CSC 461: Lecture 2 1 CSC461 Lecture 2: Image Formation Objectives Fundamental imaging notions Fundamental imaging notions Physical basis for image formation Physical basis for image formation Light Light Color Color Perception Perception Synthetic camera model Synthetic camera model Other models Other models

CSC 461: Lecture 2 2 Image Formation In computer graphics, we form images which are generally two dimensional using a process analogous to how images are formed by physical imaging systems In computer graphics, we form images which are generally two dimensional using a process analogous to how images are formed by physical imaging systems Cameras Cameras Microscopes Microscopes Telescopes Telescopes Human visual system Human visual system

CSC 461: Lecture 2 3 Elements of Image Formation Objects Objects Viewer Viewer Light source(s) Light source(s) Attributes that govern how light interacts with the materials in the scene Attributes that govern how light interacts with the materials in the scene Note the independence of the objects, viewer, and light source(s) Note the independence of the objects, viewer, and light source(s)

CSC 461: Lecture 2 4 Light Light is the part of the electromagnetic spectrum that causes a reaction in our visual systems 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 nm (nanometers) Generally these are wavelengths in the range of about nm (nanometers) Long wavelengths appear as reds and short wavelengths as blues Long wavelengths appear as reds and short wavelengths as blues

CSC 461: Lecture 2 5 Ray Tracing and Geometric Optics One way to form an image is to follow rays of light from a point source determine which rays enter the lens of the camera. However, each ray of light may have multiple interactions with objects before being absorbed or going to infinity. One way to form an image is to follow rays of light from a point source determine which rays enter the lens of the camera. However, each ray of light may have multiple interactions with objects before being absorbed or going to infinity.

CSC 461: Lecture 2 6 Luminance and Color Images Luminance Luminance Monochromatic Monochromatic Values are gray levels Values are gray levels Analogous to working with black and white film or television Analogous to working with black and white film or television Color Color Has perceptional attributes of hue, saturation, and lightness Has perceptional attributes of hue, saturation, and lightness Do we have to match every frequency in visible spectrum? No! Do we have to match every frequency in visible spectrum? No!

CSC 461: Lecture 2 7 Three-Color Theory Human visual system has two types of sensors Human visual system has two types of sensors Rods: monochromatic, night vision Rods: monochromatic, night vision Cones Cones Color sensitive Color sensitive Three types of cone Three types of cone Only three values Only three values (the tristimulus values) are sent to the brain Need only match these three values Need only match these three values Need only three primary colors Need only three primary colors

CSC 461: Lecture 2 8 Additive and Subtractive Color Additive color Additive color Form a color by adding amounts of three primaries Form a color by adding amounts of three primaries CRTs, projection systems, positive film CRTs, projection systems, positive film Primaries are Red (R), Green (G), Blue (B) Primaries are Red (R), Green (G), Blue (B) Subtractive color Subtractive color Form a color by filtering white light with cyan (C), Magenta (M), and Yellow (Y) filters Form a color by filtering white light with cyan (C), Magenta (M), and Yellow (Y) filters Light-material interactions Light-material interactions Printing Printing Negative film Negative film

CSC 461: Lecture 2 9 Shadow Mask CRT

CSC 461: Lecture 2 10 Pinhole Camera A pinhole camera is a box with a small hole in the center of one side of the box A pinhole camera is a box with a small hole in the center of one side of the box The film is placed inside the box on one side opposite the pinhole The film is placed inside the box on one side opposite the pinhole The pinhole permits only a single ray of light The pinhole permits only a single ray of light Assume camera orients along z-axis and the pinhole is at the origin Assume camera orients along z-axis and the pinhole is at the origin Projection – (xp, yp, -d) is the projection of (x, y, z) Projection – (xp, yp, -d) is the projection of (x, y, z) Use trigonometry to find projection of a point These are equations of simple perspective x p = -x/z/dy p = -y/z/dz p = -d

CSC 461: Lecture 2 11 Pinhole Camera Field or angle of view – the angle made by the largest object that the camera can image on its film plane Field or angle of view – the angle made by the largest object that the camera can image on its film plane Depth of field – the distance that can be seen: from the object to the pinhole Depth of field – the distance that can be seen: from the object to the pinhole Ideal pinhole camera – infinite depth of field Ideal pinhole camera – infinite depth of field Two disadvantages Two disadvantages Pinhole is too small Pinhole is too small Camera can not be adjusted to have a different angle of view Camera can not be adjusted to have a different angle of view

CSC 461: Lecture 2 12 Synthetic Camera Model center of projection image plane projector p projection of p Constructs: Object Viewer Light Film plane Principles: Object specification independent of viewer specification Image can be computed using simple trigonometric calculation The angle of view can be changed by moving the film plane – clipping window

CSC 461: Lecture 2 13 Advantages Separation of objects, viewer, light sources Separation of objects, viewer, light sources Two-dimensional graphics is a special case of three-dimensional graphics Two-dimensional graphics is a special case of three-dimensional graphics Leads to simple software API Leads to simple software API Specify objects, lights, camera, attributes Specify objects, lights, camera, attributes Let implementation determine image Let implementation determine image Leads to fast hardware implementation Leads to fast hardware implementation

CSC 461: Lecture 2 14 Global vs Local Lighting Cannot compute color or shade of each object independently Cannot compute color or shade of each object independently Some objects are blocked from light Some objects are blocked from light Light can reflect from object to object Light can reflect from object to object Some objects might be translucent Some objects might be translucent

CSC 461: Lecture 2 15 Why not ray tracing? Ray tracing seems more physically based so why don’t we use it to design a graphics system? Ray tracing seems more physically based so why don’t we use it to design a graphics system? Possible and is actually simple for simple objects such as polygons and quadrics with simple point sources Possible and is actually simple for simple objects such as polygons and quadrics with simple point sources In principle, can produce global lighting effects such as shadows and multiple reflections but is slow and not well-suited for interactive applications In principle, can produce global lighting effects such as shadows and multiple reflections but is slow and not well-suited for interactive applications