Computer Graphics Module Review

1. Computer GraphicsModule Review CO2409 Computer Graphics

2. Lecture Contents • 2D Graphics & Geometry • 3D Geometry & Maths • Rendering Pipeline – Key Concepts • Programmable Pipeline / Shaders • Depth / Stencil Buffers & Shadows • Animation

3. Pixels & Colour • A computer display is made of a grid of small rectangular areas called pixels • Pixel colour is usually specified as red, green and blue components: • Integers (0-255) or floats (0.0 to 1.0) • The RGB ‘colour space’ is a cube • Another colour space is HLS • Hue =colour from spectrum, Lightness = brightness of the colour Saturation = intensity of the colour • Can be pictured as a double cone • More intuitive for artists

4. Bitmaps / Sprites / Alpha Channels • A bitmap is rectangle of pixels stored off-screen for use in an image • A sprite describes a particular use of bitmaps • When used as distinct elements in a larger scene • As well as RGB colours, we can store per-pixel values specifying transparency • Alpha data, or the alpha channel, making RGBA • Can use alpha to blend pixels onto viewport FinalColour = Alpha * SourceColour + (1-Alpha) * ViewportColour • Alpha can also be used for alpha testing • Used for cutout sprites

5. Further Blending • Other ways of blending pixels onto the viewport: • Multiplicative blending equation is: FinalColour = SourceColour * ViewportColour • A darkening effect, suitable for representation of glass, shadows, smoke etc • Additive blending equation is: FinalColour = SourceColour + ViewportColour • This is a lightening effect, mainly used for the representation of lights

6. Basic Geometry Definitions • In both 2D and 3D we have used these definitions: • A vertex: a single point defined by coordinates on the axes of a coordinate system • E.g A(10, 20) • An edge: a straight line joining two vertices • E.g. AB • A vector: a movement within a coordinate system • E.g. AB (from A to B) or V(5, 10) • A normal is any vector whose length is 1 • A polygon: a single closed loop of edges • E.g. ABC(from C to A implied)

7. Coordinate Systems • A coordinate system is a particular set of choices for: • The location of the origin • Orientation and scale of the axes • We later called this a space • E.g. Model space, World space • A vertex will have different coordinates in two different coordinate systems • The viewport is a particular coordinate system that corresponds to the visible display area

8. Rendering • Rendering is converting geometry into pixels • In general rendering is a two stage process: • Convert the geometry into 2D viewport space (geometry transformation / vertex shaders) • Set the colour of the pixels corresponding to this converted geometry (rasterising / pixel shaders) • Looked briefly at 2D rendering: • Render 2D lines by stepping through the pixels • Render polygons with multiple lines • Render circles with equation + many lines • Detail about filling polygon pixels is beyond scope of module

9. Maths/C++ for Graphics Apps • Be aware of numeric limitations in C++, e.g: • int limits can be exceeded • float / double have limited precision • Repeated calculations with float can build up errors • Other languages have similar limitations • C++ automatically converts between numeric types, issuing warnings when it does • Don’t ignore, may not be what is required • Several math functions used for graphics: • Max, min, remainders, modulus / absolute value, powers, cos, sin… • Know the library functions used

10. 3D Geometry - Meshes / Normals • A mesh is a set of polygons making a 3D object • A mesh is usually defined together with a set of face and/or vertex normals • A face normal is a normalised vector at right angles to a polygon • Used to specify the plane of the polygon • But not especially common • A vertex normal can be average of all the face normals of the polygons containing that vertex • Or can have multiple for sharp edges • Used frequently, most notably for lighting

11. Matrices • A matrix (plural matrices) is a rectangular table of numbers: • They have special rules of arithmetic • A coordinate system matrix is used to represent a model’s position/orientation: • Transformation matrices used to convert between spaces, or move/orient models: • E.g. world matrix converts from model->world space • Basic transforms: translate, rotation, scale • Of central importance to 3D graphics • Will always be exam questions on matrices

12. DirectX / Rendering Pipeline • Graphics APIs perform a ‘pipeline’ of operations: • This is the DirectX 10 pipeline: • Input is 3D model geometry data • Geometry stored as lists of vertex data • Customised for different techniques • Output is viewport pixels • Pipeline process: • Convert to world then viewport space, applying lighting • Scan through resultant 2D polygons, one pixel at a time • Render pixels using light colours, textures and blending

13. World Matrix • Mesh vertices are stored in model space • The local space for the mesh with a convenient origin and orientation • For each model we store a world matrix that transforms the model geometry into world space • This defines the position and orientation of the model • Has a special form containing the local axes of the model

14. View Matrix • For each camera we store a view matrix that transforms the world space geometry into camera space • Camera space defines the world as seen by the camera • X right, Y up and Z in the direction it is facing • For technical reasons this matrix is actually the inverse of a normal world matrix • But it has a similar form and can be used in a similar way

15. Projection Matrix • Cameras also have a second matrix, the projection matrix • Defining how the camera space geometry is projected into 2D • It includes the field of view and viewport distance of the camera • Viewport distance is also called the near clip distance • This matrix flattens camera space geometry into 2D viewport space • Then the projected 2D geometry is scaled/offset into pixel coordinates

16. Lighting / Shading • Geometry colour can come from: • Vertex or face colours and/or dynamic lighting • Two shading modes used • Hard or smooth edges • 3 basic light types • Directional, Point and Spot • Lighting is calculated by combining: • Ambient light – global background illumination • Diffuse light – direct lighting • Specular light – reflection of light source (highlights) • Equations are often examined

