references from inventories or turn direct model geometry into an ordinary catalog part.

Project-local filenames are not necessarily safe export identifiers. COLLADA and POV-Ray identifiers use Project::MakeExportNameFragment() and MakeUniqueExportIdentifier(): preserve ASCII letters/digits, collapse invalid runs into separators, handle empty/long names, and disambiguate collisions. Other formats have their own naming rules. Keep these transformations at the output boundary; do not sanitize lookup names, LDraw references, or stored IDs to satisfy an exporter. HTML output names likewise must not become unchecked paths derived from arbitrary submodel filenames.

Project replacement, settings, and shutdown

Opening a candidate retains the current project until parsing succeeds. A failed open discards the candidate. A successful switch releases old owners, cancels old-only queued work, and preserves shared asset identities and requests. Neither path demotes request priorities.

Pending jobs remain in the normal piece, texture, and synth queues while paused; mPaused prevents dispatch. Resume refreshes queued piece source snapshots before allowing workers to take jobs again.

The library constructor receives the effective startup stud settings; application color preferences are applied before loading sources and colors, including the built-in fallback library. UpdateStudStyleAndColors() handles runtime changes and always rebuilds affected geometry and drains the resulting work when settings change. The preferences dialog saves a changed color-config path before this setter runs, so simultaneous stud/color changes load the new configuration once. Color-only changes call LoadColors() directly.

Stud-style changes pause dispatch, finish running jobs, discard staging built with old settings, refresh sources, and rebuild affected published meshes and generated geometry. The existing style-change path resumes and drains work. Published ordinary meshes are rebuilt selectively using HasStyleStud, including flags propagated from nested includes; a style change does not indiscriminately invalidate every library mesh. Pending snapshots must still be refreshed so obsolete settings cannot publish later. Library reload and destruction destroy the loader before releasing sources, registries, or GL resources. The loader destructor calls its private CancelAndDrain(), which stops dispatch, cancels pending work, joins workers, discards obsolete results, and releases notification holds without requiring successful upload or delivering public settlement signals.

Performance rationale and verification

The shared-buffer decision has measured support, not just architectural preference. September 2026 macOS Debug measurements found a first cold cube frame at 219 ms and a first real-mesh frame at 846 ms for a 1,689-reference fighter model; its cold repacks took 6–7 ms. On an Apple M4 renderer, a larger 6,696-instance scene at 1280×720 had synchronized median draw times of 75.59 ms uncached versus 35.55 ms with shared buffers, with a 4.43 ms repack. These are historical local measurements, not portable performance guarantees. GPU timer queries, logical GL texture storage, and process RSS do not establish exact physical driver allocation or residency.

Reconsider buffer ownership only with evidence of repeated repacks exceeding a frame budget, sustained uncached-draw stalls, or substantially greater memory pressure. Cross-platform/GPU profiling and longer soak runs are optional follow-ups, not unfinished streaming implementation stages.

For changes that affect these contracts, use controlled delays and failure injection with real library/loader/model APIs. Useful acceptance scenarios are:

Area	What to verify
Cold opening and interaction	Quick cube frame, incremental textured replacement, stable selection/picking, ancestor bounds, and no automatic interactive camera changes.
Scoped waits and priorities	Occupy the workers, then request an unrelated preview; its dependencies progress ahead of queued ordinary work. Shared requests build/decode once; aging permits background progress.
Cache and dependency failures	Cold/warm/corrupt-cache equivalence; missing nested includes and missing/corrupt PNGs fail explicitly. Same-name project textures stay isolated.
Upload and context state	Hidden preview or CLI image completes before a normal draw/event loop. Exercise upload retries and alternate contexts/surfaces/framebuffers.
Ownership and replacement	Same-name active/candidate/preview models, failed opens, shared assets, last-reference release, external-file undo, and no stale success callbacks after deletion.
Synth lifecycle	Initial loading cube, repeated pending edits, retained old mesh, cancellation/deletion, nested thumbnails, and final geometry matching the latest control points.
Output policy	Strict preview failure, permissive geometry/image output, empty scenes, missing parts-list thumbnails, explicit cameras, and restored steps/state after render/write errors.
Reload and shutdown	Stud changes during loading, library reload, exit with jobs pending, and no-renderer drains. Test both shared-buffer and VBO-disabled rendering where relevant.

Historical local Debug and application AddressSanitizer runs exercised these areas, including 120 lifecycle cycles and 2,400 control-point edits. System/Qt frameworks were not instrumented and leak detection was disabled. Those runs are evidence for the tested implementation, not current proof or a replacement for checking new paths; temporary harnesses are not a maintained repository test suite.

Invariants and maintenance checks

• Keep workers on copied source/settings/control-point data and private results. Published meshes, texture pointers, global color registration, GL upload, model mutations, and consumer notifications belong on the UI thread.
• Protect queue decisions with the loader mutex; release it before publication, GL upload, or settlement callbacks. Condition waits release the queue mutex while sleeping. Do not hold library locks needed by workers while draining.
• Preserve in-flight holds through stale-result disposal and deferred notification. Validate request identity and cancellation before publishing; deleting an owner must not let a worker or callback access that deleted model/piece.
• Preserve unresolved dependency names and diagnostics through caches. A failed include/texture must never be treated as successful partial asset loading.
• Distinguish container state, library mesh readiness, and per-instance synth readiness. Choose a caller's failure policy explicitly; settling and succeeding are separate decisions.
• When changing this code, exercise shared mesh/texture promotion, last-owner cancellation, failed and successful project replacement, pending synth edits, missing includes/PNGs, cache rebuilds, context restoration, and reload/shutdown. Cover both strict previews and permissive outputs; previous validation is not proof that a new lifecycle path is safe.