1/12/11. Basic Graphics Programming. What is OpenGL. OpenGL is cross-platform. How does OpenGL work. How does OpenGL work (continued) The result

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CSCI 480 Computer raphics Lecture 2 asic raphics Programming What is OpenL A low- level graphics API for 2D and 3D interac<ve graphics.

Ch. 2] University of Southern California http://www-bcf.usc.

Linux: Mesa, a freeware implementa<on. How does OpenL work From the programmer s point of view: 1. Specify geometric objects 2.

1 CSCI 480 Computer raphics Lecture 2 asic raphics Programming What is OpenL A low- level graphics API for 2D and 3D interac<ve graphics. Descendent of L (from SI) raphics Pipeline OpenL API Primitives: Lines, Polygons Attributes: Color Example January 12, 2011 Jernej arbic [Angel Ch. 2] University of Southern California OpenL is cross-platform Same code works with liele/no modifica<ons Implementa<ons: Windows, Mac: ships with the OS Linux: Mesa, a freeware implementa<on. How does OpenL work From the programmer s point of view: 1. Specify geometric objects 2. Describe object properties Color How objects reflect light 3 4 How does OpenL work (continued) The result 3. Define how objects should be viewed where is the camera what type of camera 4. Move camera or objects around for animation the result 5 6 1

2 OpenL is a state machine State variables: color, current viewing position, line width, material properties... OpenL Library Organization L (raphics Library): core graphics capabilities LU (OpenL Utility Library): utilities on top of L LUT (OpenL Utility Toolkit): input and windowing These variables (the state) then apply to every subsequent drawing command They persist until set to new values by the programmer 7 8 raphics Pipeline Immediate-mode rendering Application generates stream of geometric primitives (polygons, lines) Primitives+ material properties Translate otate Scale Is it visible on screen? 3D to 2D Convert to Shown pixels on the screen (framebuffer) System draws each one into the framebuffer Entire scene redrawn anew every frame Contrast: off-line rendering (e.g., Pixar enderman) 9 10 The pipeline is implemented by OpenL, graphics driver and the graphics hardware raphics Pipeline Efficiently implementable in hardware (but not in software) OpenL programmer does not need to implement the pipeline. Each stage can employ multiple specialized processors, working in parallel, busses between stages However, pipeline is reconfigurable if needed shaders 11 #processors per stage, bus bandwidths arefully tuned for typical graphics use Latency vs throughput 12 2

Vertices Transformer Vertices in world coordinates void glvertex3f(lfloat x, Lfloat y, Lfloat z) Vertex (x, y, z) sent down the pipeline Function call returns Use Ltype for portability and

4] 13 14 Clipper Projector Mostly automatic (must set viewport) Complex transformation [Angel Ch. 5] Orthographic Perspective 15 16 asterizer Interesting algorithms [Angel Ch.

3 Vertices Transformer Vertices in world coordinates void glvertex3f(lfloat x, Lfloat y, Lfloat z) Vertex (x, y, z) sent down the pipeline Function call returns Use Ltype for portability and consistency glvertex234sfid[v](type coords) Transformer in world coordinates Must be set before object is drawn! glotatef(45.0, 0.0, 0.0, -1.0); glvertex2f(1.0, 0.0); Complex [Angel Ch. 4] Clipper Projector Mostly automatic (must set viewport) Complex transformation [Angel Ch. 5] Orthographic Perspective asterizer Interesting algorithms [Angel Ch. 7] To window coordinates Antialiasing Primitives Specified via vertices eneral schema glegin(type); glvertex3f(x1, y1, z1);... glvertex3f(xn, yn, zn); type determines interpretation of vertices Can use glvertex2f(x,y) in 2D

4 Example: Draw Square Outline Points and Line Segments Type = L_LINE_LOOP glegin(l_line_loop); glvertex3f(0.0, 0.0, 0.0); glvertex3f(1.0, 0.0, 0.0); glvertex3f(1.0, 1.0, 0.0); glvertex3f(0.0, 1.0, 0.0); Calls to other functions are allowed between glegin(type) and glegin (L_POINTS); glvertex3f( ); glvertex3f( ); Draw points Polygons Polygons enclose an area Polygon estrictions OpenL Polygons must be simple OpenL Polygons must be convex (a) simple, but not convex endering of area (fill) depends on attributes All vertices must be in one plane in 3D (c) convex (b) non-simple Why Polygon estrictions? Non-convex and non-simple polygons are expensive to process and render Convexity and simplicity is expensive to test ehavior of OpenL implementation on disallowed polygons is undefined Some tools in LU for decomposing complex polygons (tessellation) Triangles are most efficient Polygon Strips Efficiency in space and time educes visual artefacts Polygons have a front and a back, possibly with different attributes!

