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Mouse Selection in OpenGL Scene

, 25 Oct 2002 CPOL
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One another approach for picking objects with the mouse in OpenGL scene.

Sample Image - OpenGLMouseSellection.jpg


This article explains how to implement mouse selection in OpenGL scene, as well as moving, rotating and zooming OpenGL scene and sterilization in MDI applications. The project uses MFC Template classes to hold object primitives. There are several methods how you can implement selection in OpenGL scene. Here is a short description.


Selection method is OpenGL mechanism and it works in the following way. Draw your scene into the framebuffer, and then you enter selection mode and redraw the scene. When you're in selection mode, the contents of the framebuffer doesn't change until you exit selection mode. When you exit selection mode, OpenGL returns a list of the primitives that intersect the viewing volume. A selection hit is caused by intersection between primitive and viewing volume. The list of primitives is actually returned as an array of integers. You construct the name stack by loading names onto it as you issue primitive drawing commands while in selection mode. Thus, when the list of names is returned, you can use it to determine which primitives might have been selected on the screen by the user.


Feedback is similar to selection in that once you're in either mode, no pixels are produced and the screen is frozen. Drawing does not occur; instead, information about primitives that would have been rendered is sent back to the application. The key difference between selection and feedback modes is what information is sent back. In feedback mode, information about transformed primitives is sent back to an array of floating-point values. The values sent back to the feedback array consist of tokens that specify what type of primitive (point, line, polygon, image, or bitmap) has been processed and transformed, followed by vertex, color, or other data for that primitive.

Pick ray

Yet another method involves shooting a pick ray through the mouse location and testing for intersections with the currently displayed objects. OpenGL doesn't contain any function for ray intersections. To generate pick ray, you need OpenGL matrix transformation, and some knowledge about linear algebra.


The method which I implemented for mouse selection involves projection of small area from the object (primitive) to the screen and testing the mouse click. The code below shows how you can project the object point to the screen.

//Needs to retrieve OpenGL transformation matrices before projection
glGetIntegerv(GL_VIEWPORT, m_spheres->viewport);
glGetDoublev(GL_MODELVIEW_MATRIX, m_spheres->modelMatrix);
glGetDoublev(GL_PROJECTION_MATRIX, m_spheres->projMatrix);

m_spheres->winy=m_spheres->viewport[3] - (GLint) y - 1;

Every shape object contains projected point and rectangle. When you want to select shape, use the LButtonDown message handler and test its point in rectangle. The code below shows the test for Sphere object.

void CMyView::OnLButtonDown(UINT nFlags, CPoint point) 
    m_LeftButtonDown = TRUE;
    m_LeftDownPos = point;

    CMyDoc* pDoc = (CMyDoc*)GetDocument();
    CTypedPtrList<CObList,CSphere*>& sphereList =pDoc->m_SphereList;
    POSITION pos = sphereList.GetHeadPosition();
    while (pos != NULL)
        CSphere* spheres = sphereList.GetNext(pos);

    CView::OnLButtonDown(nFlags, point);


The problem of the implementation is how much can be the projected area of the object. If you set projected area too big, it will be possible that two different areas can be merged. If projected area is too small it's hard to pick it with the mouse.


  1. OpenGL Programming Guide The Official Guide to Learning OpenGL, Version 1.1
  2. Open GL Super Bible (Publisher: Macmillan Computer Publishing) Author(s): Waite group Press ISBN: 1571690735 Publication date: 08/01/96


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Bahrudin Hrnjica
Software Developer (Senior)
Bosnia And Herzegovina Bosnia And Herzegovina
Senior Software Developer at and Microsoft MVP for Visual C#.
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Comments and Discussions

GeneralMy vote of 5 PinmemberMohamed Aissiou21-Dec-12 21:24 
Generalabout gluUnProject Pinmemberfeifly26-Apr-10 17:02 
Questioni want help about mousesel project Pinmembersuuunil25-May-09 20:18 
GeneralGood Job~! Pinmemberdefineconst30-Apr-09 0:11 
Questionanother way? PinmemberPorolfi4-Jun-07 23:31 
I've just started looking for a simple API to do some 3D rendering. Accepting user input is a requirement as well. OpenGL seems OK, but I'm wondering if there is a simpler way to do object picking. I think the renderer could maintain a buffer that stores an 'object ID' (supplied at runtime by the user of the OpenGL API) for each rendered pixel, similarly to the depth buffer. Then I could simply pass the mouse pointer coordinates and the API could return the 'object ID'. Is there any way to do this? Maybe in other 3D APIs? Thanks for replies.
AnswerRe: another way? PinmemberPorolfi5-Jun-07 1:55 
Questionhow about gluUnProject PinmemberMerlinblack11-Apr-05 17:26 
AnswerRe: how about gluUnProject PinmemberBahrudin Hrnjica13-Apr-05 21:02 
GeneralRe: how about gluUnProject PinmemberMerlinblack14-Apr-05 18:47 
GeneralRe: how about gluUnProject PinmemberBahrudin Hrnjica17-Apr-05 20:52 
GeneralFinishing the project in VC6? Thank you very much! Pinmemberaqing19-Jun-04 19:57 
GeneralRe: Finishing the project in VC6? Thank you very much! PinmemberBahrudin Hrnjica20-Jun-04 8:45 
GeneralRe: Finishing the project in VC6? Thank you very much! Pinmemberaqing20-Jun-04 17:51 
GeneralRe: Finishing the project in VC6? Thank you very much! PinsussAnonymous21-Jun-04 3:34 
GeneralRe: Finishing the project in VC6? Thank you very much! Pinmemberaqing25-Jun-04 21:48 
Generalfile not found Pinmembergok16-Feb-05 20:33 
GeneralProblem existing after converting the code form the .NET to VC6 by prjconverter Pinmemberaqing31-May-04 23:23 
GeneralRe: Problem existing after converting the code form the .NET to VC6 by prjconverter PinmemberBahrudin Hrnjica1-Jun-04 3:26 
GeneralRe: Problem existing after converting the code form the .NET to VC6 by prjconverter PinmemberMerlinblack11-Apr-05 17:42 
QuestionHow to run 2 different(or same) OpenGL objects in one DialogBox? Pinmemberwerter13-May-04 22:17 
AnswerRe: How to run 2 different(or same) OpenGL objects in one DialogBox? PinmemberBahrudin Hrnjica25-May-04 10:54 
GeneralOpenGL + JAI / JAVA Pinmemberprabu_univ12-Apr-04 3:10 
Questionhow to run i t Pinmemberwjiybb25-Nov-03 21:21 
AnswerRe: how to run i t PinmemberBahrudin Hrnjica26-Nov-03 12:22 
GeneralRe: how to run i t Pinmemberinfinitecmdz23-Oct-06 23:20 

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