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State Management and Drawing Geometry Objects (OpenGL Book Ch 2)

State Management and Drawing Geometry Objects (OpenGL Book Ch 2). Objective. Clear the window to an arbitrary color Force any pending drawing to complete Draw with any geometric primitive Turn states on and off and query state variables Control the display of those primitives

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State Management and Drawing Geometry Objects (OpenGL Book Ch 2)

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  1. State Management and Drawing Geometry Objects (OpenGL Book Ch 2)

  2. Objective • Clear the window to an arbitrary color • Force any pending drawing to complete • Draw with any geometric primitive • Turn states on and off and query state variables • Control the display of those primitives • Specify normal vectors • Use vertex arrays • Save and restore state variables

  3. Clearing the Window glClearColor(0.0, 0.0, 0.0, 0.0); glClearDepth(1.0); glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // or run more slowly with, // glClear(GL_COLOR_BUFFER_BIT); // glClear(GL_DEPTH_BUFFER_BIT);

  4. Specifying a Color • Pseudocode set_current_color(red); draw_object(A); draw_object(B); set_current_color(green); // wasted set_current_color(blue); draw_object(C); • glColor3f(1.0, 0.0, 0.0);

  5. Forcing Completion of Drawing • glFlush(); • Forces previously issued OpenGL commands to begin execution. • glFinish(); • Forces all previously issued OpenGL command s to complete. This command doesn’t return until all effects from previous commands are fully realized.

  6. What are Points, Lines, and Polygon? • Points • represented by a vertex • Lines • refer to line segments • Polygons • must be simple, convex, and planar • Rectangles • glRectf(x1, y1, x2, y2); • glRectfv(*pt1, *pt2);

  7. Curves and Curved Surface Approximating Curves

  8. Specifying Vertices glVertex2s(2, 3); glVertex3d(0.0, 0.0, 3.14); glVertex4f(2.4, 1.0, -2.2, 2.0); GLdouble v[3] = {1.0, 9.0, 8.0}; glVertex3dv(v);

  9. Drawing Geometric Primitives glBegin(GL_POLYGON); glVertex2f(0.0, 0.0); glVertex2f(4.0, 3.0); glVertex2f(6.0, 1.5); glVertex2f(4.0, 0.0); glEnd();

  10. OpenGL Geometric Primitives • GL_POINTS • GL_LINES • GL_LINE_STRIP • GL_LINE_LOOP • GL_TRIANGLES • GL_TRIANGLE_STRIP • GL_TRIANGLE_FAN • GL_QUADS • GL_QUAD_STRIP • GL_POLYGON

  11. Geometric Primitive Types

  12. Valid Commands between glBegin(), glEnd() • glVertex*() • glColor*(), glIndex*() • glNormal*() • glTexCoord*() • glEdgeFlag*() • glMaterial*() • glArrayElement() • glEvalCoord*(), glEvalPoint*() • glCallList(), glCallLists() • and any C or C++ codes

  13. Basic State Management • glEnable(GL_DEPTH_TEST); • glDisable(GL_FOG) • if (glIsEnabled(GL_FOG)) ... • glGetBooleanv(); • glGetIntegerv(); • glGetFloatv(); • glGetDoublev(GL_CURRENT_COLOR,x); • glGetPointerv();

  14. Point and Line Details glPointSize(2.0); glLineWidth(2.0); glLineStipple(1,0xAAAA); glLineStipple(2,0xAAAA); glEnable(GL_LINE_STIPPLE);

  15. Stippled Lines

  16. Polygon Details • Drawing polygons as points, outlines, or solids glPolygonMode(GL_FRONT, GL_FILL); glPolygonMode(GL_BACK, GL_LINE); glPolygonMode(GL_FRONT_AND_BACK, GL_POINT);

  17. Reversing and Culling Polygon Faces glFrontFace(GL_CCW); glFrontFace(GL_CW); glCullFace(GL_BACK); glCullFace(GL_FRONT); glCullFace(GL_FRONT_AND_BACK);

  18. Stippling Polygons glEnable(GL_POLYGON_STIPPLE); // Define stipple patternflyhere... glPolygonStipple(fly);

  19. Marking Polygon Boundary Edges glPolygonMode(GL_FRONT_AND_BACK, GL_LINE); glBegin(GL_POLYGON); glEdgeFlag(GL_TRUE); glVertex3fv(V0); glEdgeFlag(GL_FALSE); glVertex3fv(V1); glEdgeFlag(GL_TRUE); glVertex3fv(V2); glEnd();

