Chapter 8

1. Chapter 8 Discrete Techniques Buffers, images, textures, etc. University of North Carolina – Chapel Hill COMP 136

2. Announcements • Code snippet changes • Assignment 2 due today! University of North Carolina – Chapel Hill COMP 136

3. Buffers • Per-pixel data is stored in buffers on graphics card • Color Buffer (front and back) • Depth Buffer • Accumulation Buffer (more bpp than Color Buffer) • Stencil Buffer • Others (overlay planes, auxiliary buffers, color indices) University of North Carolina – Chapel Hill COMP 136

4. Image Formats v. Buffers • JPEG, GIF, TIFF, etc. all have a file format • Can include compression • Auxiliary data (ppi for instance) • Buffers (as we see them) are just a long series of floats, ints or bytes • unsigned char img[256][256][3]; University of North Carolina – Chapel Hill COMP 136

5. Writing to Buffers I • Send a primitive (tri, line) to graphics card • The OpenGL pipeline interfaces with the buffers, finalizing values for buffers • What if we want direct write to buffer? • glRasterPos3f(x,y,z) • glRasterPos2i(x,y) • glBitmap(width,height,x0,y0,xi,yi,bitmap) • glDrawPixels(width,height,format,type,image) • glReadPixels(x,y,width,height,format,type,image) • glCopyPixels(x,y,width,height,buffer) University of North Carolina – Chapel Hill COMP 136

6. Writing to Buffers II • The Writing Mode alters how such operations affect the buffer • Most common: overwrite and XOR • XOR used for swapping University of North Carolina – Chapel Hill COMP 136

7. Complexity Problems Switching gears now • Real world exhibits high amounts of complexity • How could we model each of these? University of North Carolina – Chapel Hill COMP 136

8. Buffer Mapping Techniques • Once we have buffers, we can use them to affect the standard pipeline • Texture maps • Paint the image onto triangles/quads • Bump maps • Alter the normal on triangle/quad • Reflection/Environment maps • Quick and dirty (i.e. inaccurate) reflections University of North Carolina – Chapel Hill COMP 136

9. Texture Mapping I • Easiest case: Repeating University of North Carolina – Chapel Hill COMP 136

10. Texture Mapping II • Specialized textures University of North Carolina – Chapel Hill COMP 136

11. Texture Mapping III • Problem I • We have an image (stored as a buffer in our code) • How do we map that image onto a triangle? • HINT: Done after rasterization • From image to world space? • From world space to image? • This one done generally University of North Carolina – Chapel Hill COMP 136

12. Texture Mapping IV • Sampling issues: • Mipmapping creates multiple resolutions of an image • Hand it 256x256, it creates 128x128, 64x64, 32x32, 16x16, 8x8, 4x4, 2x2, 1x1 • Samples these depending on polygon’s on screen size University of North Carolina – Chapel Hill COMP 136

13. Texture Mapping V • Whole field of research in sampling and reconstruction • Beyond the scope of this class, but very instructive University of North Carolina – Chapel Hill COMP 136

14. Assigning Texture Coordinates I • We have a square texture • How do we map it to triangles? • We want a relatively even sharing of texels on each triangle University of North Carolina – Chapel Hill COMP 136

15. Assigning Texture Coordinates II • What effect do we want? University of North Carolina – Chapel Hill COMP 136

16. Assigning Texture Coordinates III • The book proposes the following systems: University of North Carolina – Chapel Hill COMP 136

17. Assigning Texture Coordinates IV University of North Carolina – Chapel Hill COMP 136

18. Creating Textures I • Sometimes they are created by hand • Most repeating textures are done this way • The robot above was too (as well as most video games)! • Automatic generation • active research • Data gathering from real world University of North Carolina – Chapel Hill COMP 136

19. Creating Textures II • Identical copy-paste textures are too regular • Take an example image and extend it with plausible results Courtesy of Vivek Kwatra University of North Carolina – Chapel Hill COMP 136

20. Creating Textures III University of North Carolina – Chapel Hill COMP 136

21. Creating Textures IV • 3D scanners can now obtain position information from real scenes • How do we color them? University of North Carolina – Chapel Hill COMP 136

22. OpenGL Functions • glTexImage2D • glEnable(GL_TEXTURE_2D) • glTexParameter • glBindTexture • gluBuild2DMipmaps • glTexEnv • glHint(GL_PERSPECTIVE_CORRECTION, GL_NICEST) • glTexCoord2df(s,t) University of North Carolina – Chapel Hill COMP 136

