计算机图形学（七）Discrete Techniques

• Uses of Texturing
  • Simulating materials
  • Reducing Geometric Complexity

1. Buffer in OpenGL

• Define a buffer: spatial resolution $(n\times m)$ and its depth (or precision) $k$, the number of bits/pixel
• OpenGL Frame Bufferes: Back buffer, Front buffer
• OpenGL Buffers
  • Color buffers: Front, Back, Auxiliary 辅助, Stereo 立体
  • Depth
  • Stencil: Holds masks
  • Most RGBA buffers 8 bits per component
  • Latest are floating point (IEEE)

2. Writing in Buffers

• The frame buffer is part of this memory

• Writing Model

  • read desrination pixel before writing source

  

  • Bit Writing Modes

    • Source and destination bits are combined bitwise
    • $s$, $d$, 从小到大, 16 possible function(glLoicOp(mode); --default:mode=GL_COPY)

    

  • XOR Mode

    • XOR is especially useful for swapping blocks of memory such as menus that are stored off screen
    • $S$ represents screen and $M$ represents a menu the sequence
      • $S\leftarrow S\oplus M$; $M\leftarrow S\oplus M$; $S\leftarrow M\oplus M$
      • Swaps the $S$ and $M$

• The Pixel Pipeline

  • OpenGL has a separate pipeline for pixels

  

• Buffer Selection

  • OpenGL can read from any of the buffers (front, back, depth)
  • Default to the back buffer, change with glReadBuffer
  • format of the pixels in the frame buffer is different from that of processor memory and these two types of memory reside in different places
    • Need packing and unpacking
    • Reading can be slow
  • Drawing through texture function

• OpenGL Pixel Functions

  • glReadPixels(x,t,width,height,format,type, myimage)
  • eg. glReadPixells(0,0,512,512,GL_RGB,GL_UNSIGNED_BYTE,myimage)

• Deprecated Functionality

  • Replace by use of texture functionality, glBltFrameBuffer, frame buffer objects
  • GPUs now include a large amount of texture memory that we can write into
  • Advantage: fast (not under control of window system)

3. Mapping Methods

• 3 Types
  • Texture Mapping
    • Uses images to fill inside of polygons
  • Environment (reflection mapping)
    • Uses a picture of the environment for texture maps
    • Allows simulation of highly specular surfaces
  • Bump mapping
    • Emulates altering normal vectors during the rendering process
    • to process each fragment independently with a fragment shader
• Where does mapping take place
  • Mapping techniques are implemented at the end of the rendering pipeline
  • Very efficient because few polygons make it past the clipper
• Coordinate Systems
  • Parametric coordinates
    • May be used to model curves and surfaces
  • Texture coordinates
    • Used to identify points in the image to be mapped
  • Object or World Coordinates
    • Conceptually, where the mapping takes place
  • Window Corrdinates
    • Where the final image is really produces
• Mapping
  • Backward Mapping
    • $s=s(x,y,z)$; $t=t(x,y,z)$
  • Two-part Mapping
    • first map the texture to a simple intermediate surface
    • Cylindrical Mapping
    • Sherical Map
    • Box Map

4. Texture Mapping in OpenGL

• 3 steps to applying a texture
  • specify the texture
  • Assign texture coordinates to vertices
  • specify texture parameters
• Specify the Texture
  • Glubyte my_texels[512][512]
  • glEnable(GL_TEXTURE_2D)
  • Define Image as a Texture
    • glTexImage2D(target,level,components,w,h,border,format,type,texels)
      • format: 是否使用平滑
    • eg. glTexImage2D(GL_TEXTURE_2D,0,3,512,512,0,GL_RGB,GL_UNSIGNED_BYTE,my_texels)
• Maping a Texture
  • glTexCoord*() specified at each vertex
• Parameter of Texture Mapping
  • Interpolation:
    • glHint(GL_PERSPECTIVE_CORRECTION_HIT,hint)
    • GL_DONT_CARE(default), GL_NICEST, GL_FASTNEST(don’t perform the perspective correction to maximize speed)
  • OpenGL has a variety of parameters that determine how texture is applied
    • Wrapping Mode: determine what happpens if $s$ and $t$ are outside the $(0,1)$ range
      • glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_EDGE)
      • glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP_REPEAT)
    • Filter Mode: allow us to use area averageing instead of point samples
      • More than one texel can cover a pixel (minification) or more than one pixel can cover a texel (magnification)
      • glTexParameteri(GL_TEXTURE,GL_TEXTURE_MAG_FILTER,GL_NEAREST)
      • glTexParameteri(GL_TEXTURE,GL_TEXTURE_MIN_FILTER,GL_LINEAR)
    • Mipmapping: allow us to use textures at multiple resolutions，处理纹理缩小的问题
      • Mipmapping allows for prefiltered texture maps of decreasing resolutions
      • Lessens interpolation errors for smaller textured objects
      • glGenerateMipMap(GL_TEXTURE_2D)
      • glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST_MIPMAP_NEAREST)
      • point sampling, linear filtering, mipmapped point sampling, mipped linear filtering
    • Environment parameters: determine how texture mapping interacts with shading

5. Texture Objects in OpenGL

• Texture Object

  GLuint textures[1];
  glEnable(GL_TEXTURE_2D); //open texture computing
  glGenTextures(1, texName); // generates a texture name
  glBindTexture(GL_TEXTURE_2D,texName[0]); // Create texture objects with texture data and state
  
  glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_EDGE);
  glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_REPEAT); //texture parameters
  glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR); glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
  //Define a texture image from an array of texels in CPU memory
  glTexImage2D(GL_TEXTURE_2D,0,GL_RGB,TEX_WIDTH,TEX_HEIGHT, 0, GL_RGB, GL_UNSIGNED_BYTE, texImage);
  glActiveTexture( GL_TEXTURE0 );

• Linking with Shader

• Applying Textures

  • sampler

  in vec4 color; //color from rasterizer
  in vec2 texCoord; //texure coordinate from rasterizer uniform sampler2D texture; //texture object from application
  void main() {
  gl_FragColor = color * texture2D( texture, texCoord );
  }

• Mirror image

  glTexParameteri(GL_TEXTURE_ 2D,GL_TEXTURE_WRAP_S,GL_MIR RORED_REPEAT);
  glTexParameteri(GL_TEXTURE_ 2D,GL_TEXTURE_WRAP_S,GL_MIR RORED_REPEAT); glTexParameteri(GL_TEXTURE_ 2D,GL_TEXTURE_WRAP_T,GL_MIR RORED_REPEAT);

6. The Example of Texture

• OpenGL new version can read texture depth with GL_DEPTH_STENCIL

7. Blend Model 合成技术

• Use A component of RGBA color to store opacity

• Draw partially transparent faces

  glEnable(GL_DEPTH_TEST)
  //Draw all opacity
  glEnable(GL_BLEND)
  glDepthMask(GL_FALSE) //depth only read
  glBlendFunc(GL_SRC_ALPHA, GL_ONE)
  //Draw all transparence from back to front 
  //compare depth but not write to depth, blending with opacity
  glDepthMask(GL_TRUE) 
  glDisable(GL_BLEND)

• Billboard

  • A billboard is a plane object (usually simple, like a rectangle) on which an image is texture mapped
  • The image often includes zero-alpha areas so they can be “seen through”
  • The object is rotated to face the viewer so that the viewer sees the image in 3D space, simulating a full 3D object