CSC 461: Lecture 6 1 CSC461 Lecture 6: 2D Programming in OpenGL Objectives: Fundamental OpenGL primitives Attributes Viewport

CSC 461: Lecture 6CSC 461: Lecture 6 11

CSC461 Lecture 6:

2D Programming in OpenGL

Objectives:Objectives:

Fundamental OpenGL primitivesFundamental OpenGL primitives AttributesAttributes ViewportViewport


OpenGL PrimitivesOpenGL Primitives

GL_QUAD_STRIPGL_QUAD_STRIP

GL_POLYGONGL_POLYGON

GL_TRIANGLE_STRIPGL_TRIANGLE_STRIP GL_TRIANGLE_FANGL_TRIANGLE_FAN

GL_POINTSGL_POINTS

GL_LINESGL_LINES

GL_LINE_LOOPGL_LINE_LOOP

GL_LINE_STRIPGL_LINE_STRIP

GL_TRIANGLESGL_TRIANGLES


Polygon IssuesPolygon IssuesOpenGL will only display polygons correctly that areOpenGL will only display polygons correctly that are

SimpleSimple: edges cannot cross: edges cannot crossConvexConvex: All points on line segment between two : All points on line segment between two

points in a polygon are also in the polygonpoints in a polygon are also in the polygonFlatFlat: all vertices are in the same plane: all vertices are in the same plane

User program must check if above trueUser program must check if above trueTriangles satisfy all conditionsTriangles satisfy all conditions

nonsimple polygon nonconvex polygon


Drawing a SphereDrawing a Sphere

Sphere description:Sphere description:X(X(θθ,,ФФ)=sin)=sinθθcoscosФФy(y(θθ,,ФФ)=cos)=cosθθcoscosФФz(z(θθ,,ФФ)=sin)=sinФФ

ApproximationApproximation A set of polygonsA set of polygons Efficiently usingEfficiently using

• Quad stripsQuad strips• Triangle stripsTriangle strips


Drawing a Sphere Drawing a Sphere (Cont.)(Cont.)

Fix Fix θθ and change and change ФФ get circles of get circles of constant longitudeconstant longitude

Fix Fix ФФ and change and change θθ get circles of get circles of constant latitudeconstant latitude

Generate points at fixed increments of Generate points at fixed increments of θθ and and ФФ quadrilaterals quadrilaterals

Color corresponds to increments of 20 Color corresponds to increments of 20 degrees in degrees in θθ and 20 degrees in and 20 degrees in ФФ


Source code: Drawing quadrilateralsSource code: Drawing quadrilateralsc=M_PI/180;c=M_PI/180;for (phi=-80.0; phi<=80.0; phi+=20.0) {for (phi=-80.0; phi<=80.0; phi+=20.0) {

glBegin(GL_QUAD_STRIP);glBegin(GL_QUAD_STRIP);for (theta=-180.0; theta<=180.0; theta+=20.0) {for (theta=-180.0; theta<=180.0; theta+=20.0) {

x = sin(c*theta)*cos(c.phi);x = sin(c*theta)*cos(c.phi);y = cos(c*theta)*cos(c*phi);y = cos(c*theta)*cos(c*phi);z = sin(c*theta);z = sin(c*theta);glVertex3d(x,y,x);glVertex3d(x,y,x);

x = sin(c*theta)*cos(c*(phi+20.0));x = sin(c*theta)*cos(c*(phi+20.0));y = cos(c*theta)*cos(c*(phi+20.0));y = cos(c*theta)*cos(c*(phi+20.0));z = sin(c*(phi+20.0));z = sin(c*(phi+20.0));glVertex3d(x,y,z);glVertex3d(x,y,z);

}}glEnd();glEnd();


