三維繪圖程式設計 3D Graphics Programming Design 第三章三維繪圖狀態設定和幾何元件繪製嘉大資工系盧天麒

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1 三維繪圖程式設計 3D Graphics Programming Design 第三章三維繪圖狀態設定和幾何元件繪製嘉大資工系盧天麒 1

2 In this chapter, you will learn How to access values in the OpenGL state machine The types of primitives available in OpenGL How to modify the way primitives are handle and displayed 2

3 State functions OpenGL provides a number of multipurpose functions that allow to query the OpenGL state machine Most of which begin with glget 3

4 Querying numeric states 4 functions to retrieve values glgetbooleanv(glenum pname, GLboolean *params) glgetdoublev(glenum pname, GLdouble *params) glgetfloatv(glenum pname, GLfloat *params) glgetintegerv(glenum pname, GLint *params) The parameter pname The state setting you are query params An array to hold all values associated with the setting 4

5 Enabling and disabling states Turn the states on and off glenable(glenum cap) gldisable(glnum cap ) Over 40 capabilities to enable and disable GL_BLEND, GL_TEXTURE_2D, GL_LIGHTING 5

6 glisenabled( ) Find out whether a particular OpenGL capability is on or off It is usually easier to use GLboolean glisenabled(glenum cap); The function can be called with any of the values accepted by glenable( ) / gldisable( ) GL_TRUE and GL_FALSE 6

7 Querying String Values You can find out the details of the OpenGL implementation being used at runtime const GLubyte *glgetstring(glenum name); 7

8 Finding Errors Passing incorrect values to OpenGL functions Cause an error flag to be set Querying the error flag can help you to more easily track down problems in your code You can do this through GLenum glgeterror( ); 8

9 Finding Errors The returns one of the values in the following 9

10 Giving OpenGL a Hint Allow developers a level of control over the trade-off between image quality and speed The glhint( ) function allows you to specify your desired level of trade-off void glhint(glenum target, GLenum hint); 10

11 Giving OpenGL a Hint The target parameter specifies the behavior you want to control 11

12 Giving OpenGL a Hint The hint parameter can be one of the three options GL_FASTEST sacrificing quality GL_NICEST losing performance GL_DONT_CARE OpenGL driver will decide what it thinks is best 12

13 Handling Primitives Basic geometric entities Points, lines, and triangles The first is void glbegin(glenum mode); void glend(); You use glbegin( ) to tell OpenGL two things Ready to start drawing The primitive type you want to draw 13

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15 Handling Primitives glbegin( ) / glend( ) blocks may not be nested Not all OpenGL functions cab be used inside a glbegin( ) / glend( ) block Only variations of the function listed in the following table may be used 15

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17 Handling Primitives Draw with OpenGL void glvertex{234}{dfis}( ); void glvertex{234}{dfis}v( ); For example glvertex2i(5, 20); glvertex3f(1.5, 0.5, 10.0); GLfloat v[3]={1.5, 0.5, 10.0}; glvertex3fv(v); 17

18 Drawing Points in 3D Draw a point in OpenGL glbegin(gl_points); glvertex3f(0.0, 0.0, 0.0); glend(); Draw two points glbegin(gl_points); glvertex3f(0.0, 0.0, 0.0); glvertex3f(0.0, 1.0, 0.0); glend(); 18

19 Modifying Point Size To change the point size void glpointsize(glfloat size); The default size is 1.0 Find out the current selected size GLfloat oldsize; glgetfloatv(gl_point_size, &oldsize); 19

20 Antialiasing Points There is finite number of pixels on the screen Cause the edges of primitives to look jagged Antialiasing provides a means of smoothing out the edges Give edges a more realistic look 20

21 Antialiasing Points If you want to use antialiasing, you can Turn it on by passing GL_POINT_SMOOTH to glenable( ) Turn it off again by passing the same parameter to gldisable( ) 21

22 Antialiasing Points If you are unsure whether point antialiasing is enabled or disabled, you find out by calling glget( ) with GL_POINT_SMOOTH, or with glisenabled(gl_point_smooth) Here is an example If (!glisenable(gl_point_smooth)) glenable(gl_point_smooth); 22

