1 Angel: Interactive Computer Graphics5E © Addison- Wesley 2009 Image Formation 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

2 Angel: Interactive Computer Graphics5E © Addison- Wesley 2009 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, the viewer, and the light source(s) Note the independence of the objects, the viewer, and the light source(s)

3 Figures are extracted from Angel's book (ISBN x) The Human Visual System vs The Pinhole camera Human Visual System Visible Spectrum Pinhole camera

4 Figures are extracted from Angel's book (ISBN x) The Synthetic-camera Model

Synthetic Camera y (x,y,z) X Z d X p, Y p, -d

Synthetic Camera Projection Geometry Z X Y COP d (0,0,0) x,y,z x’ = x (d/z) y’ = y(d/z) z’ = d (x’,y’,z’) Projection Plane Projected Point

7 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Models and Architectures

8 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 Practical Approach Process objects one at a time in the order they are generated by the application Process objects one at a time in the order they are generated by the application Can consider only local lighting Can consider only local lighting Pipeline architecture Pipeline architecture All steps can be implemented in hardware on the graphics card All steps can be implemented in hardware on the graphics card application program display

Vertices Vertices in world coordinates Vertices in world coordinates void glVertex3f(GLfloat x, GLfloat y, GLfloat z) void glVertex3f(GLfloat x, GLfloat y, GLfloat z) Vertex (x, y, z) sent down the pipeline Vertex (x, y, z) sent down the pipeline

Transformer Transformer in world coordinates Transformer in world coordinates Must be set before object is drawn! Must be set before object is drawn! glRotatef(45.0, 0.0, 0.0, -1.0); glRotatef(45.0, 0.0, 0.0, -1.0); glVertex2f(1.0, 0.0); glVertex2f(1.0, 0.0); Complex [Angel Ch. 4] Complex [Angel Ch. 4]

Transformation Matrices in OpenGL Stack Current Stack CurrentLoad Matrix Vertices 3D Model Vertices 3D 2D ModelviewProjection Matrix Mode

12 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 Projection Projection is the process that combines the 3D viewer with the 3D objects to produce the 2D image Projection is the process that combines the 3D viewer with the 3D objects to produce the 2D image Perspective projections: all projectors meet at the center of projection Perspective projections: all projectors meet at the center of projection Parallel projection: projectors are parallel, center of projection is replaced by a direction of projection Parallel projection: projectors are parallel, center of projection is replaced by a direction of projection

Projector

Orthographic Projection If d = z -  and d  x’ = x y’ = y z = d x’ = x (d/z) y’ = y(d/z) z’ = d

15 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 Primitive Assembly Vertices must be collected into geometric objects before clipping and rasterization can take place Line segments Line segments Polygons Polygons Curves and surfaces Curves and surfaces

Clipper

17 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 Clipping Just as a real camera cannot “see” the whole world, the virtual camera can only see part of the world or object space Objects that are not within this volume are said to be clipped out of the scene Objects that are not within this volume are said to be clipped out of the scene

18 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 Rasterization If an object is not clipped out, the appropriate pixels in the frame buffer must be assigned colors If an object is not clipped out, the appropriate pixels in the frame buffer must be assigned colors Rasterizer produces a set of fragments for each object Rasterizer produces a set of fragments for each object Fragments are “potential pixels” Fragments are “potential pixels” Have a location in frame bufffer Have a location in frame bufffer Color and depth attributes Color and depth attributes Vertex attributes are interpolated over objects by the rasterizer Vertex attributes are interpolated over objects by the rasterizer

Rasterizer

20 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 Fragment Processing Fragments are processed to determine the color of the corresponding pixel in the frame buffer Fragments are processed to determine the color of the corresponding pixel in the frame buffer Colors can be determined by texture mapping or interpolation of vertex colors Colors can be determined by texture mapping or interpolation of vertex colors Fragments may be blocked by other fragments closer to the camera Fragments may be blocked by other fragments closer to the camera Hidden-surface removal Hidden-surface removal

21 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 API Contents Functions that specify what we need to form an image Functions that specify what we need to form an image Objects Objects Viewer Viewer Light Source(s) Light Source(s) Materials Materials Other information Other information Input from devices such as mouse and keyboard Input from devices such as mouse and keyboard Capabilities of system Capabilities of system

22 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 Object Specification Most APIs support a limited set of primitives including Most APIs support a limited set of primitives including Points (0D object) Points (0D object) Line segments (1D objects) Line segments (1D objects) Polygons (2D objects) Polygons (2D objects) Some curves and surfaces Some curves and surfaces Quadrics Quadrics Parametric polynomials Parametric polynomials All are defined through locations in space or vertices All are defined through locations in space or vertices

23 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 Example glBegin(GL_POLYGON) glVertex3f(0.0, 0.0, 0.0); glVertex3f(0.0, 1.0, 0.0); glVertex3f(0.0, 0.0, 1.0); glEnd( ); type of object location of vertex end of object definition

24 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 Camera Specification Six degrees of freedom Six degrees of freedom Position of center of lens Position of center of lens Orientation Orientation Lens Lens Film size Film size Orientation of film plane Orientation of film plane

25 Angel: Interactive Computer Graphics 5E © Addison- Wesley 2009 Lights and Materials Types of lights Types of lights Point sources vs distributed sources Point sources vs distributed sources Spot lights Spot lights Near and far sources Near and far sources Color properties Color properties Material properties Material properties Absorption: color properties Absorption: color properties Scattering Scattering Diffuse Diffuse Specular Specular