Drawing Image RGB Color Distribution Using OpenGL

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This article allows ones to:

  • draw an image RGB color distribution using OpenGL in a
    dialog,
  • use OpenGL display lists,
  • draw antialiased smooth colored lines,
  • implement mouse interaction (rotation),
  • store and load an image from resource.

We assume that our aim is to display the RGB color
distribution of the following image (Figure 1) in an interactive
way. In particular, it would be nice to display the RGB cube and
the image color cloud.

Figure 1: Our goal is to display
the RGB color distribution of this 24 bits BMP color image in an
intuitive way. OpenGL will help us.

Figure 2: Illustrates two viewpoints of the
color distribution in a dialog. The viewpoint is modifed using
the left mouse button that implements the x/y rotation, the
dialog may be resized and a popup menu allows us to load and view
the current image (Figure 3).

Figure 3: Two examples of color
distribution in the RGB cube seen under two distinct viewpoints.
In particular, the red cloud is linked to the nose of the baboon
seen in Figure 1. Notice that the lines are antialiased, each
saturated color is drawn with a sphere , and the cloud is painted
using the GL_POINTS primitive.

Figure 4: A popup menu allows to
load a 24 bits RGB image, change the OpenGL clear color (recalled
back color), and view the current image in a dialog window.

Initialization of the Rendering Engine

Following lines make the rendering engine to be initialized in a dialog.

//*********************************
// OnCreate 
//*********************************
int CColorDlg::OnCreate(LPCREATESTRUCT lpCreateStruct)
{
 // Init OpenGL engine
 HWND hWnd = GetSafeHwnd();
 HDC hDC = ::GetDC(hWnd);

 if(SetWindowPixelFormat(hDC)==FALSE)
  return 0;

 if(CreateViewGLContext(hDC)==FALSE)
  return 0;

 // The lines are antialiased
 glEnable(GL_LINE_SMOOTH);
 glEnable(GL_BLEND);
 glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
 glHint(GL_LINE_SMOOTH_HINT,GL_NICEST);
 glLineWidth(1.5); // required
 glPointSize(1.0);
 glPolygonMode(GL_FRONT,GL_LINE);
 glPolygonMode(GL_BACK,GL_LINE);
 glShadeModel(GL_SMOOTH);

 // Precalculate display lists
 BuildListCube();
 BuildListCloud();

 return 0;
}
//**********************************************
// OpenGL
//**********************************************
BOOL CColorDlg::SetWindowPixelFormat(HDC hDC)
{
 PIXELFORMATDESCRIPTOR pixelDesc;

 pixelDesc.nSize = sizeof(PIXELFORMATDESCRIPTOR);
 pixelDesc.nVersion = 1;

 pixelDesc.dwFlags = PFD_DRAW_TO_WINDOW
                   | PFD_SUPPORT_OPENGL
                   | PFD_DOUBLEBUFFER
                   | PFD_STEREO_DONTCARE;

 pixelDesc.iPixelType = PFD_TYPE_RGBA;
 pixelDesc.cColorBits = 32;
 pixelDesc.cRedBits = 8;
 pixelDesc.cRedShift = 16;
 pixelDesc.cGreenBits = 8;
 pixelDesc.cGreenShift = 8;
 pixelDesc.cBlueBits = 8;
 pixelDesc.cBlueShift = 0;
 pixelDesc.cAlphaBits = 0;
 pixelDesc.cAlphaShift = 0;
 pixelDesc.cAccumBits = 64;
 pixelDesc.cAccumRedBits = 16;
 pixelDesc.cAccumGreenBits = 16;
 pixelDesc.cAccumBlueBits = 16;
 pixelDesc.cAccumAlphaBits = 0;
 pixelDesc.cDepthBits = 32;
 pixelDesc.cStencilBits = 8;
 pixelDesc.cAuxBuffers = 0;
 pixelDesc.iLayerType = PFD_MAIN_PLANE;
 pixelDesc.bReserved = 0;
 pixelDesc.dwLayerMask = 0;
 pixelDesc.dwVisibleMask = 0;
 pixelDesc.dwDamageMask = 0;

