OpenGL Basics Compared to DirectX

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OpenGL Basics Compared to DirectX

• By Jin Li
• Jan 20th, 2005
• Introduction to Computer Graphics

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Review of the major similarities and differences

• DirectX is more than just a graphics API, OpenGL
  is strictly a graphics API
• Both use the traditional hardware rendering
  pipeline, which is based on the z buffer.
• Both describe vertices as a set of data
  consisting of coordinates. Graphics primitives,
  are defined as an ordered set of vertices. How
  each API handles vertices to form primitives is
  different

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• OpenGL
• Cross-platform open standard, easier to
  understand
• No support for create graphic user interface,
  needs other supports.
• DirectX
• A Windows-specific proprietary standard, harder
  to understand,
• But with Managed DirectX (DirectX wrapped into
  very handy .NET classes), easier to use.
• Full support, a Multimedia API.

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• OpenGL
• First, from SGI (Silicon Graphics Inc.) Now, ARB(
  Architecture Review Board)
• New functionalities, through vendor-specific
  extensions. The base API changes only slowly.
• DirectX
• Developed by Microsoft with some other vendors
• New functionalities, through API changes, quite
  severely.

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How to Start

• Windows Since Window 98, the library has
  already been included.
• LINUX/BSD There are several free
  implementations, GLX , Mesa, TinyGL, Ygl

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GL UTILITIES (GLU)

• OpenGL Utility Library, included in Standard
  OpenGL
• It includes a set of functions in
• Texture mipmapping from a base image,
• Draw quadric surfaces and NURBS
• Camera management

The GL Utility Library

• Extends functionality of OpenGL library
• General independent window support
• Manages keyboard, mouse, window
• Also provides some higher primitives
• Simple non-convex polygo
• Texturing
• They need to be downloaded.

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A fully installed Windows OpenGL development
environment
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For UNIX or UNIX-like operating systems

• You need to point the compiler and linker to
  their location with the appropriate -I and -L
  options.
• -lglut -lGLU -lGL -lXmu -lX11 is typical.

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OpenGL basics

• API conventions

• ???? Function names begin with gl and use mixed
  case
• glBegin, glEnd, glFrontFace
• ???? Constants begin with GL and use only upper
  case
• GL_TRIANGLES, GL_BACK, GL_LIGHT0
• ???? Types begin with GL and use only lower case
• GLbyte, GLubyte, GLint, GLfloat,...

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• Setup an OpenGL project in Microsoft Visual
  Studio .NET 2003
• Create and manage drawing windows through GLUT.

Display Handler
Main Event Loop
Keyboard Handler
Mouse Handler
Event Driven Programming
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• Clearing the window (Clear color buffer and depth
  buffer)
• glClear(GL_COLOR_BUFFER_BIT GL_DEPTH_BUFFER_BIT)
  
• glClearDepth (1.0)
• Specifying a Color
• glColor3f
• glClearColor() takes four parameters, the fourth
  is alpha value.
• Forcing Completion of Drawing (glFlush() and
  glFinish())
• glFlush() causes all OpenGL commands currently
  queued to be submitted to the hardware for
  execution.
• glFinish() with the addition that glFinish() will
  block until all commands submitted have been
  executed.

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• Hidden-Surface Removal (Using a depth buffer)
• OpenGL
• glEnable(GL_DEPTH_TEST)
• glClear(GL_DEPTH_BUFFER_BIT)
• DirectX
• presentParams.EnableAutoDepthStencil true
• presentParams.AutoDepthStencilFormat
  DepthFormat.D16
• device.Clear(ClearFlags.Target
  ClearFlags.ZBuffer, Color.CornflowerBlue, 1.0f,
  0)
• In both cases, there are more precision at the
  front of the depth buffer. This will cause the
  problem of less precision in large scene.

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• Describing Points, Lines, and Polygons
• Using glBegin() and glEnd()
• different from device.BeginScene() and
  device.EndScene() in DirectX.
• Specifying Vertices
• ???? glVertex234sifdv()
• glVertex3i(Glint x,Glint y,Glint z)
• glVertex3fv(Glfloat v)
• ???? argument types
• GL byte y, GL short GL int , GL float l GL
  double o
• unsigned byte (ub), short ( us ), ), int (ui)
• ???? v indicates vector

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• Drawing Primitives
• OpenGL
• glBegin(GLenum mode)
• Primitive types
• ???? Points GL_POINTS
• ???? Lines GL_LINES,GL_LINE_STRIP,GL_LINE_LOOP
• ???? Triangles
• GL_TRIANGLES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN
• ???? Quadrilaterals and all other polygons
  GL_QUADS,GL_QUAD_STRIP,GL_POLYGON
• ????Ordering of vertices (corners)
• GL_CCW lt Default GL_CW

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• Culling Polygon Faces
• OpenGL
• glEnable(GL_CULL_FACE) .
• glFrontFace(GLenum mode) Define the orientation
  of front-facing polygons
• GL_CW and GL_CCW are accepted.
• DirectX
• Sounterclockwise cull mode is the default for
  Direct3D
• device.RenderState.CullMode Cull.None(Turn
  off culling) Cull.None, Cull.CounterClockwise,
  Cull.Clockwise

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Viewing and Transformations

• OpenGL has only two matrices
• MODELVIEW and PROJECTION.
• glMatrixMode(GL_MODELVIEW)
• glMatrixMode(GL_PROJECTION)

