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 Specify normal vectors Use vertex arrays Save and restore state variables

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);

Specifying a Color Pseudocode glColor3f(1.0, 0.0, 0.0); 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);

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.

glutReshapeFunc(reshape) Example code void reshape (int w, int h) { glViewport (0, 0, w, h); glMatixMode (GL_PROJECTION); glLoadIdentity(); glOrtho2D(0, w, 0, h); }

What are Points, Lines, and Polygon? 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);

Curves and Curved Surface Approximating Curves

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);

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();

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

Geometric Primitive Types

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

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

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

Stippled Lines

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);

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

Stippling Polygons glEnable(GL_POLYGON_STIPPLE); // Define stipple pattern fly here... glPolygonStipple(fly);

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

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

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(); } 7 3 2 1 5 6 4 X Y Z

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

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);

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)

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);

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]);

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(); 7 3 2 1 5 6 4 X Y Z

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);

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.

Next … Vectors, Matrices and Homogeneous coordinate system Transformations