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EngiNet™
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©2001 The Research Foundation of the State University of New York
CS 560
Computer Graphics
Professor Richard Eckert
Lecture # 6
February 7, 2001
1 Lecture 5 1. Lowest Level (earliest)-- Assembly/machine Wednesday, 2-7-01 language
l Programs drive hardware directly Computer Graphics Software –Fast, but non-portable –Difficult to program –Prone to errors
2. Medium Level (General Programming Packages) l B. Standard Graphics Packages –Sets of specifications l A. Extensions to high level languages-- – Supposedly language/platform graphics libraries independent – e.g., Borland's BGI, Windows' GDI, Silicon – Usually with bindings for many high level Graphics GL, Microsoft's DirectX languages – Still have platform dependencies – Syntax for accessing graphics functions – Easier to program – Examples: GKS, PHIGS, OpenGL – Usually slower, but with optimized compilers, not so bad
3. Special-purpose We'll be working at level 2 for Application packages most of this course l e.g., Corel Draw, 3D Studio, Harvard Graphics, Photoshop l Good for what they do, but specific uses
2 Describing positions of 2-D Cartesian System objects – Measure distance along two mutually perpendicular axes l Need coordinate systems – Position given by 2 numbers (x,y) l 2-D and 3-D Cartesian systems used universally
3-D Cartesian System Other commonly-used – Measure distance along three mutually systems perpendicular axes – Position given by three numbers (x,y,z) l Depend on symmetry – Spherical (rho, theta, phi) – Cylindrical (r, theta, z) l Conversion formulas used
Types of coordinate 1. Modeling coordinate systems system (MCS) l System used by programmer (modeler) to describe a single object l Can be 2-D or 3-D l Depends on object being modeled l Origin, scale picked according to object l Varies from object to object
3 2. World Coordinate System 3. Device Coordinate (WCS) System (DCS) l Reference system used to position l Coordinate system used by output device objects in a scene l Units usually pixels l Can be 2-D or 3-D l Always 2-D l Origin, scale, units specific to scene l Varies according to HW platform l Coordinate transformations map from l Graphics SW maps from WCS to DCS an MCS to the WCS – Called the “Viewing Transformation” – Effectively positions objects in scene l If WCS is 3-D, a projection transformation is – Called the “Modeling Transformation” also involved
4. Normalized Device Graphics Transformation Coordinates (NDCS) Pipeline – 2-D system l MCS ------> WCS ------> NDCS ------> DCS – 0 <= x <= 1; 0<= y <= 1 modeling xform viewing xform HW-depend. xform – Intermediate between WCS and DCS l The first two transformations are device independent – Hardware-independent
A Window A Viewport
l A rectangular area (2-D) l A rectangular portion of the screen or Rectangular parallelepiped (3-D) l Expressed in device coordinates l Expressed in WC – e.g., xvmin, yvmin, xvmax, yvmax – e.g., l A window is mapped to a viewport xwmin,xwmax,ywmin,ywmin,zwmin,zwmax l Specifies a part of the scene of interest
4 Window to Viewport Clipping Transformation
– mapping from a window to a viewport l Removal of parts of scene outside a window or viewport l Graphics system may perform clipping with respect to a window or a viewport
Animation Techniques using BASIC COMPONENTS OF A windows/viewports/clipping GRAPHICS SOFTWARE l Zooming SYSTEM – Change window size from frame to frame, clip, and transform l Examples from: Windows & OpenGL • scene appears to approach or recede from viewer l Panning – Translate window from frame to frame, clip, and transform • scene appears to move across the screen
1. Output Primitives l Lines l Building blocks for drawing pictures Windows: l Plotting a pixel--most primitive MoveTo(x,y); // Set Curr. Pos. SetPixel(x,y,colref); // Windows--plots pixel colref = GetPixel(x,y); // returns pixel color LineTo(x,y); // line from CP to (x,y)
glBegin (GL_POINTS); // OpenGL glBegin (GL_LINES); //OpenGL glVertex2f (x, y); // 2==>2D, f==>floating pt glVertex2f(x1,y1); //endpoint vertices glEnd(); // current drawing color used glVertex2f(x2,y2); //appear in pairs glEnd()
5 l l Polylines and Polygons Other primitives
Polyline(lppts,num_pts); // Windows--use pts –Windows // array, number of points Polygon(lppts,num_pts); • Lots of other primitives • See prior notes on Windows glBegin (GL_POLYGON); // OpenGL programming glVertex2f(x1,y1); // polygon vertex … // more vertices –OpenGL glEnd(); • GL_TRIANGLES, GL_LINE_STRIP, GL_QUADS, etc. -- lots more
l Text 3-D primitives
Windows: l Windows has nothing TextOut(x,y,lpszStr,cStrLngth); l OpenGL – GLU graphics library OpenGL-- Use display lists and/or • sphere, cube, cone, etc. GLUT library functions
