Computer Graphics The Rendering Pipeline Review

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Computer GraphicsThe Rendering Pipeline - Review

• CO2409 Computer Graphics
• Week 13

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Lecture Contents

• The Rendering Pipeline
• Input Geometry
• Vertex Data Primitive Data
• Vertex Processing
• Matrix Transformations
• Lighting
• Geometry Processing
• Pixel Processing
• Textures
• Pixel Rendering

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The Rendering Pipeline

• We looked at the rendering pipeline of DirectX

• Have seen each stage now
• Except tessellation not commonly used
• Will now review each stage in turn

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Vertex Data

• Initially identified the input vertex data as the
  vertex coordinates of our geometry

• (X,Y,Z) position for each vertex
• Other parts of the pipeline also need per-vertex
  data
• Vertex normals
• if using lighting to find colour at vertex
• Vertex colours
• if not using lighting
• Texture coordinates / UVs
• if we use textures
• Multiple sets if we blend multiple textures

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Vertex Data Formats

• Input vertex data is more than just a simple list
  of vertex positions
• Data required depends on how we render
• With textures, lighting etc.
• Use custom vertex data formats for flexibility,
  e.g.

// Vertex coord colour // Fixed colour polys
with // No lighting or texture struct
CUSTOMVERTEX1 FLOAT vx, vy, vz DWORD
colour
// Vertex coord, normal // texture coord Lit
and // textured polys struct CUSTOMVERTEX2
FLOAT vx, vy, vz FLOAT nx, ny, nz FLOAT
u, v
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Vertex Buffers

• A vertex buffer is just an array of vertices
• Defining the set of vertices in our geometry
• Each vertex a custom format as above
• The buffer is managed by DirectX
• We must ask DirectX to create and destroy them
• and to access the data inside (called locking)
• Stored in main memory or video card memory
• We have some control when we create them
• First saw vertex buffers in the week 5 lab

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Primitive Data / Index Buffers

• Primitive data is data defining the triangles in
  the geometry
• Or other primitives, such as lines or points
• The vertex data itself can define the primitives
• Each triplet of vertices defining a triangle

• More often use an index buffer
• An array of integers that index the vertex buffer
• Triplet of indexes for each triangle
• No duplication of vertices
• Saw index buffers in week 9

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Vertex Processing

• Vertex primitive data defines the 3D geometry
• Next step is to convert this data into 2D
  polygons
• Key operation is a sequence of matrix transforms
• Transforming vertex coordinates from 3D model
  space to 2D viewport space
• This step can also include calculation of
  lighting (using the per-vertex normals)
• And animation, which we havent covered yet
• Some vertex data (e.g. UVs) is not used at this
  stage and is simply passed through to later
  stages
• We looked at vertex shaders for vertex processing

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From Model to World Space

• Mesh vertices are stored in model space
• A local space with a convenient origin and
  orientation

• Each model has a world matrix
• That transforms the model geometry from model
  space to world space

• This matrix defines position and orientation for
  the model
• Has a special form containing the local axes of
  the model

• These axes are often extracted from the matrix

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From World to Camera Space

• We view the models through a camera
• Which is part of the scene

• A camera has a view matrix
• Transforms the world space geometry into camera
  space
• Camera space defines the world as seen by the
  camera
• Camera looks down its Z axis
• The view matrix is the inverse of a normal world
  matrix
• But has a similar form and can be used in a
  similar way

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Camera Space to Viewport

• Each camera has a second matrix, the projection
  matrix
• Defining how the camera space geometry is
  projected into 2D
• Defines the camera viewport / near clip distance
  field of view
• Final step is to scale the projected 2D geometry
  into viewport pixel coordinates
• Performed internally by DirectX
• Covered this material weeks 6-9

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Lighting

• We looked at the mathematical lighting models
  used to illuminate 3D geometry (week 11)
• We showed how lighting can be calculated while
  vertices are being transformed
• In the same vertex shader
• The calculations need a normal for each vertex

• Discarded after vertices are converted to 2D
• Several effects combined for final vertex colour
• Ambient, diffuse, specular

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Geometry Processing

• This stage processes 2D polys before rendering
• Off-screen polygons are discarded (culled)
• Partially off-screen polygons are clipped
• Back-facing polygons are culled if required
  (determined by clockwise/anti-clockwise order of
  viewport vertices)
• These steps occur with minimal programming
• This stage also scans through the pixels of the
  polygons (called rasterising / rasterizing)
• The input data is interpolated and passed to the
  later pixel-based stages
• 2D coordinates, colours, UVs etc.

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Textures and Pixel Processing

• Next each pixel is worked on to get a final
  colour
• The texture and UV data has been passed through
  from the previous steps
• The UVs map the textures onto the 2D polygons
• Textures can be filtered (texture sampling) to
  improve their look
• Textures covered in week 12
• The texture colours are combined with the polygon
  colour to produce final pixel colours
• The polygon colour came from the original vertex
  data or from the lighting calculations

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Pixel Rendering

• The final step in the rendering pipeline is the
  rendering of the pixels to the viewport
• This involves blending the final polygon colour
  with the existing viewport pixel colours
• Havent covered this in 3D yet, similar to sprite
  blending
• Also the depth buffer values are tested / written
• We saw the depth buffer in the week 8 lab
• Will see them in more detail shortly

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Where Next?

• Modern apps use the programmable pipeline
• Vertex processing is performed by a Vertex Shader
• Matrix transformations, lighting
• Pixel processing by a Pixel Shader
• Combining texture and polygon colours
• There is a fixed pipeline that avoids shaders
• Little flexibility, rarely used nowadays
• Shaders allows a much wider range of pipeline
  behaviour for advanced techniques and effects
• In the next few weeks we will look at some of
  these advanced shader techniques