23 Effect of Distance Points always occupy the same amount of space on the screen For some applications of points, you will want the points to be smaller as they get farther away Such as particle systems 23

24 Effect of Distance You can do this through the use of the glpointparameter( ) function void glpointparameter{if}(enum pname, type param); void glpointparameter{if}v(enum pname, const type *params); 24

25 A Pointy Example Each point s size increases in the row Each point separated by 0.5 units float pointsize = 0.5; // draw a line of points of increasing size for (float point = -4.0; point < 5.0; point+=0.5) { glpointsize(pointsize); // set the point size // draw the point glbegin(gl_point); glvertex3f(point, 0.0, 0.0); glend(); // increase the point size for the next point pointsize += 1.0; } 25

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27 Drawing Lines in 3D Passing GL_LINES to glbegin( ) glbegin(gl_lines); glvertex3f(-2.0, -1.0, 0.0); glvertex3f(3.0, 1.0, 0.0); glend(); 27

28 Modifying Line Width The default line width is 1.0 void gllinewidth(glfloat width); GLfloat oldwidth; glgetfloatv(gl_line_width, &oldwidth); If (oldwidth < 1.0) gllinewidth(5.0); 28

29 Antialiasing Lines Antialiasing for lines works very much as it does with points You can turn it on and off by passing GL_LINE_SMOOTH to glenable( ) and gldisable( ) The current state can be determined by passing GL_LINE_SMOOTH to glget( ) or glisenabled( ) It is disabled by default 29

30 Antialiasing Lines Here is an example of how to do that GLfloat sizes[2]; GLfloat granularity; glgetfloatv(gl_line_width_range, sizes); GLfloat minlinewidth = sizes[0]; GLfloat maxlinewidth = sizes[1]; glgetfloatv(gl_line_width_granularity, &granularity); 30

31 Drawing Polygons in 3D To change the way polygons are drawn void glpolygonmode(glenum face, GLenum mode); Polygons modes GL_POINT, GL_LINE, GL_FILL OpenGL handles the front and back faces of polygons separately Polygons are drawn filled by default glpolygonmode(gl_front, GL_FILL); glpolygonmode(gl_back, GL_LINE); 31

32 Polygon Mode Example Five squares rotating clockwise at the same rate Starting from the left glpolygonmode(gl_front, GL_LINE); glpolygonmode(gl_back, GL_POINT); glpolygonmode(gl_front_and_back, GL_FILL); glpolygonmode(gl_back, GL_LINE); glpolygonmode(gl_front_and_back, GL_LINE); 32

33 Polygon Face Culling It is possible to have OpenGL to eliminate polygons that the viewer can not see Culling OpenGL can do this for you automatically 33

34 Polygon Face Culling First, passing GL_CULL_FACE to glenable( ) Then you need to specify which face you want culled Call glcullface( ) void glcullface(glenum mode); Mode can be GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK GL_BACK is the default setting 34

35 Polygon Face Culling The next step is telling OpenGL how to determine whether a polygon is front or back facing Polygon winding You have to proceed either clockwise or counterclockwise around its vertices 35

36 Polygon Face Culling By default Counterclockwise (CCW) ordering as frontfacing Clockwise (CW) ordering as back-facing The default behavior can be changed using glfrontface( ) void glfrontface(glenum mode); Mode should GL_CCW or GL_CW 36

37 Hiding Polygon Edges Wire-frame or all the edges of your polygons show up For example, you may not want the viewer to see the diagonal line void gledgeflag(glboolean isedge); The flag is set to GL_TRUE (the default) Drawn The flag is set to GL_FLASE Not to draw 37

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39 Antialiasing Polygons As with points and lines, you can also choose to antialias polygons By passing GL_POLYGON_SMOOTH to glenable( ) and gldisable( ) The current state can be determined By passing the same parameter to glget( ) or glisenable( ) It is disabled by default 39

40 Antialiasing Polygons Here is an example of how to enable polygon antialiasing if (!glisenabled(gl_polygon_smooth)) glenable(gl_polygon_smooth); 40

41 Triangles Triangles are the preferred polygon form Three points define a plane A triangle is always convex A triangle can not cross over itself glbegin(gl_triangles); glvertex3f(-2.0, -1.0, 0.0); glvertex3f(3.0, 1.0, 0.0); glvertex3f(0.0, 3.0, 0.0); glend(); The number of vertices defined isn t a multiple of 3, then the extra vertices are discarded 41