23. OpenGL Demo? • Will I do a demo? University of North Carolina – Chapel Hill COMP 136

24. Environment Maps I • Reflection/refraction of light can be “approximated” • Draw 6 sides of box from center of box • When rendering clear/shiny surface, texture with texture coordinates on box created via reflection approximation University of North Carolina – Chapel Hill COMP 136

25. Environment Maps II See Color Plates 29 and 30 in 4th edition Textbook for a sample environment map. University of North Carolina – Chapel Hill COMP 136

26. Bump Maps I • If we use Phong Shading (calculating lighting at each fragment based on interpolated normal) we can bump map • Bump map perturbs the standard normal by some amount at each fragment University of North Carolina – Chapel Hill COMP 136

27. Bump Maps II These are basic teapots and planes University of North Carolina – Chapel Hill COMP 136

28. Compositing/Multiple Passes • Some advanced OpenGL techniques are “multi-pass” • This means you render the whole scene two (or more) times before showing the result to the viewer • OpenGL also has the ability to “Render to Texture” so that the viewer never sees the results of a pass, but can be read by later passes University of North Carolina – Chapel Hill COMP 136

29. Alpha Blending I • glColor4f(R,G,B,A) • We’ve always set A to 1 • glEnable(GL_BLEND); • glBlendFunc(…) • You can draw to the frame buffer without overwriting! University of North Carolina – Chapel Hill COMP 136

30. Alpha Blending II • Problems: • Describe z-buffer • What happens to the z-buffer when you draw a clear object? • Color(x,y) = (1-A)C0 + AC1 • Where C0 is the original color, C1 is the new color • What happens when multiple alpha values hit the same pixel? • Is it commutative? University of North Carolina – Chapel Hill COMP 136

31. Alpha Blending III • So, order matters • Which is better: front to back or back to front? University of North Carolina – Chapel Hill COMP 136

32. Depth Cueing and Fog • OpenGL “fog” or “haze” gives the appearance of objects disappearing in the distance • See p. 431 in the text for how University of North Carolina – Chapel Hill COMP 136

33. *Not in the text Shadow Volumes I Light Source Viewer Object1 Object3 Shadow Volume Object2 Ground Plane University of North Carolina – Chapel Hill COMP 136

34. Shadow Volumes II • Track the silhouette edges of the objects (when viewed from the light) • Render all objects to the depth buffer • Ambient light only for color • Render all shadow volume faces to the Stencil Buffer • Increment at each pixel for front face shadow vol • Decrement for back face shadow vol • Don’t write to the depth buffer…but do check with it • Render all objects again • Diffuse/Specular color but only if pixel is ZERO in Stencil Buffer • Check against the depth buffer, no need to write! University of North Carolina – Chapel Hill COMP 136

35. Accumulation Buffer I • The color buffers (front and back) only have 8bpp for each of R, G, and B • When doing multiple passes, you may want better precision than that • Accumulation Buffer has more • How big depends on your machine University of North Carolina – Chapel Hill COMP 136

36. Accumulation Buffer II • Motion blur • See code on page 432 of text (4th ed) • Essentially, you draw the scene multiple times each at 1/n color brightness • The standard buffer would loose a lot of the color quality University of North Carolina – Chapel Hill COMP 136

37. Accumulation Buffer III • Image Processing • Convolution kernels - kxk array that multiplied and summed across the image • Blur kernel [1 2 1] 1/20 * [2 4 2] [1 2 1] • Sharpen kernel [0 -1 0] [-1 5 -1] [0 -1 0] University of North Carolina – Chapel Hill COMP 136

38. Accumulation Buffer IV • Depth of Field • The blue plane is the focal plane • The things on the blue plane will always render to the same pixels • Those off the plane will blur by varying amounts (depending on distance from the plane) University of North Carolina – Chapel Hill COMP 136

39. Antialiasing • Aliasing AKA “jaggies” • Antialiasing - smooth the jaggies • Multiple renders from slightly different gluLookAt vals into Accumulation buffer • BUT, OpenGL does it via alpha • See p 430 in the text for sample code University of North Carolina – Chapel Hill COMP 136

40. Conclusions I • We’ve just completed what we will cover of OpenGL • There are many other techniques we haven’t covered • OpenGL is always growing with “extensions” • Full OpenGL description (-exten.) • OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 1.4, Fourth Edition University of North Carolina – Chapel Hill COMP 136

41. Conclusions II • For real-time graphics, DirectX is another option • With what we’ve learned in this class, you could learn DX on your own • DirectX9 SDK free download: • http://msdn.microsoft.com/directx/sdk/ • Sample code and documentation comes with it University of North Carolina – Chapel Hill COMP 136