StPendl
Jul 30, 2009
=OpenGL 3D Graphics in Liberty BASIC= ==Lesson Eight: Transparent Surfaces and Fog== //by Robert McAllister//==Transparent surfaces:=====Transparent surfaces:=== You can use a transparent surface to simulate a window or maybe the surface of some water. Back in Lesson One the alpha value for glColor4fv was briefly mentioned. Here we will use it to control the transparency of a surface. The acceptable values are between 0 & 1, with 0 being completely transparent and 1 being solid. Before this will work a few OpenGL states need to be set. The first is to enable blending. This tells OpenGL that if there is a transparent surface in the scene that it needs to blend the colors wherever it interacts with another surface. The second call needed is to glBlendFunc and it tells OpenGL which method to use when blending the colors. The only thing to keep in mind is that the transparent surface needs to be created last in order to work. [[code format="vbnet"]] 'transparent triangle GL.BLEND = 3042 GL.SRC.ALPHA = 770 GL.ONE.MINUS.SRC.ALPHA = 771 CALL glEnable GL.BLEND CALLDLL #gl , "glBlendFunc" ,_ GL.SRC.ALPHA AS long ,_ GL.ONE.MINUS.SRC.ALPHA AS long ,_ ret AS long FOR a = 1 TO 360 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ CALL glRotatef a , 0 , 1 , 0 'solid black line CALL glBegin GL.LINES CALL glColor4fv 0 , 0 , 0 , 1 CALL glVertex -1 , 0 , -.5 CALL glVertex 1 , 0 , -.5 CALL glEnd 'transparent triangle CALL glBegin GL.TRIANGLES CALL glColor4fv 0 , 0 , 1 , .3 ' make the surface 30 percent solid CALL glVertex -1 , -1 , 0 CALL glVertex 0 , 1 , 0 CALL glVertex 1 , -1 , 0 CALL glEnd CALL RefreshView CALL Pause 10 NEXT a WAIT [[code]]==Fog:=====Fog:=== Fog is an easy to add special effect with OpenGL. It can be used to add depth perception to a scene. The first step is to make a call to glEnable with GL.FOG as the argument. As with all other enabled functions, it can be turned off with a call to glDisable. Next is to set the color of the fog. Again, the values for the individual color should be between 0 & 1. The default values are 0,0,0,0. Then we need to set the density of the fog. This value should also be between 0 & 1, with the default value being 1. [[code format="vbnet"]] 'fog GL.FOG = 2912 GL.FOG.DENSITY = 2914 GL.FOG.COLOR = 2918 CALL glEnable GL.FOG STRUCT fogColor , red AS ulong , green AS ulong , blue AS ulong , alpha AS ulong fogColor.red.struct = R4( .5 ) fogColor.green.struct = R4( .5 ) fogColor.blue.struct = R4( .5 ) fogColor.alpha.struct = R4( 1 ) CALLDLL #gl , "glFogfv" ,_ ' set fog color to gray GL.FOG.COLOR AS long,_ fogColor AS STRUCT ,_ ret AS void density = R4( .25 ) CALLDLL #gl , "glFogf" ,_ ' set the density of the fog GL.FOG.DENSITY AS long,_ density AS ulong ,_ ret AS void FOR a = 1 TO 345 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ CALL glColor4fv 0 , 0 , 0 , 1 CALL glBegin GL.QUADS CALL glVertex -2 , -1 , -2 CALL glVertex -2 , -1 , 2 CALL glVertex 2 , -1 , 2 CALL glVertex 2 , -1 , -2 CALL glEnd CALL glRotatef a , 0 , 1 , 0 CALL glColor4fv 0 , 0 , 1 , 1 CALL glBegin GL.TRIANGLES CALL glVertex -1 , -1 , -2 CALL glVertex 0 , 1 , -2 CALL glVertex 1 , -1 , -2 CALL glEnd CALL glBegin GL.TRIANGLES CALL glVertex -1 , -1 , 0 CALL glVertex 0 , 1 , 0 CALL glVertex 1 , -1 , 0 CALL glEnd CALL glBegin GL.TRIANGLES CALL glVertex -1 , -1 , 2 CALL glVertex 0 , 1 , 2 CALL glVertex 1 , -1 , 2 CALL glEnd CALL RefreshView CALL Pause 15 NEXT a WAIT [[code]] In the next lesson we will dig into "OpenGL calls and argument types"