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Real-Time Rendering 靜宜大學資工研究所 蔡奇偉副教授 2010©

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**大綱 Introduction Graphics Rendering Pipeline Architecture**

Application Stages Geometry Stages Rasterizer Stages

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**What are the results that are achievable with Computer Graphics?**

Tomas Akenine-Mőller © 2002

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**State-of-the-Art Real-Time Rendering 2001**

Tomas Akenine-Mőller © 2002

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**State-of-the-Art Real-Time Rendering 2001**

Tomas Akenine-Mőller © 2002

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**Which one is a real photo, and which one is CG?**

Tomas Akenine-Mőller © 2002

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**Complexity: 8 million triangles…**

Tomas Akenine-Mőller © 2002

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**Photo-realistic Rendering…**

Tomas Akenine-Mőller © 2002

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Introduction Real-time rendering is concerned with making images rapidly on the computer. The rate at which images are displayed is measured in frames per second (fps) or Hertz (Hz). Usually between 15 fps and 72 fps. Interactivity, 3D rendering, graphics acceleration hardware. Include applications such as computer games, CAD, 3D animation software (3D Max or Maya), …, etc.

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**Graphics Rendering Pipeline**

The main function of the pipeline is to generate, or render, a two-dimensional image, given a virtual camera, 3D objects, light sources, shading equations, textures, and more.

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**Pipeline Architecture**

A pipeline consists of several stages. The speed of the pipeline is determined by the slowest stage, no matter how fast the other stages may be. conceptual stages functional stages pipeline stages

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Conceptual Stages Application Geomrtry Rasterizer

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**The APPLICATION stage Executed on the CPU Examples:**

Geometry Rasterizer The APPLICATION stage Executed on the CPU Means that the programmer decides what happens here Examples: Collision detection Speed-up techniques Animation Most important task: send rendering primitives (e.g. triangles) to the graphics hardware Tomas Akenine-Mőller © 2002

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**Rendering Primitives Use graphics hardware for real time…**

These can render points, lines, triangles. A surface is thus an approximation by a number of such primitives. Tomas Akenine-Mőller © 2002

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**You say that you render a ”3D scene”, but what is it?**

First, of all to take a picture, it takes a camera – a virtual one. Decides what should end up in the final image A 3D scene is: Geometry (triangles, lines, points, and more) Light sources Material properties of geometry Textures (images to glue onto the geometry) A triangle consists of 3 vertices A vertex is 3D position, and may include normals and more. Tomas Akenine-Mőller © 2002

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Virtual Camera Defined by position, direction vector, up vector, field of view, near and far plane. point dir near far fov (angle) Create image of geometry inside gray region Used by OpenGL, DirectX, ray tracing, etc. Tomas Akenine-Mőller © 2002

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Application Geometry Rasterizer The GEOMETRY stage Task: ”geometrical” operations on the input data (e.g. triangles) Allows: Move objects (matrix multiplication) Move the camera (matrix multiplication) Compute lighting at vertices of triangle Project onto screen (3D to 2D) Clipping (avoid triangles outside screen) Map to window Tomas Akenine-Mőller © 2002

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**Animate objects and camera**

Application Geometry Rasterizer Animate objects and camera Can animate in many different ways with 4x4 matrices Example: Before displaying a torus on screen, a matrix that represents a rotation can be applied. The result is that the torus is rotated. Same thing with camera (this is possible since motion is relative) Tomas Akenine-Mőller © 2002

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Application Geometry Rasterizer The RASTERIZER stage Main task: take output from GEOMETRY and turn into visible pixels on screen Also, add textures and various other per-pixel operations And visibility is resolved here: sorts the primitives in the z-direction Tomas Akenine-Mőller © 2002

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Application Geometry Rasterizer In Summery The programmer sends down primtives to be rendered through the pipeline (using API calls) The geometry stage does per-vertex operations The rasterizer stage does per-pixel operations Next, scrutinize geometry and rasterizer Tomas Akenine-Mőller © 2002

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**The GEOMETRY stages Application Geometry Rasterizer**

Tomas Akenine-Mőller © 2002

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**GEOMETRY Model Transformation**

Application Geometry Rasterizer GEOMETRY Model Transformation Originally, an object is in model space Move, orient, and transform geometrical objects into world space Example, a sphere is defined with origin at (0,0,0) with radius 1 Translate, rotate, scale to make it appear elsewhere Done per vertex with a 4x4 matrix multiplication! The user can apply different matrices over time to animate objects Tomas Akenine-Mőller © 2002

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**GEOMETRY View Transform**

Application Geometry Rasterizer GEOMETRY View Transform You can move the camera in the same manner But apply inverse transform to objects, so that camera looks down negative z-axis Tomas Akenine-Mőller © 2002

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**GEOMETRY Vertex Shading**

Application Geometry Rasterizer GEOMETRY Vertex Shading shading equation Materials + Lighting Shading Compute color at vertices Try to mimic how light in nature behaves Hard to uses empirical models, hacks, and some real theory light Geometry blue red green Rasterizer

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**GEOMETRY Projection Two major ways to do it Orthogonal Perspective**

Application Geometry Rasterizer GEOMETRY Projection Two major ways to do it Orthogonal Perspective Mimics how humans perceive the world, i.e., objects’ apparent size decreases with distance Tomas Akenine-Mőller © 2002