  20. Normal Vectors glBegin (GL_POLYGON); glNormal3fv(n0); glVertex3fv(v0); glNormal3fv(n1); glVertex3fv(v1); glVertex3fv(v2); glEnd(); • Provide Unit Normals! glEnable(GL_NORMALIZE) can be expensive

  21. Y 3 2 7 6 X 0 1 Z 4 5 Example: Drawing a unit cube Static GLfloat vdata[8][3] = { {0.0,0.0,0.0},{1.0,0.0,0.0}, {1.0,1.0,0.0},{0.0,1.0,0.0}, {0.0,0.0,1.0},{1.0,0.0,1.0}, {1.0,1.0,1.0},{0.0,1.0,1.0}}; //global!! Static GLint allIndx[6][4] = { {4,5,6,7},{1,2,6,5},{0,1,5,4}, {0,3,2,1},{0,4,5,4},{2,3,7,6}}; for (i=0; i<6; i++){ glBegin(GL_QUADS); glVertex3fv(&vdata[allIndx[i][0]][0]); glVertex3fv(&vdata[allIndx[i][1]][0]); glVertex3fv(&vdata[allIndx[i][2]][0]); glVertex3fv(&vdata[allIndx[i][3]][0]); glEnd(); }

  22. Vertex Arrays • To reduce the number of function calls • Six sides; eight shared vertices Step 1: Enabling arrays Step 2: Specifying data for the arrays Step 3: Dereferencing and rendering

  23. Step 1: Enabling Arrays glEnableClientState(GL_NORMAL_ARRAY); glEnableClientState(GL_VERTEX_ARRAY); glEnableClientState(GL_COLOR_ARRAY); glEnableClientState(GL_INDEX_ARRAY); glEnableClientState(GL_EDGE_FLAG_ARRAY); glEnableClientState(GL_TEXTURE_COORD_ARRAY); glDisableClientState(GL_NORMAL_ARRAY);

  24. Step 2: Specifying Data for the Arrays • glVertexPointer(size, type, stride, pointer) • glColorPointer(size, type, stride, pointer) • glIndexPointer(type, stride, pointer) • glNormalPointer(type, stride, pointer) • glTexCoordPointer(size, type, stride, pointer) • glEdgeFlagPointer(stride, pointer)

  25. Step 2: Specifying Data for the Arrays • glVertexPointer(size, type, stride, ptr) static GLfloat v[] = { 0.0,0.0,0.0, 1.0,0.0,0.0, 1.0,1.0,0.0, 0.0,1.0,0.0, 0.0,0.0,1.0, 1.0,0.0,1.0, 1.0,1.0,1.0), 0.0,1.0,1.0 }; glVertexPointer(3, GL_FLOAT, 0, v);

  26. Stride static GLfloat intertwined [ ] = { 1.0, 0.2, 1.0, 100.0, 100.0, 0.0, /* ………………………………*/ 0.2, 0.2, 1.0, 200.0, 100.0, 0.0 }; glColorPointer(3, GL_FLOAT, 6*sizeof(GLfloat), intertwined); glVertexPointer(3, GL_FLOAT, 6*sizeof(GLfloat), &intertwined[3]);

  27. Y 3 2 7 6 X 0 1 Z 4 5 Step 3: Dereferencing and Rendering Alternative 1: dereferencing a single array element glVertexPointer(3, GL_FLOAT, 0, v); glBegin(GL_QUADS); glArrayElement(4); glArrayElement(5); glArrayElement(6); glArrayElement(7); … glEnd();

  28. Dereferencing a List of Array Elements glVertexPointer(3, GL_FLOAT, 0, v); Static GLint allIndx[24]={4,5,6,7, 1,2,6,5, 0,1,5,4, 0,3,2,1, 0,4,5,4, 2,3,7,6}; Alternative 2: glBegin(GL_QUADS); for(int i = 0; i < 24; i++) glArrayElement(allIndx[i]); glEnd(); Alternative 3: Better still … glDrawElements(GL_QUADS,24,GL_UNSIGNED_INT,allIndx);

  29. Hints for Building Polygonal Models of Surfaces • Keep polygon orientations consistent. • all clockwise or all counterclockwise • Watch out for non-triangular polygons. • Trade-off between speed and quality. • Avoid T- intersections • There are more… Read the book.

  30. Next … Vectors, Matrices and Homogeneous coordinate system Transformations

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