CS148: Introduction to Computer Graphics and Imaging Scanline Rendering

1. CS148: Introduction to Computer Graphics and Imaging Scanline Rendering Electric Sheep screensaver by Scott Draves Image created collectively by users on the internet

2. Human Eye Light comes from light sources, is absorbed and reflected off of objects, and goes through the pupil of the eye.

3. Without the pupil light scatters everywhere and does not make a coherent image

4. With a pinhole model of the pupil light makes a coherent image inside the eye

5. Human Retina: Rods and cones • Retina contains rods and cones • Rods are responsible for light intensity • Cones are responsible for color vision

6. Three Types of Cones From http://webvision.med.utah.edu/imageswv/fovmoswv.jpeg

7. Rods Capture Intensities

8. Color Matching Experiment Adjust brightness of three primaries Lasers: R = 700 nm, G = 546 nm, B = 435 nm until a human mistakenly thinks it matches another color C = x nm Result: all colors can be matched with three colors Therefore: humans have trichromatic color vision C = R “+” G “+” B

9. Trichromatic Theory • Since the human eye works in this fashion, we work this way with images, printers, and displays. • Image formats store values in the R, G, and B channels. • The values are between 0 and 255. • The relative values give the color, and the overall values give the intensity. • This is similar to the cones of the human eye. • The monitor can be used to increase or decrease the image intensities (brightness/darkness). • This is similar to the rods of the human eye.

10. Cameras A camera works similarly to the eye. Light passes through the lens, bends to come to a point of focus, and makes an upside-down image on the film.

11. Pinhole Camera • Linear perspective with viewpoint at pinhole

12. Laws of Pinhole Cameras • Common assumptions • Light leaving an object travels in straight lines • These lines converge to a point (e.g. pinhole) • Infinite depth of field which means everything is in focus • Perspective image is formed by the intersection of these lines with an image plane • More distant objects subtend smaller visual angles and appear smaller • Objects in front occlude the objects behind them

13. OpenGL Camera OpenGL uses a pinhole camera. In OpenGL the image plane is in front of the focal point which means the image is right side up. The frustum is the volume of our view (shown in blue below). The image plane is the plane of the frustum nearest to the camera.

14. Specifying the Camera in OpenGL void gluLookAt(eye.x, eye.y, eye.z, at.x, at.y, at.z, up.x, up.y, up.z); - gluLookAt creates a camera from an eye point, a reference point indicating the center of the scene, and an up vector.

15. Specifying the Frustum in OpenGL void gluPerspective(fovy, aspect, near, far); • gluPerspective specifies a viewing frustum for the camera into the world coordinate system. • fovy specifies the field of view angle. • aspect specifies the aspect ratio of the associated image plane. • near/far specify the distance from the camera to the new/far clipping plane.

16. Drawing a Triangle in front of your Camera glColor3f(1.f,0.f,0.f); glBegin(GL_TRIANGLES); glVertex3f(0.f,0.f,0.f); glVertex3f(1.f,0.f,0.f); glVertex3f(.5f,.86f,0.f); glEnd(); • void glBegin(Glenum mode); • glBegin delimits the vertices of a primitive. • mode specifies the primitive that will be created from vertices presented between glBegin and glEnd. • void glVertex(x,y,z); • glVertex specifies a vertex to form the primitive.

17. Fundamental Primitive: Triangles Why triangles? Lowest common denominator Easily break convex polygons into triangles Optimize the implementation of one primitive Triangles have unique properties Guaranteed to be planar Guaranteed to have a well-defined interior Well-defined method (barycentric interpolation) for interpolating values in the interior

18. Triangle Meshes Stanford Bunny 69,451 triangles David, Digital Michelangelo Project 28,184,526 vertices, 56,230,343 triangles

19. OpenGL Drawing Primitives Geometric Shapes Bitmaps Images

20. Rasterization Graphics Rendering Pipeline Texturing Per-fragment ops Frame buffer ops Per-vertex ops Primitive assembly Commands Processor triangles, lines, points images Vertices Transformed vertices Primitives Fragments Shaded fragments pixels in the framebuffer Display

21. Command queue Command interpretation Unpack and perform format conversion Maintain graphics state Command Processor glBegin(GL_TRIANGLE_STRIP); glColor3f(.0f, 1.f, .0f); glVertex3f( .0f, .0f, .0f ); glVertex3f( 0.f, 1.f, .0f ); glColor3f(1.f, .0f, .0f); glVertex3f( 1.f, .0f, .0f ); glVertex3f( 1.f, 1.f, .0f ); glEnd();

22. Vertex transformation Normal transformation Texture coordinate generation Texture coordinate transformation Lighting (light sources and surface reflection) Projection Per-vertex Operations World-space triangles Screen-space shaded triangles

23. Primitive Assembly Combine transformed/shaded vertices into primitives 1 vert -> point 2 verts -> line 3 verts -> triangle Clipping Transform to window coordinates (viewport) Determine orientation (CW/CCW) Back-face cull

