OpenGL refresher. Advanced Computer Graphics 2012

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Roxanne Chambers
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1 Advanced Computer Graphics 2012

2 What you will see today Outline General OpenGL introduction Setting up: GLUT and GLEW Elementary rendering Transformations in OpenGL Texture mapping Programmable shading Review of basic Cg programming

3 General OpenGL Introduction (1) OpenGL is a computer graphics rendering API Generate high-quality color images by rendering with geometric and image primitives Create interactive applications with 3D graphics OpenGL is a cross-platform API operating system independent window system independent

4 General OpenGL Introduction (2) OpenGL architecture

5 General OpenGL Introduction (3) OpenGL provides low-level rendering functionality. Assembling primitives from vertex data. Transformations between spaces (model, world, view, etc) Rasterization of primitives (point, lines, triangles, etc). Performing lighting calculations (usually programmable) Applying textures to geometry Applications build higher-level functionality on top: Loading and displaying models Animation Scene management.

6 GLUT and GLEW We ll be building on a couple of simple libraries (GLUT and GLEW) to simplify OpenGL usage. Preliminaries Header files Libraries #include <GL/glew.h> // includes GL/gl.h and GL/glu.h #include <GL/glut.h> The OpenGL library is commonly named libgl.so gcc... -lgl -lglu -lglut -lglew

7 GLUT and GLEW (2) GLUT Configure and open window Initialize OpenGL state Register input callback functions Display Resize Input: keyboard, mouse, etc. Enter event processing loop GLEW GL Extension Wrangler Provide access to modern GPU features Standard gl.h only exposes calls up to GL 1.1, we are now at 4.2

8 Sample program GLUT and GLEW (3) void main( int argc, char** argv ) { //Get GLUT up and running int mode = GLUT_RGB GLUT_DOUBLE; glutinitdisplaymode( mode ); glutinitwindowsize( 512, 512 ); glutcreatewindow( argv[0] ); //Our initialisation code init(); } //GLUT callback functions glutdisplayfunc( display ); glutreshapefunc( resize ); glutkeyboardfunc( key ); glutidlefunc( idle ); glutmainloop();

9 GLUT and GLEW (4) Initialize GL extensions using GLEW Set up OpenGL state void init( void ) { glewinit( ); glclearcolor( 0.0, 0.0, 0.0, 1.0 ); glcleardepth( 1.0 ); glenable( GL_LIGHT0 ); glenable( GL_LIGHTING ); glenable( GL_DEPTH_TEST ); }

10 GLUT callback functions GLUT and GLEW (5) Routines that are called when something happens Window resize or redraw User input Timer Register callbacks with GLUT glutdisplayfunc( display ); glutreshapefunc( resize ); glutkeyboardfunc( key ); glutidlefunc( idle );

11 Rendering callback GLUT and GLEW (6) Draw the window contents using OpenGL void display( void ) { glclear( GL_COLOR_BUFFER_BIT ); glbegin( GL_QUADS ); glvertex3fv( &v[0] ); glvertex3fv( &v[1] ); glvertex3fv( &v[2] ); glvertex3fv( &v[3] ); glend(); glutswapbuffers(); }

12 GLUT and GLEW (7) Idle callback Called when event loop is idle Use for continous update of rendering void idle( void ) { curtime = glutget(glut_elapsed_time);... glutpostredisplay(); }

13 User Input Callbacks Process user input GLUT and GLEW (8) void keyboard( char key, int x, int y ) { switch( key ) { case 'q' : case 'Q' : exit( EXIT_SUCCESS ); break; case 'r' : case 'R' : rotate = GL_TRUE; break; } }

14 Elementary rendering OpenGL geometric primitives

15 Elementary rendering (2) All primitives are specified by their vertices Contains at least a position Possibly other data (normals, texcoords, etc).

16 Elementary rendering (3) The vertices of a primitive are stored as an array. OpenGL provides a lot of control over exactly how these arrays are laid out. Separate array per component Single array of structures

17 Transformations in OpenGL Transformation pipeline Spaces define what vertex positions are relative to. Each space has it s own coordinate system. Transforms: Modelview, Projection, Viewport Represented as 4 by 4 matrices Transform by post-multiplication with homogeneous coordinates of vertex (,,,)

18 Transformations in OpenGL (3) Modeling transform glmatrixmode(gl_modelview) Transforms coordinates from object-space to world-space; places the object within the world Object-space positions specified as homogeneous coordinates May involve rotations, translations, scaling View transform Converts world-space positions to view-space positions Typical transform is a translation which moves the eye position in world-space to the origin (of eye space) example: if the eye is positioned at (0,0,5) in world-space, then the view transform would translate by (0,0,-5)

19 Transformations in OpenGL (3) Projection transform glmatrixmode(gl_projection) Converts eye-space coordinates into clip-space coordinates Defines a view frustum representing the region of eye space where objects are viewable Perspective divide Divides x, y and z by w. The resulting coordinates are called normalized device coordinates. Viewport transform Converts normalized device coordinates to 2D pixel positions

20 Texture mapping Apply 1D 2D or 3D image to geometric primitives Uses of texturing Add surface detail Reflections Shadows

21 Texture mapping (2) How does texture mapping work? Assign (u,v) coordinates to each vertex These coordinates assign a position in the image to a vertex Coordinates are interpolated over the primitive

22 Texture mapping (3) You can control the wrap mode which is used. Textures can repeat, clamp, or be mirrored. Different filtering modes can be specified Nearest neighbour filtering Linear filtering Higher-order filtering as an advanced technique Mipmaps can be created: Smaller versions of the texture to be used in the distance. Helps combat aliasing issues.

23 Blending When two primitives overlap how should they be combined? Just use the last drawn primitive Or blend based on alpha value Additive Multiplicative Custom OpenGL gives control over this: glblendequation(...) glblendfunc(...)

24 Programmable shading Traditionally, OpenGL was controlled by configuring states. Lighting mode, texture mode, etc The last 10 years have seen the graphics pipeline become increasingly programmable Much more flexibility for the programmer. Short programs are written in GLSL or Cg. Current generation hardware allows vertex, fragment and primitive processing to be controlled. Also programmable tessellation. More to come in the future?

25 OpenGL reference pages Online resources Extension registry Cg users manual Questions?

Computer Graphics. Bing-Yu Chen National Taiwan University

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