Computer Graphics Rendering Pipeline

Transcription

1 Computer Graphics Rendering Pipeline Univ.-Prof. Dr. Matthias Harders Summer semester 2015 Content Rendering process Graphics pipeline Application programming interfaces Shader programming 1 1

2 Computer-Generated Images From 3D scenes to 2D images Input Rendering process Output 2 Examples of Rendering Paradigms Rasterization Ray tracing Volume rendering Radiosity 3 2

3 Rasterization-Based Rendering Projection of 3D geometry to 2D image plane Conversion of continuous geometry to discrete pixels 4 Several Definitions Rendering Process of synthesizing image from model Primitive Basic drawing elements (e.g. triangles) Rasterization Step of converting primitives into fragments associated with screen locations Pre-pixel data at pixel location, that can potentially change pixel color Pixel Single picture element of raster (grid) image (i.e. point sample) Framebuffer Memory holding pixel image for display, destination of rendering 5 3

4 Rasterization-Based Rendering From 3D scenes to 2D images Model Rendering pipeline Image 6 Pipeline Input Geometry representation Triangle meshes NURBS surfaces 7 4

5 Pipeline Input Light sources 8 Pipeline Input Surface appearance Reflectance functions 9 5

6 Pipeline Input Camera Near plane Far plane y z x Camera position Viewing frustum 10 Pipeline Output From video RAM to final display on device Signal conversion (e.g. DAC) Framebuffer 11 6

7 Scanline Screen Refresh Progressive scanning Start Scanline (horizontal retrace) (vertical retrace) Display End 12 Buffering Strategies Single buffer Frame Frame Frame Frame Video RAM Display A Display B Display C Display D Problem: Images displayed while being redrawn 13 7

8 Buffering Strategies Double buffer (front & back) Frame Frame Frame Frame Buffer 1 Display A Render C Display C Render E Buffer 2 Render B Display B Render D Display D 14 Buffering Strategies Double buffer (front & back) Frame Frame Frame Frame Buffer 1 Display A Render C Display C Rende Buffer 2 Render B Display B Render D Display 15 8

9 Buffering Strategies Double buffer (front & back) with v-sync Frame Frame Frame Frame Buffer 1 Display A Render C Disp Buffer 2 Render B Display B Re Problem: Vertical sync to avoid screen tearing limits visual update to screen refresh rate 16 Buffering Strategies Triple buffer Frame Frame Frame Frame Buffer 1 Display A Render D Disp Buffer 2 Render B Display B Rende Buffer 3 Render C Display C No more wait for vertical sync required 17 9

10 Rasterization-Based Rendering From 3D scenes to 2D images Model Rendering pipeline Image 18 Rendering Pipeline Main processing s Input triangles Vertices in screen space Interpolated fragments Rasterization Model Image 19 10

11 Steps of Processing Stages Model View Lighting Projection Primitive assembly Clipping Viewport mapping generation Attribute interpolation Texturing tests blending 20 Stage Input coordinates (x, y, z) Topology (v 1, v 2, v 3 ) Normals (n x, n y, n z ) Material properties (RGB ) Texture coordinates (u, v) Rasterization 21 11

12 Model Transform M World space z y Model space x M View Transform y = y World space V 1 V z x z x Camera space 23 12

13 Model & View Transform Inverse, objects to camera space Often Model and View composed into combined Modelview matrix Apply ation individually to all vertices Note: camera facing down z direction V 1 M 24 Lighting Computation of color at individual vertices Following simplified illumination model Based on light sources, material, normals 25 13

14 Projection Transform Projection to normalized device coordinates (NDC) y View volume y z x y z x Ppersp ( 1,1,1) z (1, 1,1) (1, 1,1) x (1,1, 1) (1, 1, 1) Portho Canonical view volume (NDC cube ( 1, 1, 1) to (1, 1, 1)) 26 Primitive Assembly Operations so far on vertices only Aggregate vertices into primitives (e.g. triangles), based on connectivity information Further geometry processing possible 27 14

15 Clipping Drop triangles/parts outside canonical view volume Also: cull/clip primitives based on borders y y New vertices x x Unit cube 28 Viewport Mapping Transformation to screen coordinates y Width x y x Height Unit cube 29 15

16 Viewport Mapping Primitive x/y-coordinates translated and scaled to pixel positions on screen z-coordinate mapped from ( 1,1) to (0,1) z values to be used for depth testing Clipping and viewport mapping usually automatic 30 Stage Output Vertices in screen space (x, y) Normalized z-depth colors from lighting Normals (n x, n y, n z ) Texture coordinates (u, v) Rasterization 31 16