24. Setup (per-triangle) Sampling (triangle = {fragments}) Interpolation (interpolate colors and coordinates) Rasterization Triangles Fragments

25. Textures are arrays indexed by floats (Sampler) Texture address calculation Texture bilinear interpolation and filtering Texturing Texture Fragments Fragments

26. Per-fragment Operations Combine texture sampler outputs Per-fragment shading Fragments Textured Fragments

27. Operation Test window ownership Test scissor and stencil mask Test alpha Test depth Blending or compositing Frame buffer Operations Textured Fragments Framebuffer Pixels

28. Testing Depth using Z-Buffer • Initialize z-buffer to zmax • Interpolate z across the triangle • Draw fragment if closer if(frag.Z<Z[frag.X][frag.Y]){ Z[frag.X][frag.Y]=frag.Z; C[frag.X][frag.Y]=frag.C; } http://en.wikipedia.org/wiki/File:Z_buffer.svg

29. Frame Buffering Frame Buffer • Store image in a buffer to separate refresh rate from drawing rate Single-buffer • Draw into display buffer directly • May see picture being drawn Double-buffer • Display “front” buffer • Draw into “back” buffer (can’t see drawing) • Swap front and back (wait until vertical sync) Triple buffer • Avoid waiting for vertical sync

30. Framebuffer Example Framebuffer: 1440 x 900 All coordinates are integers; they refer to pixel locations in the framebuffer

31. Window Example Framebuffer: 1440 x 900 The window is the portion of the display usable by the application (under control of the “window system”) Window (512 x 512) All coordinates are integers; they refer to pixel locations in the framebuffer

32. Viewport Example Framebuffer: 1440 x 900 The viewport is the portion of the window that can be drawn in, no pixels will appear outside the viewport Viewport (256x256) Window (512 x 512) All coordinates are integers; they refer to pixel locations in the framebuffer

33. Specifying the Viewport in OpenGL void glViewport(x, y, w, h); • glViewport specifies the location and the size of the viewport in the screen coordinate system. Here x, y, w, and h are all integers. • x and y specify the lower left corner of the viewport rectangle. • w and h specify the width and height of the viewport. w/h should agree with the aspect ratio you set in gluPerspective.

34. Windows = Virtual Framebuffers Like virtual memory enables us to share the physical memory, virtual framebuffers allows us to share the physical framebuffer Abstract properties • Location and size on the screen • Stacking order

35. Window Coordinate Systems top = 1.0 Window (512 x 512) left = -1.0 right = 1.0 bottom = -1.0 Each window has a user coordinate system A 2D coordinate system is specified by assigning coordinates to the edges of the window left need not be less than right ...

36. Exposure Parts of the window that are not visible can become visible • When opening • When unhiding • When brought to front How do you update the visible parts of the window? • Redraw entire window (or a part of it) • Save the drawn window in a “backing store”, and copy onto screen when needed

37. Display Gamma correction Analog to digital conversion Display Framebuffer Pixels Light

38. CPU glBegin(GL_TRIANGLES) glVertex3fv(v1); glVertex3fv(v2); glVertex3fv(v3); glEnd() -90 GPU OpenGL -90

39. Graphics processing unit A graphics processing unit (GPU), also occasionally called visual processing unit (VPU), is a specialized electronic circuit designed to rapidly manipulate and alter memory to accelerate the building of images in a frame buffer intended for output to a display. NVIDIA’s GeForce GTX 690 VisionTek Radeon 7970

40. What’s in a GPU? Rasterizer Framebuffer Ops Shader Core Shader Core Shader Core Shader Core Shader Core Shader Core Shader Core Shader Core Primitive Assembly Tex Tex Tex Work Distributor Tex

41. Modern PC 4MB L2 Cache 512MB video Memory (GDDR3) 3.0 Ghz Intel Core2 Duo NVIDIA GeForce 8800 GTX (575 MHz) (16 cores) Core 1 Core 2 2GB main memory (DDR2) System board (Intel D975) 12.8 GB/sec PCIe Bus (v1 = 4 GB/sec) 84 GB/sec NVIDIA 8800GTX

42. Frame buffer Video out Game Machines – Xbox 360 L2 Cache 3.2 Ghz PowerPC CPU 500 Mhz ATI GPU IO Chip Core 1 Core 2 Core 3 48 3D Cores 512 MB memory controllers/ethernet/ audio/DVD/etc. Display (TV)

43. Video out Game Machines – PS3 3.2 Ghz Cell L2 Cache 256 MB video Memory (GDDR3) SPU4 SPU3 SPU2 SPU1 SPU7 SPU6 SPU0 SPU5 550 Mhz NVIDIA RSX GPU PPC Core IO Chip 256 MB Memory (XDR) Multiple 3D cores controllers/ethernet/ audio/DVD/etc. Display (TV)

44. Hybrid CPU-GPUs Intel Sandybridge Apple A5

45. NVIDIA GPU Performance