OpenCascade Technology Overview: Visualization
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OpenCascade is a visualization library with components for presentation and selection. It includes Application Interactive Services (AIS) which manages interactive objects for display and selection using a high-level API. Presentation requires presentable objects, a viewer, and an interactive context. Selection provides point, rectangle, and polyline selection and is managed by a selection manager using a bounding volume hierarchy. Standard interactive objects include shapes, dimensions, and relations. MeshVS provides mesh visualization and MeshVS_Mesh is a standard interactive object for displaying meshes.
OpenCascade Technology Overview: Visualization
- 2. Introduction • Components: presentation (顯示) and selection (選取). • Application Interactive Services (AIS): 管理OCCT顯示技術 核心 (interactive objects, for presentation and selection). 提供GUI與OCCT核心功能之間的連結.
- 3. Presentation • 顯示需要3項要素: • Presentable object (AIS_InteractiveObject class). 提供電腦圖 學需要的顯示資訊. StdPrs and Prs3d packages. • Viewer. Zoom, translate or rotate with view. • Interactive context (AIS_InteractiveContext). High-level API, 控制整個顯示流程. • Packages: • AIS, PrsMgr (low-level to be implemented). • StdPrs (顯示points, curves, shapes…), Prs3d (顯示wireframe, shading, hidden line removal). • V3d (3d viewer), Graphic3d (graphics structure), Visual3d, DsgPrs.
- 4. Selection • OCCT提供3種選取功能: point, rectangle, polyline. • Sensitive entity: 被選取物件中定義可選取元素. • Selectable object: 代表被選取物件, 含sensitive entities與 selection modes. • Modes: 0 (AIS_Shape), 1 (vertices), 2 (edges), 3 (wires), 4 (faces), 5 (shells), 6 (constituent solids).
- 5. Selection Manager • Selection Manager: High-level manager, 管理所有顯示物件 的選取流程. • Viewer selector: viewer對應的selection功能管理實作. • Selection Struct: BVH (Bounding Volume Hierarchy)資料結構. Selectable objects with sensitive entities.
- 6. Selection Algorithm • Selection Frustums. • 3 level BVH (Bounding Volume Hierarchy) traversal. • 由BVH中的entities與frustums做重疊判斷，決定哪些entities被選取到.
- 7. Selection Packages • SelectBasics: base classes (entity, owner, …) • Select3D: standard sensitive entities (box, circle, curve, face, ...). • SelectMgr: 管理完整的選取功能流程. • 指定selection mode啟動與關閉; selection mode對應的運算 interface (ComputeSelection() API); selection filter; 維護更新BVH 資料結構. • StdSelect: SelectMgr對應之功能實現classes. • _BRepOwner, _BRepSelectionTool (AIS topological shape 對應功 能實作). • _ViewerSelector3d. • _FaceFilter, _EdgeFilter.
- 8. Select Highlighting • Highlighting: • 舊做法, 每個entity owner需準備對應的Prd3d_Presentation object. 會消耗較多記憶體, 並影響繪圖效能. • 新改良方案, 由interactive object自己實現highlighting. SelectMgr_SelectableObject::HilightSelected(). (如何實現優化? 需 要再確認) • SelectMgr_EntityOwner::IsAutoHilight() return true: 使用舊 highlighting顯示機制.
- 9. Application Interactive Service • 負責管理顯示與選取功能. • Interactive Context: 管理繪圖功能與選取功能. • Neutral Point (預設初始狀態), Local Contexts (新暫存狀態). • Interactive Object: 在viewer中被顯示和可被選取的代表物 件.
- 10. Interactive Object • Presentations: 一個object可以擁有多個presentations物 件(顯示內容)，透過 PrsMgr_PresentableObject::Compute() API建構. • 支援Hidden Line Removal顯示模式. • Presentation modes: Datum, Relation, Object, None. • Selection: AIS_InteractiveObject::ComputeSelection() API, 定義sensitive entities. • Graphic attributes (AIS_Drawer): color, width, material, … • Hierarchy: scene graph概念, children指繼承 transformations. • Instancing: (不是實現 OpenGL instancing rendering..) AIS_ConnectiveInteractive代表個別object, AIS_MultiConnectedInteractive代表一群instance objects的集合.
- 11. Interactive Context • 管理共用繪圖屬性, modes for presentations and selections, 管理local context. • Neutral Point: Interactive Context的預設初始狀態. • Local Context: 新建立的暫存狀態, local contexts以stack的 方式被存取管理. • AIS_InteractiveContext::OpenLocalContext() API開啟新建立的local context. 並管理新context自己的顯示與選取模式等. • Local context被關閉後, 所有該context產生的暫存interactive objects都會一起被移除.
