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Polygon Details

Polygon Details. What is the Polygon?. a polygon is a plane figure specified by a set of three or more coordinate positions, called vertices, that are connected in sequence by straight-Line segments, called the edges or sides of the polygon. Rectangle. Rectangle is a polygon.

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Polygon Details

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  1. Polygon Details

  2. What is the Polygon? • a polygon is a plane figure specified by a set of three or more coordinate positions, called vertices, that are connected in sequence by straight-Line segments, called the edges or sides of the polygon.

  3. Rectangle • Rectangle is a polygon. • OpenGL provides a filled-rectangle drawing primitive, glRect • We need 2 points to define one rectangle. • gl. glRect{sifd}( x1, y1, x2, y2); x1,y1 x2,y2

  4. Rectangle ex. • gl.glColor3f(0,1f,0); • gl.glRecti(100,50,-100,-50); OUTPUT

  5. Polygon • A polygon has two sides -front and back. • If vertices of polygons appear in counterclockwise order called front-facing. • Because OpenGL by default use counterclockwise then also we use counterclockwise to draw polygon. • We bracket each set of vertices between a call to glBegin() and a call to glEnd().

  6. Polygon Ex. gl.glBegin(GL2.GL_POLYGON); gl.glVertex2i(-50,75); gl.glVertex2i(-100,0); gl.glVertex2i(-50,-75); gl.glVertex2i(50,-75); gl.glVertex2i(100,0); gl.glVertex2i(50,75); gl.glEnd(); OUTPUT

  7. Class Activity

  8. Geometric Drawing Primitives

  9. Geometric Drawing Primitives

  10. Polygon face Mode • Each polygon shape contains of two faces, Front and Back. • By default both of them are Fill • The PolygonModeuse to control the polygon faces such as (Fill, Line or Point). Fill Mode Line Mode Point Mode

  11. Polygon face Mode gl.glPolygonMode(GL2.GL_FRONT,GL2.GL_FILL); gl.glPolygonMode(GL2.GL_BACK,GL2.GL_LINE); gl.glPolygonMode(GL2.GL_FRONT_AND_BACK, GL2.GL_POINT); • To change the face of polygon form Front to Back or Back to Front we use gl.glFrontFace(GL2.GL_CW); (ClockWise) gl.glFrontFace(GL2.GL_CCW); (CounterClockWise)

  12. Polygon face Mode Ex. gl.glPolygonMode(GL2.GL_FRONT,GL2.GL_FILL); gl.glPolygonMode(GL2.GL_BACK,GL2.GL_LINE); gl.glFrontFace(GL2.GL_CW); gl.glBegin(GL2.GL_POLYGON); gl.glVertex2i(0,100); gl.glVertex2i(-75,0); gl.glVertex2i(0,-100); gl.glVertex2i(75,0); gl.glEnd(); OUTPUT

  13. Class Activity

  14. PolygonOffset • If you want to highlight the edges of a solid object, you might draw the object with polygon mode set to GL_FILL, and then draw it again, but in a different color and with the polygon mode set to GL_LINE • The problem: Depth values are not the same • This undesirable effect can be eliminated by using polygon offset, which adds an appropriate offset to force coincident z values apart, separating a polygon edge from its highlighting line

  15. PolygonOffset

  16. PolygonOffset

  17. Culling Polygon Faces • To discard front-or back-facing polygons, use the command glCullFace() and enable culling with glEnable(). • We can discard GL_FRONT,GL_BACK or both faces GL_FRONT_AND_BACK. • Culling enabled using glEnable() with GL_CULL_FACE, and disabled using glDisable()

  18. Culling Polygon Faces Ex. • gl.glPolygonMode(GL2.GL_FRONT,GL2.GL_FILL); • gl.glPolygonMode(GL2.GL_BACK,GL2.GL_LINE); • gl.glFrontFace(GL2.GL_CCW); • gl.glCullFace(GL2.GL_FRONT); • gl.glEnable(GL2.GL_CULL_FACE); • gl.glBegin(GL2.GL_POLYGON); • gl.glVertex2i(0,100); • gl.glVertex2i(-75,-50); • gl.glVertex2i(75,-50); • gl.glEnd(); • gl.glDisable(GL2.GL_CULL_FACE);

  19. Discarding Boundary Edges • gl.glEdgeFlag(boolean flag) use to discard the boundary edges of polygon, this method only use with LINE mode of polygon. • used between glBegin() andglEnd() . • boolean flag can be true or false if flag is false then all edges after the glEdgeFlag() command is discarded . • When we want to disable discarding edges we change the flag to true.

  20. EdgeFlag() Ex. • gl.glPolygonMode(GL2.GL_FRONT,GL2.GL_LINE); • gl.glPolygonMode(GL2.GL_BACK,GL2.GL_FILL); • gl.glFrontFace(GL2.GL_CCW); • gl.glBegin(GL2.GL_POLYGON); gl.glEdgeFlag(false); • gl.glVertex2i(100,50); • gl.glEdgeFlag(true); • gl.glVertex2i(-75,50); • gl.glEdgeFlag(false); • gl.glVertex2i(-100,-50); • gl.glEdgeFlag(true); • gl.glVertex2i(75,-50); gl.glEnd();

  21. HomeWork

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