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robmcal
Jul 13, 2009
=OpenGL 3D Graphics in Liberty BASIC= ==Lesson Five: Complex Surfaces== //by Robert McAllister// OpenGL makes it easy to create complex surfaces such as line strips, triangle strips and quad strips. A strip uses the last set of coordinates of one object as the first set for the next object. By far the easiest to understand is the Line Strip. The comments after each vertex explain it best. [[code format="vbnet"]] ' a line strip CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ CALL glColor4fv 1 , 0 , 0 , 1 CALL glBegin GL.LINE.STRIP CALL glVertex 0 , 0 , 0 'start of first line CALL glVertex 1 , 1 , 0 'end of first line, start of second line CALL glVertex .5 , 1.5 , 0 'end of second line, start of third line CALL glVertex -1 , .5 , 0 'end of third line, start of fourth line CALL glVertex -1.5 , 1.5 , 0 'end of fourth line CALL glEnd CALL RefreshView WAIT [[code]] For a triangle strip the first three vertices describe the first triangle. Then the fourth vertex along with the previous two describe the next triangle, and so on. Since I couldn't make up my mind which color to use for the first triangle, this snippet also demonstrates how to convert normal RGB values for use with OpenGL. [[code format="vbnet"]] ' a triangle strip CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ COLORDIALOG "", chosen$ red = val(word$(chosen$,1))/255 green = val(word$(chosen$,2))/255 blue = val(word$(chosen$,3))/255 CALL glColor4fv red , green , blue , 1 CALL glBegin GL.TRIANGLE.STRIP CALL glVertex -1 , -1 , 0 'first set triangle coordinates CALL glVertex -1 , 1 , 0 'second set triangle coordinates CALL glVertex 1 , 1 , 0 'third set triangle coordinates CALL glColor4fv 1 , 1 , 0 , 1'change color to make it easier to tell triangles apart CALL glVertex .5 , 1.5 , 0 'coordinate for second triangle, along with the previous two CALL glColor4fv 0 , 0 , 0 , 1 CALL glVertex 2 , 1.25 , 0 'coordinate for third triangle, along with the previous two CALL glEnd CALL RefreshView WAIT [[code]] The same principle applies for quad strips. [[code format="vbnet"]] ' a quad strip CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ CALL glColor4fv 1 , 0 , 1 , 1 CALL glBegin GL.QUAD.STRIP 'coordinates for first quad CALL glVertex -.5 , -.5 , 0 CALL glVertex 0 , -.5 , 0 CALL glVertex -.25 , 0 , 0 CALL glVertex .25 , 0 , 0 CALL glColor4fv 0 , 0 , 0 , 1 'coordinates for second quad, along with the previous two CALL glVertex .25 , .5 , 0 CALL glVertex .75 , .5 , 0 CALL glColor4fv 0 , 1 , 0 , 1 'coordinates for third quad, along with the previous two CALL glVertex .25 , 1 , 0 CALL glVertex .75 , 1 , 0 CALL glEnd CALL RefreshView WAIT [[code]] OpenGL also has the ability to create polygons, shapes with 3 or more sides. There is no practical limit to the number of sides in a polygon but there shouldn't be any concave areas (indentations). [[code format="vbnet"]] ' polygon GL.POLYGON = 9 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ CALL glColor4fv 0 , 0 , 1 , 1 CALL glBegin GL.POLYGON CALL glVertex -1.5 , 0 , 0 CALL glVertex -.3 , 1.1 , 0 CALL glVertex 1.6 , .3 , 0 CALL glVertex 1.5 , -1.3 , 0 CALL glVertex -.7 , -1.4 , 0 CALL glEnd CALL RefreshView WAIT [[code]] In the next lesson we will start"[[OpenGL3D_5|Creating"[[OpenGL3D_6|Creating shapes]]"