1. 1 91.427 Computer Graphics I, Fall 2008 Image Formation

2. 2 91.427 Computer Graphics I, Fall 2008 Objectives Fundamental imaging notions Physical basis for image formation ­Light ­Color ­Perception Synthetic camera model Other models

3. 3 91.427 Computer Graphics I, Fall 2008 Image Formation In computer graphics form images ­generally two dimensional ­process analogous to how images formed by physical imaging systems ­Cameras ­Microscopes ­Telescopes ­Human visual system

4. 4 91.427 Computer Graphics I, Fall 2008 Elements of Image Formation Objects Viewer Light source(s) Attributes govern how light interacts with materials in scene Note independence of objects, viewer, and light source(s)

5. 5 91.427 Computer Graphics I, Fall 2008 Light Light is the part of the electromagnetic spectrum that causes a reaction in our visual systems Generally these are wavelengths in the range of about 350-750 nm (nanometers) Long wavelengths appear as reds and short wavelengths as blues

6. 6 91.427 Computer Graphics I, Fall 2008 Ray Tracing and Geometric Optics One way to form image: follow rays of light from point source find which rays enter lens of camera But, each ray ==> multiple interactions with objects before being absorbed or going to infinity

7. 7 91.427 Computer Graphics I, Fall 2008 Luminance and Color Images Luminance Image ­Monochromatic ­Values are gray levels ­Analogous to working with black and white film or television Color Image ­Perceptional attributes: hue, saturation, and lightness ­Have to match every frequency in visible spectrum? No!

8. 8 91.427 Computer Graphics I, Fall 2008 Three-Color Theory Human visual system: two types of sensors ­Rods: monochromatic, night vision ­Cones Color sensitive Three types of cones Only three values (tristimulus values) sent to brain Need only match these three values ­==> Need only three primary colors

9. 9 91.427 Computer Graphics I, Fall 2008 Shadow Mask CRT

10. 10 91.427 Computer Graphics I, Fall 2008 Additive and Subtractive Color Additive color ­Form color by adding amounts of three primaries CRTs, projection systems, positive film ­Primaries = Red (R), Green (G), Blue (B) Subtractive color ­Form color by filtering white light with Cyan (C), Magenta (M), and Yellow (Y) filters Light-material interactions Printing Negative film

11. 11 91.427 Computer Graphics I, Fall 2008 Pinhole Camera x p = -x/z/dy p = -y/z/d Use trigonometry to find projection of point at (x,y,z) Equations of simple perspective z p = d

12. 12 91.427 Computer Graphics I, Fall 2008 Synthetic Camera Model center of projection image plane projector p projection of p

13. 13 91.427 Computer Graphics I, Fall 2008 Advantages Separation of objects, viewer, light sources 2-D graphics is special case of 3-D graphics Leads to simple software API ­Specify objects, lights, camera, attributes ­Let implementation determine image Leads to fast hardware implementation

14. 14 91.427 Computer Graphics I, Fall 2008 Global vs Local Lighting Cannot compute color or shade of each object independently ­Some objects blocked from light ­Light can reflect from object to object ­Some objects might be translucent

15. 15 91.427 Computer Graphics I, Fall 2008 Why not ray tracing? Ray tracing more physically based ==> Why not use to design graphics system? Possible & simple for simple objects E.g., polygons & quadrics with simple point sources In principle, can produce global lighting effects E.g., shadows & multiple reflections But ray tracing slow & not well-suited for interactive applications