Drawing Drawing polespoles

c=M_PI/180.0;c=M_PI/180.0;glBegin(GL_TRANGLE_FAN);glBegin(GL_TRANGLE_FAN);

glVertex3d(x,y,z);glVertex3d(x,y,z);z=sin(c*80.0);z=sin(c*80.0);for (theta=-180.0; theta<=180.0; theta+=20) {for (theta=-180.0; theta<=180.0; theta+=20) {

x = sin(c*theta)*cos(c.80.0);x = sin(c*theta)*cos(c.80.0);y = cos(c*theta)*cos(c*80.0);y = cos(c*theta)*cos(c*80.0);glVertex3d(x,y,z);glVertex3d(x,y,z);

}}

glEnd()glEnd()X=y=0;X=y=0;z=-1;z=-1;glBegin(GL_TRIANGLE_FAN);glBegin(GL_TRIANGLE_FAN);

glVertex3d(x,y,z);glVertex3d(x,y,z);z=-sin(c*80.0);z=-sin(c*80.0);for (theta=-180.0; theta<=180.0; theta+=20) {for (theta=-180.0; theta<=180.0; theta+=20) {

x = sin(c*theta)*cos(c.80.0);x = sin(c*theta)*cos(c.80.0);y = cos(c*theta)*cos(c*80.0);y = cos(c*theta)*cos(c*80.0);glVertex3d(x,y,z);glVertex3d(x,y,z);

}}

glEnd();glEnd();


AttributesAttributes

Attributes are part of the OpenGL and determine Attributes are part of the OpenGL and determine the appearance of objectsthe appearance of objects

Color (points, lines, polygons)Color (points, lines, polygons) Size and width (points, lines)Size and width (points, lines) Stipple pattern (lines, polygons)Stipple pattern (lines, polygons) Polygon modePolygon mode

• Display as filled: solid color or stipple patternDisplay as filled: solid color or stipple pattern• Display edgesDisplay edges


RGB colorRGB colorEach color component stored separately in Each color component stored separately in the frame bufferthe frame buffer

Usually 8 bits per component in bufferUsually 8 bits per component in bufferNote in Note in glColor3fglColor3f the color values range the color values range from 0.0 (none) to 1.0 (all), while in from 0.0 (none) to 1.0 (all), while in glColor3ubglColor3ub the values range from 0 to 255 the values range from 0 to 255


Indexed ColorIndexed ColorColors are indices into tables of RGB valuesColors are indices into tables of RGB valuesSelecting color from tableSelecting color from table

glIndexi(glIndexi(elementelement))Setting color into table: Setting color into table:

glutSetColor(int color, GLfloat r, GLfloat b, GLfloat g)glutSetColor(int color, GLfloat r, GLfloat b, GLfloat g)Requires less memoryRequires less memory

indices usually 8 bitsindices usually 8 bits not as important nownot as important now

• Memory inexpensiveMemory inexpensive• Need more colors for shadingNeed more colors for shading


Color and StateColor and StateThe color as set by The color as set by glColorglColor becomes part of becomes part of

the state and will be used until changedthe state and will be used until changedColors and other attributes are not part of the Colors and other attributes are not part of the

object but are assigned when the object is object but are assigned when the object is renderedrendered

We can create conceptual We can create conceptual vertexvertex colors colors by code by code such assuch as

glColor(…)glColor(…) glVertex(…)glVertex(…) glColor(…)glColor(…) glVertex(…)glVertex(…)


Smooth ColorSmooth ColorDefault is Default is smoothsmooth shading shading

OpenGL interpolates vertex colors across visible OpenGL interpolates vertex colors across visible polygonspolygons

Alternative is Alternative is flat shadingflat shadingColor of first vertex Color of first vertex

determines fill colordetermines fill colorCodeCode

glShadeModel(GL_SMOOTH)glShadeModel(GL_SMOOTH) glShadeModel(GL_FLAT)glShadeModel(GL_FLAT)


ViewportsViewportsDo not have use the entire window for the Do not have use the entire window for the image: image: glViewport(x,y,w,h)glViewport(x,y,w,h)

Values in pixels (screen coordinates)Values in pixels (screen coordinates)

Documents

CSC 461: Lecture 6 1 CSC461 Lecture 6: 2D Programming in OpenGL Objectives: Fundamental OpenGL primitives Attributes Viewport