 m_GLPixelIndex = ChoosePixelFormat(hDC,&pixelDesc);
 if(m_GLPixelIndex==0) // Choose default
 {
  m_GLPixelIndex = 1;
  if(!DescribePixelFormat(hDC,m_GLPixelIndex,
  sizeof(PIXELFORMATDESCRIPTOR),&pixelDesc))
  return FALSE;
 }

 if(!SetPixelFormat(hDC,m_GLPixelIndex,&pixelDesc))
  return FALSE;

 return TRUE;
}

//*********************************
// CreateViewGLContext 
//*********************************
BOOL CColorDlg::CreateViewGLContext(HDC hDC)
{
 m_hGLContext = wglCreateContext(hDC);

 if(m_hGLContext==NULL)
  return FALSE;

 if(wglMakeCurrent(hDC,m_hGLContext)==FALSE)
  return FALSE;

 return TRUE;
}
//*********************************
// OnDestroy 
//*********************************
void CColorDlg::OnDestroy()
{
 CDialog::OnDestroy();

 if(wglGetCurrentContext() != NULL)
  wglMakeCurrent(NULL,NULL);

 if(m_hGLContext != NULL)
 {
  wglDeleteContext(m_hGLContext);
  m_hGLContext = NULL;
 }

 glDeleteLists(1,2);
}

RENDERING

The RGB cube and the cloud are rendered in two distinct display lists. The cube is a fixed part, while the color cloud is rebuild each time a new image is loaded.

//*********************************
// OnPaint 
//*********************************
void CColorDlg::OnPaint()
{
 // ** Draw scene **
 CPaintDC dc(this);
 RenderScene();
 SwapBuffers(dc.m_ps.hdc); // double buffer
}
//*********************************
// RenderScene 
//*********************************
void CColorDlg::RenderScene()
{
 ::glClearColor((float)GetRValue(m_BackColor)/255.0f,
                (float)GetGValue(m_BackColor)/255.0f,
                (float)GetBValue(m_BackColor)/255.0f,1.0f);
 ::glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

 ::glPushMatrix();
 ::glTranslated(0.0,0.0,-8.0);
 ::glRotated(m_xRotate, 1.0, 0.0, 0.0);
 ::glRotated(m_yRotate, 0.0, 1.0, 0.0);
 ::glCallList(1); // cube
 ::glCallList(2); // clouds
 ::glPopMatrix();
}

//***********************************************
// BuildList
//***********************************************
void CColorDlg::BuildListCube(BOOL list)
{
 GLUquadricObj* pQuadric = gluNewQuadric();

 if(list)
  ::glNewList(1,GL_COMPILE_AND_EXECUTE);

 float x = m_Size;

 // RGB cube
 glBegin(GL_LINE_LOOP);
  glColor3ub(0,0,0);
  glVertex3d(-x,-x,-x);
  glColor3ub(255,0,0);
  glVertex3d(x,-x,-x);
  glColor3ub(255,255,0);
  glVertex3d(x,x,-x);
  glColor3ub(0,255,0);
  glVertex3d(-x,x,-x);
 glEnd();

 glBegin(GL_LINE_LOOP);
  glColor3ub(0,0,255);
  glVertex3d(-x,-x,x);
  glColor3ub(255,0,255);
  glVertex3d(x,-x,x);
  glColor3ub(255,255,255);
  glVertex3d(x,x,x);
  glColor3ub(0,255,255);
  glVertex3d(-x,x,x);
 glEnd();

 glBegin(GL_LINES);
  glColor3ub(0,0,0);
  glVertex3d(-x,-x,-x);
  glColor3ub(0,0,255);
  glVertex3d(-x,-x,x);
  glColor3ub(255,0,0);
  glVertex3d(x,-x,-x);
  glColor3ub(255,0,255);
  glVertex3d(x,-x,x);
  glColor3ub(255,255,0);
  glVertex3d(x,x,-x);
  glColor3ub(255,255,255);
  glVertex3d(x,x,x);
  glColor3ub(0,255,0);
  glVertex3d(-x,x,-x);
  glColor3ub(0,255,255);
  glVertex3d(-x,x,x);
 glEnd();