17. Textures • A texture is a bitmap wrapped around a model • The wrapping is specified by assigning a texture coordinate (UV) to each vertex in the geometry • This is texture mapping • The UVs for the texture range from 0-1 • UVs outside this range will be wrapped, mirrored, etc. depending on the texture addressing mode • Each pixel in the bitmap appears as a square on the geometry called a texel • Textures and texels can be smoothed using texture filtering and mip-mapping

18. Vertex / Index Data • Customised vertex data is stored in vertex buffers • Coordinate (always), normal, UVs, colour etc. • Can use vertex data alone to store geometry • Each triplet of vertices is a triangle (triangle list) • More efficient to use an additional index buffer • Store the set of unique vertices only • Define the triangles using triplets of indices • Can also use triangle strips: • First triplet defines the first triangle • Each further vertex/index is used with the previous two to form a further triangle

19. Programmable Pipeline / Shaders • Three pipeline stages can be programmed directly: • The vertex, geometry and pixel processing stages • We did not look at programmable geometry processing • Programs usually written in a high-level language (HLSL) • Called shaders, the vertex shader and the pixel shader • We write a shader for every rendering technique needed • Shaders can be compiled at runtime and are loaded as needed

20. Vertex Shaders • We define shaders in terms of: • Inputs - from previous stages • Outputs - to later stages • Shaders have a “typical ” usage, but are actually very flexible • Vertex shaders operate on each vertex in the original 3D geometry. Their typical usage is to: • Transform and project the vertex into viewport space • Perhaps apply animation or lighting to the vertex • At a minimum, a vertex shader expects vertex coordinates as input, but may have other input too: • Normals, UVs, vertex colours, etc. • A vertex shader must at the very least output a viewport position for the current vertex, although they often output much more

21. Pixel Shaders • Pixel Shaders operate on each pixel in the final 2D polygons. Their typical usage is to: • Sample (and filter) any textures applied to the polygon • Combine the texture colour with the existing polygon colour (from lighting and/or geometry colours) • Input for a pixel shader is usually the output from the vertex shader • After rasterization • A pixel shader must at least output a final pixel colour to be drawn/blended with the viewport A Random Image

22. Advanced Shaders • Advanced shaders can be used to implement high-quality lighting and rendering techniques • A key technique is per-pixel lighting • Vertex lighting exhibits problems on large polygons • Instead, have the vertex shader pass the vertex position and normal on to the pixel shader • These are interpolated for the pixel shader, which the uses the normal lighting equations on them • Have covered several other shader techniques: • Specular mapping, normal mapping, parallax mapping, cell shading

23. Graphics Architecture • The basic graphics architecture for all modern PCs and game consoles is similar • Comprised of a main system and a graphics unit • With one processor each (CPU & GPU) • Fast local RAM for each processor • Interface between these two systems is often slow • GPU is a dedicated graphics microprocessor • Much faster than a CPU for graphics related algorithms • GPU runs at the same time as the CPU • These are concurrent systems

24. Depth Buffers – Z-Buffers • A depth buffer is a rectangular array of depth values that matches the back-buffer • Used to sort rendered primitives in depth order • A z-buffer stores the viewport space Z coordinate of each pixel in the back buffer • Calculated per vertex (range 0.0 to 1.0) • Interpolated per-pixel • Pixels are only rendered if their z value is less than the existing value in the z-buffer • Z values are not distributed evenly • Different depth pixels can get same z value • Causes visual artefacts (z-fighting)

25. Stencil Buffers • The stencil-buffer is a buffer of additional per-pixel values associated with the depth buffer • Usually 1 or 8 bits embedded in the depth values • A mask controlling drawing to the back-buffer • Test stencil values before writing each pixel • Result determines whether to write to back-buffer and/or stencil • Customisable tests - a very flexible system • We used it for a mirror:

26. Rendering to Textures • Some special effects can be performed by rendering the scene into a texture • Rather than into the back-buffer/viewport • Process needs two (or more) rendering passes • Set up a special render target texture and render the scene onto it • Then render the scene again normally (to the back buffer), but with some polygons using the render texture • Quality is limited by texture size

27. Shadow Techniques • Basic shadows (e.g. blob-shadows) easy and useful • Advanced techniques may be static or dynamic • Static shadow maps are pre-calculated darkening textures applied over the main model textures • Use high-quality (slow) techniques offline to generate these textures • Called baking the shadows • Also sample static lighting at points where dynamic models will be • Can help light them at run-time

28. Dynamic Shadow Mapping • Dynamic Shadow Mapping is an extension of render-to texture techniques used for shadows • The scene is rendered into a texture (a shadow map), but from the point of view of the light • Treat the light like a camera • Then the scene is rendered normally, but each pixel first tested against shadow map • The pixel is not lit if in shadow from the light • Spotlights straightforward, point / directional complex

29. Rigid Body Animation • Rigid body animation concerns models made of several distinct parts • We assume that the parts form a hierarchy • Each part in the hierarchy has a world matrix • Defining its position and orientation - just like a model • Each part’s matrix is stored relative to its parent • And defines the joint with the parent • This is called a Matrix or Transform Hierarchy • Can be rendered using a recursive process • Or the parts can be stored in a depth-first list and rendered using an iterative process and a stack

30. Soft Body Animation • Soft body animation or skinning concerns models that stretch and flex as they animate • We define an independent hierarchy of bones assumed to underlie the geometry – the skeleton • Again each bone has a parent-relative world matrix • Each vertex can be influenced by more than one bone • Each influence carries a weight (0-1) • Sum of the weights for each vertex is 1 • Linearly blend the vertex world position from each bone influence using weights • A weighted average of the bone influences