Pixels, Bitmaps, Fonts, and Images
  • Objectives:
    • Position and draw bitmapped data
    • Read pixel data between the framebuffer and memory
    • Copy pixel data from one color buffer to another or to another location in the framebuffer
    • Control pixel data formatting 
    • Pixel data transformations



    Image from http://www.dtc.army.mil/hpcw/1999/tucker/index.html



     
     
     
     

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

  Polygons, Bitmaps, and Image Data
  • So far most of our discussion has concerned geometric objects
  • Other data may be managed by OpenGL
    • Bitmaps (usually used for font characters)
    • Image data
  • Bitmaps and image data are arranged in retangular arrays of pixels
    • Bitmaps are 1-bit deep
    • Image data are generally multi-component
    • Bitmaps generally function as color masks (or blend masks)

     
     
     

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

  Bitmaps and Fonts
  • A bitmap is a rectangular array of 0's and 1's 
    • Serves as a drawing mask 
    • Applied to a rectangular portion of a window
    • 0's in the bitmap do not affect the framebuffer
  •  glRasterPos() and glBitmap()  are used to position and draw bitmaps on the screen.
    • GLubyte rasters[24] = {
   0xc0, 0x00, 0xc0, 0x00, 0xc0, 0x00, 0xc0, 0x00, 0xc0, 0x00,
   0xff, 0x00, 0xff, 0x00, 0xc0, 0x00, 0xc0, 0x00, 0xc0, 0x00,
   0xff, 0xc0, 0xff, 0xc0};

void init(void)
{
   glPixelStorei (GL_UNPACK_ALIGNMENT, 1);
   glClearColor (0.0, 0.0, 0.0, 0.0);
}

void display(void)
{
   glClear(GL_COLOR_BUFFER_BIT);
   glColor3f (1.0, 1.0, 1.0);
   glRasterPos2i (20, 20);
   glBitmap (10, 12, 0.0, 0.0, 11.0, 0.0, rasters);
   glBitmap (10, 12, 0.0, 0.0, 11.0, 0.0, rasters);
   glBitmap (10, 12, 0.0, 0.0, 11.0, 0.0, rasters);
   glFlush();
}
     

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

  Current Raster Postition
  • The current raster position is the origin where the next bitmap or image is to be drawn
    • Defines the lower-left pixel
  •  To render in screen coordinates the modelview and projection matrices should be defined as such:


    • glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0.0, (GLfloat) width, 0.0, (GLfloat)
      height);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
     

  •  Current raster position can be queried with glGetFloatv(GL_CURRENT_RASTER_POSITION)

    •  

     
     
     
     
     
     
     
     

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

  Drawing the Bitmap
  • glBitmap(GLsizei width, GLsizei height, GLfloat xbo, GLfloat ybo, GLfloat xbi, GLfloat ybi, GLubyte *bitmap);


    •  
  • The current raster position is incremented by xi and yi
  • Floating point values are rounded to integer pixel values
  • NOTE!  Fonts cannot be rotated - aligned to framebuffer

    •  

     
     
     
     
     

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

  Defining the Color of the Bitmap
  • Color is bound before raster position is defined

    •  

     

    glColor3f(1.0, 1.0, 1.0);
    glRasterPos3fv(position);
    glColor3f(1.0, 0.0, 0.0);
    glBitmap(...)

    The font color is WHITE (!)
     
     
     

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

  End of Presentation
  • Fonts can be used in display list definitions

    •  

     


     

  •  Refer to font.c in examples

    •  

     
     
     
     
     

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

  Images
  • Images are bits which may contain more than one bit of data per pixel
  • Examples
    • Phototextures
    • Portions of a framebuffer previously rendered
    • Synthetic data from a software renderer (image or non-image) (zbuffer or stencil data for example)
    • Instrumentation data



    Image from http://sti-ultrasound.com/ultra.htm



     

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

  Reading, Writing and Copying Pixel Data
  • To manipulate pixel data there are three basic commands:
    • glReadPixels() - reads a rectangular array from the framebuffer to processor memory
    • glDrawPixels() - writes pixel data into the framebuffer at the current raster position
    • glCopyPixels() - copys pixel data from one part of the framebuffer to another (never passes through processor memory)


     
     
     

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

  Reading Pixel Data to Memory
  • Reading and writing data from the framebuffer is straightforward
    • If you want to modify the data, you need to know the format in which it is stored


    void glReadPixels(GLint x, GLint y, GLsizei width, 
      GLsizei  height, GLenum format, GLenum type, 
      GLvoid *pixels);