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**GEOMETRY Projection Also done with a matrix multiplication!**

Application Geometry Rasterizer GEOMETRY Projection Also done with a matrix multiplication! Pinhole camera (left), analog used in CG (right) Tomas Akenine-Mőller © 2002

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**GEOMETRY Clipping Square (cube) after projection**

Application Geometry Rasterizer GEOMETRY Clipping Square (cube) after projection Clip primitives to square

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**GEOMETRY Screen Mapping**

Application Geometry Rasterizer GEOMETRY Screen Mapping Screen mapping, scales and translates square so that it ends up in a rendering window These screen space coordinates together with Z (depth) are sent to the rasterizer stage

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**GEOMETRY Summary camera space world space model space world space**

Application Geometry Rasterizer GEOMETRY Summary camera space world space model space world space compute lighting projection image space clip map to screen Tomas Akenine-Mőller © 2002

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**The RASTERIZER stages Application Geometry Rasterizer**

Tomas Akenine-Mőller © 2002

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**The RASTERIZER Triangle Setup**

Application Geometry Rasterizer The RASTERIZER Triangle Setup In this stage the differentials and other data for the triangle's surface are computed. This data is used for scan conversion, as well as for interpolation of the various shading data produced by the geometry stage. This process is performed by fixed-operation hardware dedicated to this task. Tomas Akenine-Mőller © 2002

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**The RASTERIZER Triangle Traversal (Scan Conversion)**

Application Geometry Rasterizer The RASTERIZER Triangle Traversal (Scan Conversion) Find pixels inside the triangle Or on a line, or on a point Do per-pixel operations on these pixels: Interpolation Texturing Z-buffering And more… Tomas Akenine-Mőller © 2002

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**The RASTERIZER Interpolation**

Application Geometry Rasterizer The RASTERIZER Interpolation Interpolate colors over the triangle Called Gouraud interpolation blue red green Tomas Akenine-Mőller © 2002

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**The RASTERIZER Pixel Shading**

Application Geometry Rasterizer The RASTERIZER Pixel Shading Using the interpolated shading data as input The end result is one or more colors to be passed on to the next stage is executed by programmable GPU cores Texturing is one of the most important techniquies

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**The RASTERIZER Texturing**

Application Geometry Rasterizer The RASTERIZER Texturing One application of texturing is to glue images onto geometrical object Uses and other applications More realism Bump mapping Pseudo reflections Store lighting Almost infinitely many uses + =

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Example of texturing

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**The RASTERIZER Merging**

Application Geometry Rasterizer The RASTERIZER Merging Pixels are stored in color buffer Combine the fragment color with the pixel color Highly configurable Resolving visibility (done with Z-buffer) alpha channel for transparency stencil buffer for masking accumulation buffer for motion blur, depth of field, soft shadows, or antialiasing

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**The RASTERIZER Z-buffering**

Application Geometry Rasterizer The RASTERIZER Z-buffering The graphics hardware is pretty stupid It just draws triangles However, a triangle that is covered by a more closely located triangle should not be visible Assume two equally large tris at different depths Triangle 1 Triangle 2 Draw 1 then 2 incorrect Draw 2 then 1 correct

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**The RASTERIZER Z-buffering**

Application Geometry Rasterizer The RASTERIZER Z-buffering Would be nice to avoid sorting… The Z-buffer (aka depth buffer) solves this Idea: Store z (depth) at each pixel When scan-converting a triangle, compute z at each pixel on triangle Compare triangle’s z to Z-buffer z-value If triangle’s z is smaller, then replace Z-buffer and color buffer Else do nothing Can render in any order Have difficulty to handle transparancy Tomas Akenine-Mőller © 2002

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**The RASTERIZER Double Buffering**

Application Geometry Rasterizer The RASTERIZER Double Buffering The monitor displays one image at a time So if we render the next image to screen, then rendered primitives pop up And even worse, we often clear the screen before generating a new image A better solution is double buffering Tomas Akenine-Mőller © 2002

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**The RASTERIZER Double Buffering**

Application Geometry Rasterizer The RASTERIZER Double Buffering Use two buffers: one front and one back The front buffer is displayed The back buffer is rendered to When new image has been created in back buffer, swap front and back Tomas Akenine-Mőller © 2002

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Double Buffering Front Buffer 儲存螢幕目前顯示的內容，程式計算出的下一個畫面則先寫入 Back Buffer 中。 Copy 更新畫面時，在垂直回歸時間內，把 Back Buffer 拷貝至 Front Buffer： back front 程式寫入 顯示在螢幕上 back front 拷貝 (vertical retrace) back front 顯示在螢幕上

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**register 的值是在垂直回歸時才更新。**

Flipping 目前的顯示卡通常都具有緩衝區交換的硬體功能，可以避免拷貝而很有效率地把 Front Buffer 和 Back Buffer 互換： back front 程式寫入 顯示在螢幕上 register A B register 的值是在垂直回歸時才更新。 front back 程式寫入 顯示在螢幕上 register A B

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**And, lately… Programmable shading has become a hot topic**

Vertex shaders Pixel shaders Adds more control and much more possibilities for the programmer Application Geometry Rasterizer HARDWARE Vertex shader program Pixel shader

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