2. Attributes (State Variables) 3. Transformations l Done with matrix math l Properties of primitives – how they appear l Setting windows/viewports – e.g., color, line style, text style, fill patterns – Window-to-viewport transformation l Usually modal l Moving objects – values retained until changed – Geometric Transformations l Windows -- see prior notes – e.g., translation, rotation, scaling l Changing coordinate system l OpenGL-- glProperty(); – ‘Property’ is state variable to set l Changing viewpoint • e.g., glColor3f (R,G,B); l Different types of projections
6 4. Segmentation l Windows – window-to-viewport transformation l Dividing scene into component parts • done with Mapping Modes for (later) manipulation – programmer must implement others l Windows--strictly immediate mode l OpenGL is very rich – (DirectX has support for retained mode) – glLoadMatrix(), glRotatef(), glTranslatef(), l OpenGL has Display lists glScalef(), glViewport (), glFrustum(), – Groups of OpenGL commands that have glOrtho(), gluPerspective(), etc. been stored for later execution – Can be hierarchical l PHIGS uses hierarchical segments
5. Input/Interaction Input/Interaction in OpenGL l Obtain data from input devices or graphics system l Auxiliary libraries (aux, GLX, GLUT) – – So user can manipulate scene interactively Use underlying windowing system – Provide input-handling callback ftns l Windows: Built into event-driven, • e.g., auxMouseFunc () message-based paradigm • glutMouseFunc() – take pointer to callback function – programmer must write these
7. Storing/retrieving/ 6. Control/Housekeeping manipulating bitmapped Images l BitBlT--Bit Block Transfer l Initialize graphics system, put window up, etc. l Windows-- – Device Dependent Bitmaps l Windows--Extensive support • BitBlt(), StretchBlt(), etc. – RegisterClass(), CreateWindow(), etc. – But very slow l OpenGL – Device Independent Bitmaps--faster – Use of auxiliary libraries – DirectX-- flipping surfaces--fastest! • e.g., auxInitWindow(), auxInitDisplayMode (), auxMainLoop(ptr to ftn. that draws scene) l OpenGL-- – glReadPixels(); glDrawPixels(); glCopyPixels();
7 8. Photorealism Intro. to OpenGL for Windows
l Industry standard for high-quality 3-D l Hidden surfaces, lighting, shading, graphics applications reflection properties, etc. l Available on many HW and OS l Windows--Very little support platforms – DirectX (Direct3D)--Quite a bit of support l “Thin” software interface to underlying graphics HW l OpenGL--A lot of support! – e.g., light sources, lighting models, l Implementing on Windows brings material properties, blending, workstation-class graphics to PC antialiasing, fog, depth buffer, etc. l Real 3-D graphics to Windows
Steps in using OpenGL in Rendering Context (RC) Windows Applications l OpenGL equivalent of Windows GDI DC l Get a DC for rendering location (window) l Mechanism by which OpenGL calls are l Choose & select a “pixel format” for DC rendered to the device l Create a Rendering Context (RC) for DC l Links OpenGL calls to a Windows l Associate (bind) the RC with the DC window l Draw using OpenGL function calls l Must be compatible with window’s DC l Release the RC & DC l Keeps track of current values of OpenGL state variables
Pixel Format PIXELFORMATDESCRIPTOR l Data structure used to set Pixel Format l Holds attributes for device drawing surface l Some fields: – dwFlags: “OR” of properties constants, e.g. l Describes things like: • doublebuffered, stereo, window or bitmap, etc. – Using single or double buffering – iPixelType – Direct or indirect color • color type (RGBA or indexed) – Drawing to a window or a bitmap – cColorBits: # of bitplanes – Color depth (# of bit planes) – cRedBits: # of red – ZBuffer depth – cRedShift: where red bits are – Others – etc.
8 Choosing and setting the Creating and using an RC Pixel Format
l Set up a PIXELFORMATDESCRIPTOR l Use “wgl” function to create an RC: variable (e.g., pfd) – wglCreateContext(hDC) l ChoosePixelFormat(hDC,&pfd) – Returns a handle to an OpenGL Rendering – gets DC’s pixel format closest to one Context (HGLRC) desired l Make the RC “Current” (bind RC to DC) – returns an integer pf_index – wglMakeCurrent(hDC, hRC) l SetPixelFormat(hDC, pf_index, &pfd) l Now we can draw with OpenGL calls – Set that pixel format in the DC
Cleanup l Make RC noncurrent (Unbind RC from DC) – wglMakeCurrent(hDC, NULL) l Get rid of the DC – ReleaseDC() or EndPaint() in API app. – Done automatically in MFC when OnDraw() returns l Get rid of the RC – wglDeleteContext(hRC)
Building a MINOGL Example Program Windows/OpenGL App l Displays a rectangle in different l Includes in .h file: shades of red –
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