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43 Triangles Triangle strips Call glbegin( ) with GL_TRIANGLE_STRIP Draw the first three vertices Take every specified and combine it with the previous two vertices Reduce the number of vertices from 3n to n+2 43

44 Triangles Triangle fans Call glbegin( ) with GL_TRIANGLE_FAN A series of triangles around a single central vertex The first vertex specified is the central vertex Every following adjacent pair of vertices is combined with the center vertex Draw n triangles while specifying only n+2 vertices 44

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46 Quadrilaterals Four-sided polygons To draw a square or rectangle Call glbegin( ) with GL_GUADS and specifying four or more vertices 46

47 Quadrilaterals OpenGL provides quad strips as a means of improving the speed of rendering quads They are specified using GL_QUAD_STRIP Each pair of vertices specified after the first pair defines a new quad 47

48 Polygons An arbitrary number of vertices If fewer than three vertices, nothing is drawn 48

49 Using Primitives: Triangles and Quads Example The final example for this chapter TrianglesQuads A set of six grids with a different primitive type 49

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51 Using primitives: triangles and quads example DrawPoints Void DrawPoints() { glpointsize(4.0); glbegin(gl_points); for (int x = 0; x < 4; x++) for (int z = 0; z < 4; z++) glvertex3f(x, 0, z); glend(); }

52 Using primitives: triangles and quads example DrawTriangles Void DrawTriangles() { glbegin(gl_triangles); for (int x = 0; x < 3; x++) for (int z = 0; z < 3; z++) glvertex3f(x, 0.0, z); glvertex3f((x+1.0), 0, z); glvertex3f(x, 0.0, (z+1.0)); glend(); }

53 Using primitives: triangles and quads example DrawQuads Void DrawQuads() { glbegin(gl_quads); for (int x = 0; x < 3; x++) for (int z = 0; z < 3; z++) glvertex3f(x, 0.0, z); glvertex3f((x+1).0, 0, z); glvertex3f((x+1).0, 0, (z+1)); glvertex3f(x, 0.0, (z+1.0)); glend(); }

54 Using primitives: triangles and quads example DrawTriangleStrip Void DrawTriangleStrip() { // 3 rows of triangle strips for (int x = 0; x < 3; x++) { glbegin(gl_triangle_strip); for (int z = 0; z < 3; z++) { glvertex3f(x, 0.0, z); glvertex3f((x+1).0, 0, z); glvertex3f(x, 0.0, (z+1.0)); glvertex3f((x+1).0, 0, (z+1.0)); glend();

55 Using primitives: triangles and quads example DrawTriangleFan Void DrawTriangleFan() { glbegin(gl_triangle_fan); glvertex3f(0.0, 0.0, 0.0); // center vertex of fan for (int x = 4; x > 3; x--) // bottom side glvertex3f(x-1, 0.0, 3.0); for (int z = 4; z > 0; z--) // right side glvertex3f(3.0, 0.0, z-1); glend();

56 Using primitives: triangles and quads example DrawQuadStrip Void DrawQuadStrip() { for (int x = 0; x < 3; x++) { glbegin(gl_quad_strip); for (int z = 0; z < 4; z++) { glvertex3f(x, 0.0, z); glvertex3f((x+1).0, 0, z); glend(); }

57 Attributes Save and restore all of the state information for a group void glpushattrib(glbitfield mask); void glpopattrib(void);

58 Summary You learned A little more about the OpenGL state machine You know how to use glget( ) and glisenable( ) The primitive types supported by OpenGL How to modify properties Putting points, lines, triangles, and other primitives on the screen

59 On Your Own You have been tasked to write a function that draws a 2D circle approximation with the option of drawing only the edge of the circle or drawing the circle filled at the world origin (0, 0, 0) void DrawCircleApproximation(float radius, int numberofsides, bool edgeonly); radius: the radius of the circle numberofsides: a value for the number of edges in the circle approximation

60 Reference Dave Astle and Kevin Hawkins, Beginning OpenGL Game Programming, 1st Edition, Course Technology PTR,