OpenGL Bitmaps, Images, and Texts

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OpenGL Bitmaps, Images, and Texts
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Bitmap

• OpenGL provides the basic functions of drawing 2D
  raster graphics -- texts and images
• A bitmap is a rectangular binary array for
  pixels. It is often used as mask or drawing
  texts. When you draw a bitmap,
• A pixel will be drawn with the current color if
  its bit in the map 1
• Nothing is drawn if its bit 0

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Current Raster Position

• The current raster position is the origin where
  the next bitmap/image is to be drawn. It is set
  by calling
• glRasterPos234sifd(x, y, z, w)
• Z 0 and w 1 for glRasterPos2()
• The raster position is transformed in the same
  way as coordinates in glVertex().
• If the transformed point is clipped out, the
  current raster position is invalid.
• Get the current raster position
• GLfloat pos4
• glGetFloatv(GL_CURRENT_RASTER_POSITION, pos)

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Drawing the Bitmap

• glBitmap() draws a bitmap to the framebuffer
  relative to the current raster position.
• glBitmap(width, height, x0, y0, xi, yi, bitmap)
• width, height width and height of the bitmap
• x0, y0 origin of the bitmap relative to the
  current raster position.
• xi, yi the x and y increments to the raster
  position after the bitmap is drawn
• bitmap a byte array storing the bitmap
• Example write a letter A
• glOrtho(0.0f, 800, 0.0, 600, -1.0, 1.0)
• glTranslatef(rand()800, rand()600, 0)
• glRasterPos2i(0, 0)
• glBitmap(16, 16, 0.0, 0.0, 0.0, 0.0, letterA)

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Using Images

• Images have more information for a pixel, e.g.
  RGB values
• glDrawPixels() writes a rectangular array of
  pixels into the framebuffer at the current raster
  position
• glDrawPixels(width, height, format, type,
  pixels)
• format indicates the kind of pixel data elements
• GL_COLOR_INDEX
• GL_RGB
• GL_RGBA
• GL_DEPTH_COMPONENT
• type indicates the data type of each element
• GL_BYTE
• GL_UNSIGNED_BYTE
• GL_INT
• GL_FLOAT
• pixels an array of pixel data elements

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Reading Pixels

• You may read a rectangular array pixels from the
  framebuffer to the main memory
• glReadPixels(x, y, width, height, format, type,
  pixels)
• x, y lower left corner of the framebuffer
  rectangle
• width, height width and height of the rectangle
• format the same as glDrawPixels()
• type the same as glDrawPixels()
• pixels an array of pixel data
• Reading back from framebuffer is slow!
• Example
• GLubyte imgData new GLubytewh3
• glReadPixels(0, 0, w, h, GL_RGB,
  GL_UNSIGNED_BYTE, imgData)

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Copying Pixels

• Copy pixels from one framebuffer rectangle to
  another
• glCopyPixels(x, y, width, height, buffer)
• buffer specifying the framebuffer that is used
• GL_COLOR
• GL_DEPTH
• GL_STENCIL
• The buffer rectangle specified by (x, y, width,
  height) is copied to a rectangle at the current
  raster position.
• For all three functions, the exact conversion of
  data going to or from the framebuffer depend on
  the modes in effect at the time

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Magnifying, Reducing, or Flipping

• OpenGL allows you to arbitrarily magnify, reduce,
  or even flip (reflect) an image by using
  glPixelZoom()
• glPixelZoom(xZoom, yZoom)
• glPixelZoom(-1.0f, -1.0f) // flip image
  horizontally and vertically
• glPixleZoom(0.5f, 0.5f) // reduce image size
• glPixelZoom(2.0f, 2.0f) // magnify image size
• Pixel Zoom is applied to both glDrawPixels() and
  glCopyPixels()
• Application as a magnifying glass or a sniper gun
  for games

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Managing Pixel Storage

• There are a lot of details of how to store pixels
  data in main memory, such as alignments,
  byte-ordering etc.. You can control the pixel
  storage mode using
• glPixelStoreif(pname, param)
• pname GL_PACK_ALIGNMENT or GL_UNPACK_ALIGNMENT
• Packing refers to the way that pixel data is
  written to main memory, e.g. glReadPixels
• Unpacking refers to the way that pixel data is
  read from memory, e.g. glTexImage2D
• param 1, 2, 4, 8
• Default 4 means the data for each row of the
  image begin on a 4-byte boundary
• Several other parameters available, check OpenGL
  programming guide

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Image formats

• There are hundreds of different image formats
  available. I wont cover any particular format.
• You can find on-line descriptions and loaders for
  a lot of image file formats, e.g. imagemagick.

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Bitmap Fonts

• A set of bitmaps will generated as display lists,
  and drawn using glCallLists
• OpenGL doesnt directly support displaying text.
  We need to use platform specific functions.
• glutBitmapCharacter(void font, int character)
• font Bitmap font to use, including
  GLUT_BITMAP_8_BY_13, GLUT_BITMAP_9_BY_15,
  GLUT_BITMAP_TIMES_ROMAN_10, GLUT_BITMAP_TIMES_ROMA
  N_24, GLUT_BITMAP_HELVETICA_10,
  GLUT_BITMAP_HELVETICA_12, GLUT_BITMAP_HELVETICA_18
  
• character Character to render
• Read textbook for displaying fonts with wgl
  extensions.

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Drawing bitmap fonts

• char str OpenGL Bitmap Fonts!
• //Set matrices for ortho
• //Print text
• glRasterPos2f(-0.5f, 0.0f)
• for(unsigned int i0 iltstrlen(str) i)
• glutBitmapCharacter(GLUT_BITMAP_HELVETICA_18,
  stri)
• //reset matrices
• glPopMatrix()
• glMatrixMode(GL_PROJECTION)
• glPopMatrix()

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Setting Raster Position for Text

• Set the raster position in the 3D space. Remember
  Modelview and Projection transformations are
  applied to the raster position.
• glMatrixMode(GL_PROJECTION)
• glPushMatrix()
• glLoadIdentity()
• gluOrtho2D(-1.0f, 1.0f, -1.0f, 1.0f)
• glMatrixMode(GL_MODELVIEW)
• glPushMatrix
• glLoadIdentity()
• glRasterPos2f(-0.5f, 0.0f)
• A point on the near plane.