Attributes: color, shading and reflection properties Part of the OpenL state Physics of Color Electromagnetic radiation Can see only tiny piece of the spectrum Set before primitives are drawn emain

25 26 Color Filters Eye can perceive only 3 basic colors Computer screens designed accordingly Color Spaces (ed, reen, lue) Convenient for display Can be unintuitive (3 floats in OpenL) Amplitude HSV

5 Attributes: color, shading and reflection properties Part of the OpenL state Physics of Color Electromagnetic radiation Can see only tiny piece of the spectrum Set before primitives are drawn emain in effect until changed! Color Filters Eye can perceive only 3 basic colors Computer screens designed accordingly Color Spaces (ed, reen, lue) Convenient for display Can be unintuitive (3 floats in OpenL) Amplitude HSV (Hue, Saturation, Value) Hue: what color Saturation: how far away from gray Value: how bright Other formats for movies and printing Wavelength [nanometer] Source: Vos & Walraven vs HSV imp Color Picker Example: Drawing a shaded polygon Initialization: the main function V H S int main(int argc, char** argv) glutinit(&argc, argv); glutinitdisplaymode (LUT_DOULE LUT_); glutinitwindowsize (500, 500); glutinitwindowposition (100, 100); glutcreatewindow (argv[0]); init ();

0, 0.0, 0.0); /* glshademodel (L_FLAT); */ glshademodel (L_SMOOTH); 31 32 The Display Callback The routine where you render the object Install with glutdisplayfunc(display) void display(void) glclear

6 LUT Callbacks Window system independent interaction glutmainloop processes events... glutdisplayfunc(display); gluteshapefunc(reshape); glutkeyboardfunc (keyboard); glutmainloop(); return 0; Initializing Attributes Separate in init function void init(void) glclearcolor (0.0, 0.0, 0.0, 0.0); /* glshademodel (L_FLAT); */ glshademodel (L_SMOOTH); The Display Callback The routine where you render the object Install with glutdisplayfunc(display) void display(void) glclear (L_COLO_UFFE_IT); /* clear buffer */ setupcamera(); /* set up the camera */ triangle (); /* draw triangle */ glutswapuffers (); /* force display */ Drawing In world coordinates; remember state! void triangle(void) glegin (L_TIANLES); glcolor3f (1.0, 0.0, 0.0); /* red */ glvertex2f (5.0, 5.0); glcolor3f (0.0, 1.0, 0.0); /* green */ glvertex2f (25.0, 5.0); glcolor3f (0.0, 0.0, 1.0); /* blue */ glvertex2f (5.0, 25.0); The Image glshademodel(l_flat) glshademodel(l_smooth) Flat vs Smooth Shading color of last vertex each vertex separate color smoothly interpolated Flat Shading Smooth Shading

Projection Mapping world to screen coordinates void reshape(int w, int h) glviewport (0, 0, (Lsizei) w, (Lsizei) h); glmatrixmode (L_POJECTION); glloadidentity (); if (w <= h) gluortho2d (0.0, 30.

7 Projection Mapping world to screen coordinates void reshape(int w, int h) glviewport (0, 0, (Lsizei) w, (Lsizei) h); glmatrixmode (L_POJECTION); glloadidentity (); if (w <= h) gluortho2d (0.0, 30.0, 0.0, 30.0 * (Lfloat) h/(lfloat) w); else gluortho2d (0.0, 30.0 * (Lfloat) w/(lfloat) h, 0.0, 30.0); glmatrixmode(l_modelview); Orthographic Projection 2D and 3D versions glortho2d(left, right, bottom, top) In world coordinates! Viewport Determines clipping in window coordinates glviewport(x, y, w, h) Summary A raphics Pipeline The OpenL API Primitives: vertices, lines, polygons Attributes: color Example: drawing a shaded triangle

Basic Graphics Programming

CSCI 480 Computer Graphics Lecture 2 Basic Graphics Programming January 11, 2012 Jernej Barbic University of Southern California http://www-bcf.usc.edu/~jbarbic/cs480-s12/ Graphics Pipeline OpenGL API