OpenCASCADE Customize Highlighting

eryar@163.com

The traditional way of highlighting selected entity owners adopted by Open CASCADE Technology assumes that each entity owner highlights itself on its own. This approach has two drawbacks:

l  each entity owner has to maintain its own Prs3d_Presentation object, that results in a considerable memory overhead;

l  drawing selected owners one by one is not efficient from the visualization point of view.

OpenCASCADE內置的高亮算法是每個模型都有自己的高亮實體，高亮的一般是模型的TopoDS_Edge，對于B樣條曲面還會顯示出一條UIso和VIso線。這種高亮顯示策略不一定符合所有人的需求，而且會占用額外大量的內存。

Therefore, to overcome these limitations, OCCT has an alternative way to implement the highlighting of a selected presentation. Using this approach, the interactive object itself will be responsible for the highlighting, not the entity owner.

因此，為了克服上面的缺點，OCCT提供一個自定義高亮的方式，由交互對象自己來實現高亮顯示，滿足更多個性化的需求。

On the basis of SelectMgr_EntityOwner::IsAutoHilight() return value, AIS_InteractiveContext object either uses the traditional way of highlighting (in case if IsAutoHilight() returns TRUE) or groups such owners according to their selectable objects and finally calls SelectMgr_SelectableObject::HilightSelected() or SelectMgr_SelectableObject::ClearSelected(), passing a group of owners as an argument.

Hence, an application can derive its own interactive object and redefine virtual methods HilightSelected(), ClearSelected() and HilightOwnerWithColor() from SelectMgr_SelectableObject. SelectMgr_SelectableObject::GetHilightPresentation and SelectMgr_SelectableObject::GetSelectPresentation methods can be used to optimize filling of selection and highlight presentations according to the user's needs. The AIS_InteractiveContext::HighlightSelected() method can be used for efficient redrawing of the selection presentation for a given interactive object from an application code.

當SelectMgr_SelectableObject的所有子類的IsAutoHilight()返回true時，則是由OCCT自動實現高亮選擇效果。當IsAutoHilight()返回false時，則交互對象的高亮顯示交給用戶自己來實現。具體實現步驟為從類SelectMgr_SelectableObject派生出一個子類，重寫三個虛函數：

l  HilightSelected()：交互對象選中時的高亮顯示效果；

l  ClearSelected()：清除對象選中時的高亮效果；

l  HilightOwnerWithColor()：鼠標移動到交互對象上時的高亮效果；

如上圖所示為OCCT自動高亮顯示效果，模型的每個Edge都高亮顯示；

上圖所示為自己實現模型的高亮效果，只高亮顯示模型的外邊框。這樣高亮顯示的數據少，占用內存就少，效率更高。

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