 // Spheres
 glPolygonMode(GL_FRONT,GL_FILL);
 glPolygonMode(GL_BACK,GL_FILL);
 float radius = 0.1f;

 glPushMatrix();
 glTranslated(-m_Size,-m_Size,-m_Size);
 glColor3ub(0,0,0);
 gluSphere(pQuadric,radius,12,12);
 glPopMatrix();

 glPushMatrix();
 glTranslated(m_Size,-m_Size,-m_Size);
 glColor3ub(255,0,0);
 gluSphere(pQuadric,radius,12,12);
 glPopMatrix();

 glPushMatrix();
 glTranslated(-m_Size,m_Size,-m_Size);
 glColor3ub(0,255,0);
 gluSphere(pQuadric,radius,12,12);
 glPopMatrix();

 glPushMatrix();
 glTranslated(-m_Size,-m_Size,m_Size);
 glColor3ub(0,0,255);
 gluSphere(pQuadric,radius,12,12);
 glPopMatrix();

 glPushMatrix();
 glTranslated(m_Size,m_Size,-m_Size);
 glColor3ub(255,255,0);
 gluSphere(pQuadric,radius,12,12);
 glPopMatrix();

 glPushMatrix();
 glTranslated(-m_Size,m_Size,m_Size);
 glColor3ub(0,255,255);
 gluSphere(pQuadric,radius,12,12);
 glPopMatrix();

 glPushMatrix();
 glTranslated(m_Size,-m_Size,m_Size);
 glColor3ub(255,0,255);
 gluSphere(pQuadric,radius,12,12);
 glPopMatrix();

 glPushMatrix();
 glTranslated(m_Size,m_Size,m_Size);
 glColor3ub(255,255,255);
 gluSphere(pQuadric,radius,12,12);
 glPopMatrix();

 if(list)
  ::glEndList();

 gluDeleteQuadric(pQuadric);
}

//***********************************************
// BuildListCloud
//***********************************************
void CColorDlg::BuildListCloud()
{
 TRACE("Build list cloud...");

 // Image area
 unsigned int area = m_Image.GetWidth()
                   * m_Image.GetHeight();

 TRACE("area : %d...",area);

 // Need valid image (24 bits)
 if(area == 0 ||
  m_Image.GetDepth() != 24)
 return;

 // Maximum -> area distinct colors / 2^24
 // This table is memory expansive, but short lived
 TRACE("alloc...");
 unsigned char *pTable = new unsigned char[16777216];
 memset(pTable,0,16777216); // init 0

 // Image data
 unsigned int wb32 = m_Image.GetWidthByte32();
 unsigned char *pData = m_Image.GetData();

 // Build a new list
 TRACE("build list...");
 int nb = 0;
 ::glNewList(2,GL_COMPILE_AND_EXECUTE);
 glBegin(GL_POINTS);
 float tmp = 2.0f/255.0f*m_Size;
 for(unsigned int j=0;j<m_Image.GetHeight();j++)
  for(unsigned int i=0;i<m_Image.GetWidth();i++)
  {
   unsigned char b = pData[wb32*j+3*i];
   unsigned char g = pData[wb32*j+3*i+1];
   unsigned char r = pData[wb32*j+3*i+2];
   if(!pTable[b*65536+g*256+r])
   {
    glColor3ub(r,g,b);
    float x = -m_Size+(float)r*tmp;
    float y = -m_Size+(float)g*tmp;
    float z = -m_Size+(float)b*tmp;
    glVertex3d(x,y,z);
    pTable[b*65536+g*256+r] = 1;
    nb++;
   }
  }

 glEnd();
 ::glEndList();
 TRACE("%d points...",nb);

 TRACE("cleanup...");
 delete [] pTable;
 TRACE("ok\n");
}

Downloads

Download source – 10 Kb
Download (built) demo project – 530 Kb

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