17 Steps of Processing Stages Model View Lighting Projection Primitive assembly Clipping Viewport mapping generation Attribute interpolation Texturing tests blending 32 Generation Rasterization Discretize continuous primitives Generate fragments (i.e. pixel candidates ) 33 17

18 Attribute Interpolation Rasterization Linear interpolation of per-vertex values across fragment Note: not limited to color only! 34 Rasterization Stage Output s in screen coordinates (x, y) Normalized z-depth Color Texture coordinates (u, v) Normals (n x, n y, n z ) Rasterization 35 18

19 Steps of Processing Stages Model View Lighting Projection Primitive assembly Clipping Viewport mapping generation Attribute interpolation Texturing tests blending 36 Texturing Enhance visual appearance through textures & Many other applications: bump/cube/environment/ displacement mapping, etc

20 Tests Determine visibility with z-depth values? Further tests, e.g. scissor, stencil tests, alpha 38 Blending Alpha value in RGB denotes transparency Mix fragment color with existing color value 39 20

21 Rendering Pipeline Main processing s Input triangles Vertices in screen space Interpolated fragments Rasterization Model Image 40 Parallelism in Rendering Process Model Model Model Model View Model Projection View Projection Horizontal (data) parallelism Vertical (pipeline) parallelism 41 21

22 Parallelism in Rendering Process Data parallelism Same operation applied to millions of elements (Single Instruction Multiple Data SIMD) /fragment computations independent Pipeline parallelism Increase throughput by running steps in parallel Re-balance load between pipeline steps 42 GPU Architecture (AMD RADEON R9 290) 43 22

23 Graphics Application Programming Interfaces Software interface to graphics hardware Commonly hardware-accelerated rendering Abstraction layer for hardware-specific details 44 API Abstraction Layer Paradigms Modeling and rendering via scene graphs Modeling and rendering via low-level commands glbegin(gl_triangles); glcolor3f(0.97, 0.58, 0.0); gl3f(-0.95, -0.90, 0.0); gl3f( 0.90, 0.95, 0.0); gl3f(-0.95, 0.95, 0.0); glend(); 45 23

24 Shader Programming Rendering pipeline historically hard-coded Too rigid for modern graphics hardware Solution: make parts of pipeline programmable Customization with so-called shaders Rasterization 46 Programmable Pipeline Stages Model View Lighting Projection Primitive assembly Clipping Viewport mapping generation Attribute interpolation Texturing tests blending 47 24

25 Shader Programs Small programs replacing part of fixed pipeline Partly removes abstraction Requires low-level programming More efficient use of GPU hardware Typically at least vertex and fragment shader 48 OpenGL 4.3 Pipeline with Compute Shaders 49 25

26 General Shader Usage Application sets parameters, loads and compiles shaders, sends data to GPU Shaders process data on GPU Processing one vertex or fragment at a time Shader programming in specialized language 50 Shader Languages for Graphics Programming GLSL (OpenGL Shading Language) HLSL (High Level Shading Language) Cg (C for Graphics) deprecated 51 26

27 Reading Suggestion Chapter 2 The Graphics Rendering Pipeline in T. Akenine-Möller, E. Haines, N. Hoffman, Real-time Rendering, 3 rd edition, For next week: Recap vector and matrix calculus from lecture Lineare Algebra 52 Proseminar Hints Friday, March 13 th Joint proseminar (all groups), lecture hall F, 11:15 Download, compile, and run basic OpenGL examples ( Teaching) Requires OpenGL 3.3, GLSL 3.3; C code example Compile on ZID student PCs (Makefile provided) Read through

28 Lecture Schedule Date Topic Remark (Proseminar) 4.3. Introduction Rendering pipeline Introduction to OpenGL Transformations & projections Introduction to OpenGL (no lecture) (no proseminar) Easter break Light & color Theory (Transformation & Projection) Lighting & shading Practical (Camera control) Geometry representation (National celebration day) 6.5. (no lecture) (no proseminar) Texturing Theory (Color, Lighting & Shading) Visibility Programming (Using GLM) Scan conversion & anti-aliasing Theory (Geometry & Texturing) 3.6. Advanced effects Programming (Shaders) GPUs & displays Theory (Visibility & Scan Conversion) Wrap-up Programming (Textures) 54 28