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DirectX has three matrices World, View,
Perspective device.Transform.World
device.Transform.Projection
device.Transform.View
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• Projection Transformations
• 1. Perspective Projection
• gluPerspective(GLdouble fovy, GLdouble aspect,
  GLdouble zNear, GLdouble zFar)
• (DirectX
• device.Transform.Projection Matrix.PerspectiveFo
  vLH())
• glFrustum(GLdouble left, GLdouble right, GLdouble
  bottom, GLdouble top, GLdouble near, GLdouble
  far)
• 2. Orthographic Projection-- parallel viewing
  volume
• glOrtho(GLdouble left, GLdouble right, GLdouble
  bottom, GLdouble top, GLdouble near, GLdouble
  far)

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• Viewing Transformations
• gluLookAt (eyeX , eyeY , eyeZ , centerX,
  centerY, centerZ, upX, upY, upZ)
• device.Transform.View Matrix.LookAtLH()

• Modeling transformations
• glTranslate(), glRotate(), and glScale(). ---
• device.Transform.World Matrix.RotationZ

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Multiply Current Matrix by a Rotation,Translation
or Scaling Matrix

• NEW_MATRIX CURRENT_MATRIX NEW_TRANSFORM
• void glTranslatef(float x, y, z)
• void glRotatef(float angle, x, y, z)
• void glScalef(float x, y, z)

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Order of Specification isOpposite Order of
Operation

• Specification order in program code
• First glRotatef(R2)
• Second glRotatef(R1...)
• Third glTranslatef( T)
• Resulting composed transformation R2R1T.
• Order in which operations are applied to
• vertices First T Second R1 Third R2.

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• Manipulating the Matrix Stacks
• glPushMatrix()-- It copies the current matrix and
  adds the copy to the top of the stack.
• glPopMatrix(), which discards the top matrix on
  the stack and uses this matrix as the current
  matrix.
• glLoadIdentity() command to clear the currently
  modifiable matrix for future transformation
  commands. Typically, you always call this
  command before specifying projection or viewing
  transformations

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Transformation Pipeline
Hierarchical Modelling
glTranslatef(waistx,waisty,waistz) glPushMatrix()
glRotatef(ra,rx,ry,rz) // draw right
arm glPopMatrix() glPushMatrix()
glRotatef(la,lx,ly,lz) // draw left
arm glPopMatrix()
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Lights

• Three main types of light source in OpenGL,
  directional, point and spot lights.
• To directional lights, the 4th value is different
• GLfloat lightPosition 0.7f, 0.7f, 0.7f,
  0.0f
• To point lights,
• GLfloat lightPosition 0.7f, 0.7f, 0.7f,
  1.0f

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Defining Lights
glLightfv(GLenum light, GLenum pname,
GLfloat param) glEnable(GL_LIGHTING) glEnable(GL_
LIGHT0)
Parameters
GL_AMBIENT - RGBA ambient intensity GL_DIFFUSE
- RGBA diffuse intensity GL_SPECULAR - RGBA
specular intensity GL_POSITION light position
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Materials
Defining Material Properties
glMaterialfv(GLenum face, GLenum pname,
GLfloat param)
Faces
GL_FRONT, GL_BACK, GL_FRONT_AND_BACK
Parameters
GL_AMBIENT, GL_DIFFUSE, GL_SPECULAR, GL_EMISSION,
GL_SHININESS
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Drawing Example
Adding Lighting
float lPos 1.0,1.0,1.0,1.0 glLightfv(GL_LIG
HT0,GL_POSITION,lPos) float diffuse
1.0,0.0,0.0,1.0 glMaterialfv(GL_FRONT,GL_DIFFUS
E,diffuse) glEnable(GL_LIGHTING) glEnable(GL_LI
GHT0) glShadeModel(GL_SMOOTH) // Setup
camera // Draw objects
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• Blending
• OpenGL
• Alpha values can be specified with glColor(),
  the 4th value.
• glBlendFunc(GLenum sfactor, GLenum dfactor)
• glEnable(GL_BLEND)
• GL_ZERO GL_ONE ,GL_DST_COLOR, GL_SRC_COLOR
  ,GL_ONE_MINUS_DST_COLOR ,GL_ONE_MINUS_SRC_COLOR
  ,GL_SRC_ALPHA.
• In DirectX, for example
• device.RenderState.AlphaBlendEnable true
• device.RenderState.DestinationBlend Blend.One
• device.RenderState.SourceBlend
  Blend.SourceAlpha
• device.RenderState.BlendOperationBlendOperation.M
  ax

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Texture mapping
Texture Coordinate System in OpenGL Texture
Coordinate System in DirectX
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Steps to apply a texture

• Create a texture object and specify a texture for
  that object
• (Loading a texture from file is not a built-in
  feature of OpenGL,glaux.lib is used here. )
• Generate texture names
• void glGenTextures(GLsize n,GLuint textures)
• Bind texture objects to texture data
  parameters
• void glBindTexture(GLenum target, GLuint
  textureName)
• Indicate how the texture is to be applied to each
  pixel
• int gluBuild2DMipmaps(GLenum target,GLint
  components,GLint width,GLint height,GLenum
  format,GLenum type,const void data)
• Enable texture mapping
• glEnable(GL_TEXTURE_2D)
• Draw the scene, supplying both texture and
  geometric coordinates
• glTexCoord2f(0.0f, 0.0f) glVertex3f(-1.0f,
  -1.0f, 1.0f)

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Thank You!