OGRE Programming Intermediate Tutorial: Volume Selection

1. OGRE Programming Intermediate Tutorial: Volume Selection

2. Contents • Perform volume selection. • - Click and drag the mouse across the screen to define a rectangle • - Release the mouse button to select all the objects within the selection area • - Highlight the selected objects • Introduce to graphics hardware buffers • Use two objects: • ManualObject (to create the rectangle) • PlaneBoundedVolumeListSceneQuery 2

3. Graphics Buffers - Vertex buffers: define points in 3D space. - Index buffers: connect the points from the vertex buffer to form triangles. - Though all meshes have a vertex buffer, not all meshes will have an index buffer. - Vertex and index buffers: usually stored in the video card's own memory.

4. Vertex Buffers - Define points in 3D space. - Each element in the vertex buffer is defined by several attributes: - Position: The only attribute must be set. - Optional properties: color, texture coordinates and so on. 4

5. Index Buffers • Connect the points from the vertex buffer. • Every three indexes specified in the index buffer defines a single triangle to be drawn by the GPU. • The order of the vertices in the index buffer tells the graphics card which way the triangle faces. • A triangle drawn counter-clockwise is facing to the camera. • A triangle drawn clockwise is facing away from the camera. • - Normally only the front of a triangle is rendered 5 5

6. The Selection Rectangle • Define in 2D • Be rendered and on top of all other objects on the screen in the overlay layer. • In the SelectionRectangle's constructor: • setRenderQueueGroup(RENDER_QUEUE_OVERLAY); • setUseIdentityProjection(true); • setUseIdentityView(true); • setQueryFlags(0);

7. The Selection Rectangle: Coordinate System • A new coordinate system with X and Y running from -1 to 1 inclusive • Have to transform the coordinate system (CEGUI) with [0, 1] x [0, 1] to [-1, 1] x [-1, 1]. 7

8. Definition of the Selection Rectangle • The line strip draws a line to each vertex from the previous vertex. • To create the rectangle, define 5 points (the first and the last point are the same to connect the entire rectangle): • clear(); • begin("", RenderOperation::OT_LINE_STRIP); • position(left, top, -1); • position(right, top, -1); • position(right, bottom, -1); • position(left, bottom, -1); • position(left, top, -1); • end(); 8

9. Definition of the Selection Rectangle class SelectionRectangle : public ManualObject { public: SelectionRectangle(const String &name) : ManualObject(name) { setUseIdentityProjection(true); setUseIdentityView(true); setRenderQueueGroup(RENDER_QUEUE_OVERLAY); setQueryFlags(0); } };

10. Definition of the Selection Rectangle class SelectionRectangle : public ManualObject { void setCorners(float left, float top, float right, float bottom) { //mapping to ogre3D projection plane left = left * 2 - 1; right = right * 2 - 1; top = 1 - top * 2; bottom = 1 - bottom * 2; clear(); begin("", RenderOperation::OT_LINE_STRIP); position(left, top, -1); position(right, top, -1); position(right, bottom, -1); position(left, bottom, -1); position(left, top, -1); end(); AxisAlignedBox box; box.setInfinite(); setBoundingBox(box); } }; 10

11. //Mapping to Ogre3D Projection Plane Space //BEGIN Using CEGUI //left = left * 2 - 1; //right = right * 2 - 1; //top = 1 - top * 2; //bottom = 1 - bottom * 2; //End USING CEGUI //BEGIN Using TrayMgr left = left * 2 - 1; right = right * 2 - 1; top = -1 - top * 2; bottom = -1 -bottom * 2; //End USING TrayMgr

12. Volume selection process • Define a volume selection query: • PlaneBoundedVolumeListSceneQuery *mVolQuery; • Obtain the four rays at the four corners of the selection rectangle • Define the selection volume using the four rays • define the five planes which enclosing a selection volume; volList stores the list of the planes • set the selection volume: mVolQuery->setVolumes(volList); • execute the query; perform tasks on the results

13. Volume selection : code • { • … • Ray topLeft = mCamera->getCameraToViewportRay(left, top); • Ray topRight = mCamera->getCameraToViewportRay(right, top); • Ray bottomLeft = mCamera->getCameraToViewportRay(left, bottom); • Ray bottomRight = mCamera->getCameraToViewportRay(right, bottom); • // The plane faces the counter clockwise position. • PlaneBoundedVolume vol; • int np = 100; • vol.planes.push_back(Plane(topLeft.getPoint(3), topRight.getPoint(3), bottomRight.getPoint(3))); // front plane • vol.planes.push_back(Plane(topLeft.getOrigin(), topLeft.getPoint(np), topRight.getPoint(np))); // top plane • vol.planes.push_back(Plane(topLeft.getOrigin(), bottomLeft.getPoint(np), topLeft.getPoint(np))); // left plane • vol.planes.push_back(Plane(bottomLeft.getOrigin(), bottomRight.getPoint(np), bottomLeft.getPoint(np))); // bottom plane • vol.planes.push_back(Plane(bottomRight.getOrigin(), topRight.getPoint(np), bottomRight.getPoint(np))); // right plane • PlaneBoundedVolumeList volList; • volList.push_back(vol); • mVolQuery->setVolumes(volList); • SceneQueryResult result = mVolQuery->execute(); • … … • } 13

14. How to define mVolQuery ? PlaneBoundedVolumeList volList; mVolQuery = mSceneMgr ->createPlaneBoundedVolumeQuery(volList);

15. Mapping to screen coordinates: mTrayMgr Ray mRay =mTrayMgr->getCursorRay(mCamera); Vector2 scn = mTrayMgr ->sceneToScreen( mCamera, mRay.getOrigin());

16. References: http://www.ogre3d.org/wiki/index.php/Intermediate_Tutorial_3 Note: The selection volume is not correct in the original code. Last visit 1st April, 2008