- 12. Context Presentation API • Display(), Erase(), EraseMode(), ClearPrs(), Redisplay(), Remove(), EraseAll(), Hilight(), HilightWithColor(). • API使用建議: • Display() and Erase() in Neutral Point環境, Local Context管理自己 的選取功能. • Neutral Point環境中, 建議指啟用預設selection mode 0. • Erase() 改變物件的visible flag! • Remove() 移除物件. 官方建議如果物件的Compute()步驟花的時間很 小, 可以使用Remove()取代Erase()!? (?黑人問號
- 13. Selection in Context • 設定Filters: 過濾選取對象. • StdSelect_EdgeFilter, StdSelect_FaceFilter, StdSelect_ShapeTypeFilter, AIS_TypeFilter, AIS_SignatureFilter, … • Selection API: • MoveTo() 指定滑鼠選取座標. • Select() 存取被偵測到物件, 設定為被選取狀態. • ShiftSelect() 如果前一次被偵測物件尚未被設定為被選取, 將它們加入 selected objects. • Neutral Point中, 被選取物件稱為current objects; Local context中, 被 選取物件稱為selected objects. AIS_InteractiveContext提供Iterator功 能去走訪每個被選取物件. • AIS_InteractiveContext::KeepTemporary(), SetSelectedCurrent(), 將物件轉移(保留)至Neutral Point.
- 14. Recommendations on Context • 何種情境? • 對所有物件做統一操作 (Neutral Point?). • 對某些指定物件, 或特定單一物件做操作 (open a local context). • 操作Local Context步驟: • 設定新的local context選項參數. • 載入(Load)指定物件, 正確設定它們的對應選取模式(activate selection modes). • Activate Standard modes if necessary. • 建立需要的filters. • 做選取操作 (detect/select/recover desired entities). • 完成操作後關閉指定的context.
- 15. Standard Interactive Objects • None (unknown type). • Datum (construction geometric element). • AIS_Point, AIS_Line, AIS_Circle, AIS_Trihedron, … • Object (topological shape or connection between shapes). • AIS_Shape, AIS_Triangulation, AIS_ConnectedInteractive, AIS_TexturedShape, MeshVS_Mesh, … • Relation (constraints on interactive shape and reference geometry). • AIS_ConcentricRelation, AIS_FixRelation, AIS_IDenticRelation, AIS_ParallelRelation, … • Dimensions • AIS_AngleDimension, AIS_Chamf3dDimension, AIS_DiameterDimension, AIS_DimensionOwner, …
- 16. MeshVS_Mesh • Mesh Visualization Service component. 儲存幾何網格資料. • 透過MeshVS_PrsBuilder class建構網格物件. • Read STL file:
- 17. MeshVS_Mesh Development • MeshVS package. • Access mesh data source, for application data model. • Create multiple MeshVS_PrsBuilder objects (standard and cusom ones). • Each _PrsBuilder for presentations in certain display modes respectively (wireframe, shading, shrink, …). • MeshVS_Mesh derived from AIS_InteractiveObject. Has selection highlighting mechanism (highlighter object). • MeshVS_Mesh::AddBuilder() with TreatAsHilighter = true. • MeshVS_Draw control visual attributes (color, shrink coefficients).
- 18. 3D Presentation • 功能簡介: • anti-aliasing (反鋸齒). • primitive group, light (打光), structure (scene graph概念). • view (鏡頭視角), viewer (manage views), view mapping (投影), z- buffer (hidden surface removal). • Graphic primitive: markers, polygons (holes), polylines, text. • Primitive array (support vertex buffer object, VBO). • Materials (材質設定). • Textures (貼圖).
- 19. GLSL Shader • Support GLSL shaders. 目前支援vertex and fragment shader.
- 20. V3d Package • 3D viewer管理者角色. • Viewer參數設定. • 投影設定 (perspective, orthographic). • 打光設定 (point, directional, spot, headlight lights). • Clipping planes. • 實體化物件: views, planes, light sources, graphic structures, picks. • View frustum culling. 繪圖時排除視線範圍外的物件.
- 22. Ray Tracing • 實現real time ray tracing (透過 GLSL shaders)! • 支援Features: • Hard shadows. • Refractions, reflections, transparency, texturing. • 支援non-polygon objects (lines, text, …). • 2-level BVH optimization. • TKMath/BVH package提供BVH演算法.
- 23. Presentation Features • 畫面截圖儲存至圖檔. 支援向量式影像格式 (vector image export). • 3D畫面列印. • Z-layer, depth arranging functionality. 依照不同的Z-layer可 設定各自的depth buffer參數. 還可以設定各自的rendering precision (single/double precision). • Clipping planes.
- 24. Example: Create a 3D Scene • Create attributes. • Create a 3D viewer, and corresponding view. • Create an interactive context. • Create interactive objects, and calculate their primitivies. • Display interactive objects.