Programming with OpenGL Part 2: Complete Programs Computer Graphics I, Fall

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1 Programming with OpenGL Part 2: Complete Programs Computer Graphics I, Fall

2 Objectives Refine first program Alter default values Introduce standard program structure Simple viewing 2-D viewing as special case of 3-D viewing Fundamental OpenGL primitives Attributes Computer Graphics I, Fall

3 Program Structure Most OpenGL programs have similar structure Consists of following functions -main(): defines callback functions opens one or more windows with required properties enters event loop (last executable statement) -init(): sets state variables Viewing Attributes callbacks Display function Input and window functions Computer Graphics I, Fall

4 simple.c revisited In this version, see same output, but defined all relevant state values through function calls using default values In particular, we set Colors Viewing conditions Window properties Computer Graphics I, Fall

5 main.c #include <GL/glut.h> includes gl.h int main(int argc, char** argv) { glutinit(&argc,argv); glutinitdisplaymode(glut_single GLUT_RGB); glutinitwindowsize(500,500); glutinitwindowposition(0,0); glutcreatewindow("simple"); glutdisplayfunc(mydisplay); init(); glutmainloop(); } define window properties display callback set OpenGL state enter event loop Computer Graphics I, Fall

6 GLUT functions glutinit allows application to get command line arguments and initializes system gluinitdisplaymode requests properties for window (rendering context) RGB color Single buffering Properties logically ORed together glutwindowsize in pixels glutwindowposition from top-left corner of display glutcreatewindow create window with title simple glutdisplayfunc display callback glutmainloop enter infinite event loop Computer Graphics I, Fall

7 init.c void init() { glclearcolor (0.0, 0.0, 0.0, 1.0); black clear color opaque window glcolor3f(1.0, 1.0, 1.0); fill/draw with white glmatrixmode (GL_PROJECTION); glloadidentity (); glortho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0); } viewing volume Computer Graphics I, Fall

8 Coordinate Systems Units in glvertex determined by application object or problem coordinates Viewing specifications also in object coordinates size of viewing volume determines what will appear in image Internally, OpenGL convert to camera (eye) coordinates and later to screen coordinates OpenGL also uses some internal representations usually not visible to application Computer Graphics I, Fall

9 OpenGL Camera OpenGL places camera at origin in object space pointing in negative z direction default viewing volume = box centered at origin with side of length Computer Graphics I, Fall

10 Orthographic Viewing In default orthographic view, points are projected forward along z axis onto plane z = 0 z=0 z= Computer Graphics I, Fall

11 Transformations and Viewing In OpenGL, projection carried out by projection matrix (transformation) only one set of transformation functions ==> must set matrix mode first glmatrixmode (GL_PROJECTION) Transformation functions incremental ==> start with identity matrix alter it with projection matrix that gives view volume glloadidentity(); glortho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0); Computer Graphics I, Fall

12 Two- and three-dim viewing In glortho(left, right, bottom, top, near, far) near and far distances measured from camera 2-D vertex commands place all vertices in plane z = 0 If application is in 2-D, can use function gluortho2d(left, right,bottom,top) In 2-D view or clipping volume becomes clipping window Computer Graphics I, Fall

13 mydisplay.c void mydisplay() { glclear(gl_color_buffer_bit); glbegin(gl_polygon); glvertex2f(-0.5, -0.5); glvertex2f(-0.5, 0.5); glvertex2f(0.5, 0.5); glvertex2f(0.5, -0.5); glend(); glflush(); } Computer Graphics I, Fall

14 OpenGL Primitives GL_POINTS GL_LINES GL_LINE_STRIP GL_POLYGON GL_LINE_LOOP GL_TRIANGLES GL_QUAD_STRIP GL_TRIANGLE_STRIP GL_TRIANGLE_FAN Computer Graphics I, Fall

15 Polygon Issues OpenGL only display correctly polygons that are Simple: edges cannot cross Convex: All points on segment also in polygon Flat: all vertices in same plane (planar) User program can check if above true OpenGL produce output if conditions violated but may not be what desired Triangles satisfy all conditions nonsimple polygon nonconvex polygon Computer Graphics I, Fall

16 Attributes Attributes are part of OpenGL state determine appearance of objects Color (points, lines, polygons) Size and width (points, lines) Stipple pattern (lines, polygons) Polygon mode Display as filled: solid color or stipple pattern Display edges Display vertices Computer Graphics I, Fall

17 RGB color Each color component stored separately in frame buffer Usually 8 bits per component in buffer Note in glcolor3f color values range from 0.0 (none) to 1.0 (all), whereas in glcolor3ub values range from 0 to Computer Graphics I, Fall

18 Indexed Color Colors = indices into tables of RGB values Requires less memory indices usually 8 bits not as important now Memory inexpensive Need more colors for shading Computer Graphics I, Fall

19 Color and State color as set by glcolor becomes part of state will be used until changed Colors and other attributes not part of object but assigned when object rendered can create conceptual vertex colors by code such as glcolor glvertex glcolor glvertex Computer Graphics I, Fall

20 Smooth Color Default is smooth shading OpenGL interpolates vertex colors across visible polygons Alternative is flat shading Color of first vertex determines fill color glshademodel (GL_SMOOTH) or GL_FLAT Computer Graphics I, Fall

21 Viewports Don t have to use entire window for image: glviewport(x,y,w,h) Values in pixels (screen coordinates) Computer Graphics I, Fall

Introduction to OpenGL Week 1

CS 432/680 INTERACTIVE COMPUTER GRAPHICS Introduction to OpenGL Week 1 David Breen Department of Computer Science Drexel University Based on material from Ed Angel, University of New Mexico Objectives