Question

I attempted to write the following code to draw a cylinder. The cylinder was drawn in C++ with OpenGL. And I’m given a little tool by the school that I could compile with my own model’s cpp file and then able to rotate the model with the mouse. The tool doesn’t affect the rotation of my model because it works for the other demo models. However, for some reason which I do not understand, I cannot rotate the cylinder in the Z-axis to see it in its horizontal view. So, the cylinder can only be rotated and seen in these directions:

Why I cannot rotate the cylinder to see it in this direction? The following image was rotated manually in Photoshop to illustrate the direction of view that the model couldn’t rotate to:

I don’t understand what is the reason for not being to rotate in that direction because the demo shown for other models(written by others) could be rotated freely in all directions.

This is the code that I have written to try in mymodel.cpp:

void drawCylinderObject() {
    float topRadius = 5;
    float bottomRadius = 5;
    float height = 10;
    int numOfPolygons = 50;
    float basisvec1[3] = {1, 0, 0};
    float basisvec2[3] = {0, 0, -1};
    float topPosition[3] = {0, height/2.0, 0};
    float bottomPosition[3] = {0, -height/2.0, 0};


    for(int i=0; i<numOfPolygons; i++) {
        float angle1 = (float)i/(float)numOfPolygons*2.0*M_PI;
        float angle2 = ((float)i+1.0)/(float)numOfPolygons*2.0*M_PI;
        vector<float> point1(3), point2(3), point3(3), point4(3);
        for(int j=0; j<3; j++) {
            point1[j] = topPosition[j] + topRadius * cos(angle1) * basisvec1[j] + topRadius * sin(angle1) * basisvec2[j];
        }


        for(int j=0; j<3; j++) {
            point2[j] = bottomPosition[j] + bottomRadius * cos(angle1) * basisvec1[j] + bottomRadius * sin(angle1) * basisvec2[j];

        }

        for(int j=0; j<3; j++) {
            point3[j] = bottomPosition[j] + bottomRadius * cos(angle2) * basisvec1[j] + bottomRadius * sin(angle2) * basisvec2[j];
        }

        for(int j=0; j<3; j++) {
            point4[j] = topPosition[j] + topRadius * cos(angle2) * basisvec1[j] + topRadius * sin(angle2) * basisvec2[j];
        }

        float crossvec1[3] = {point4[0]-point1[0], point4[1]-point1[1], point4[2]-point1[2]};
        float crossvec2[3] = {point2[0]-point1[0], point2[1]-point1[1], point2[2]-point1[2]};
        float normalVector1[3];

        crossProduct(crossvec2, crossvec1, normalVector1);

        glBegin(GL_POLYGON);

        glNormal3fv(normalVector1);

        glVertex3f(point1[0], point1[1], point1[2]);
        glVertex3f(point2[0], point2[1], point2[2]);
        glVertex3f(point3[0], point3[1], point3[2]);
        glVertex3f(point4[0], point4[1], point4[2]);

        glEnd();
    }
}

And the overwritten function I have is like this, also in mymodel.cpp:

void CRenderView::drawScene()
{
    //calls the above function
    drawCylinderObject();
}

What I have done basically is just to define 2 perpendicular basis unit vectors and then extend them outwards with a magnitude value. And I loop through this 360 degrees to draw the polygons to form the cylinder. But what is wrong that this way of drawing doesn’t allow me to rotate the model freely?

Edit:

The following is part of the code of how the tool draws the scene. Somehow, the tool has a huge chunk of classes. Most of its classes merely sets up the GUI of the tool and then the only part that draws it is the one below in CRenderView.cpp:

void CRenderView::OnPaint() 
{
    // Device context for painting
    CPaintDC dc(this); 

    // Model is stored in Document
    CToolDoc *pDoc = (CToolDoc *)GetDocument();
    ASSERT_VALID(pDoc);

    // Useful in multidoc templates
    HWND hWnd = GetSafeHwnd();
    HDC hDC = ::GetDC(hWnd);
    wglMakeCurrent(hDC,m_hGLContext);

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glClearColor(m_ClearColorRed,m_ClearColorGreen,m_ClearColorBlue,1.0f);

    glPushMatrix();

    // Position / translation / scale
    glTranslated(m_xTranslation,m_yTranslation,m_zTranslation);
    glRotatef(m_xRotation, 1.0, 0.0, 0.0);
    glRotatef(m_yRotation, 0.0, 1.0, 0.0);
    glScalef(m_xScaling,m_yScaling,m_zScaling);

    // Start rendering...
    drawScene();

    glPopMatrix();

    // Double buffer
    SwapBuffers(dc.m_ps.hdc);
}

protected:
    void drawScene();

void CRenderView::OnMouseMove(UINT nFlags, 
                                                            CPoint point) 
{
    if(m_LeftButtonDown)
    {
        m_yRotation -= (float)(m_LeftDownPos.x - point.x)/3.0f;
        m_xRotation -= (float)(m_LeftDownPos.y - point.y)/3.0f;
        m_LeftDownPos = point;
        InvalidateRect(NULL,FALSE);
    }
    CView::OnMouseMove(nFlags, point);
}

Answer 1

May I ask, why you rotate the cylinder by recalculating its vertices? Just generate a cylinder model once, then perform any following transformations on the modelview matrix. Also I think you’ll want to rotate about Z, not Y.

Update

That “tool” seems to be a slightly extended version of the “MFC OpenGL CView” tutorial. *yuck*

The biggest problem I see, that the CRenderView::OnPaint function has been written by someone, who doesn’t know how to properly use OpenGL.

void CRenderView::OnPaint() 
{
    // Device context for painting
    CPaintDC dc(this); 

    // Model is stored in Document
    CToolDoc *pDoc = (CToolDoc *)GetDocument();
    ASSERT_VALID(pDoc);

    // Useful in multidoc templates
    HWND hWnd = GetSafeHwnd();
    HDC hDC = ::GetDC(hWnd);
    wglMakeCurrent(hDC,m_hGLContext);

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);    
    glClearColor(m_ClearColorRed,m_ClearColorGreen,m_ClearColorBlue,1.0f);

glClearColor must be called before glClear, as it sets the value that will be applied upon the glClear call. This way round it will work on the glClear call of the next frame or not at all.

    glPushMatrix();

Which matrix is pushed here? This lacks a call to glMatrixMode.

Starting from some arbitrary matrix. And BTW where is the projection set (let me guess, in the OnSize handler, right?).

    // Position / translation / scale
    glTranslated(m_xTranslation,m_yTranslation,m_zTranslation);
    glRotatef(m_xRotation, 1.0, 0.0, 0.0);
    glRotatef(m_yRotation, 0.0, 1.0, 0.0);

You confirmed you want to rotate about the Z axis. So why won’t you do it here? There only rotation about X and Y here. Problem is: You’re using euler angles here, which have some nasty properties and are frowned upon by 3D graphics people. Better use Quaternions for representing rotations (just a suggestion).

    glScalef(m_xScaling,m_yScaling,m_zScaling);

    // Start rendering...
    drawScene();

    glPopMatrix();

    // Double buffer
    SwapBuffers(dc.m_ps.hdc);
}