OpenGL 3D Graphics in Liberty BASIC

Lesson Seven: Texture mapping

by Robert McAllister


Texture mapping applies an image to a set of coordinates instead of a color. The program is setup to use 24-bit bitmaps, though other formats can be used. The bitmaps must be sized to a power of 2 in order to work properly with OpenGL, 2x2, 4x4, 8x8, 16x16, etc.... In my own experience 256x256 gives pretty good results.

In this first sample, the call to ‘CreateBMPTexture’ loads the bitmap and gives it the name “1”. ‘glEnable GL.TEXTURE.2D’ tells OpenGL that we will be working with textures. And ‘glBindTexture GL.TEXTURE.2D , Texture’ sets texture 1 as the one we will be applying to the triangle.

In the TextureVertex calls, the first two values specify the x,y position of the bitmap to use. The remaining three are the X,Y,Z coordinates of the triangle. For the bitmap coordinates, (0,0) is the bottom left, (0,1) is the top left, (1,1) is the top right and (1,0)is the bottom right. The photo below shows the x,y bitmap coordinates used for the triangle.

 'triangle with partial bitmap texture 
 CALL glClearColor .9 , .9 , .9 , 1 
 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ 
 
 Texture = 1 
 CALL CreateBMPTexture "OpenGL_Logo.bmp" , Texture 
 CALL glEnable GL.TEXTURE.2D 
 CALL glBindTexture GL.TEXTURE.2D , Texture 
 
 CALL glBegin GL.TRIANGLES 
 CALL TextureVertex .1 , .1 , -1 , -1 , 0 
 CALL TextureVertex .5 , .9 , 0 , 1 , 0 
 CALL TextureVertex .9 , .1 , 1 , -1 , 0 
 CALL glEnd 
 
 CALL RefreshView 
 
 CALL glDisable GL.TEXTURE.2D 
 
 WAIT 

These next two examples build a textured cube and a textured cylinder with the normals applied. The cylinder example shows how to apply a texture to a Quad-strip.

 'cube with bitmap texture 
 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ 
 CALL glClearColor .8 , .8 , .8 , 1 
 Texture = 1 
 CALL CreateBMPTexture "OpenGL_Logo.bmp" , Texture ' load bitmap and put in Texture #1 
 
 Width = 1.5 
 Height = 1.5 
 Depth = 1.5 
 CubeCenterX = 0 
 CubeCenterY = 0 
 CubeCenterZ = 0 
 CALL glGenLists 1 
 CALL glNewList 1 , 4865 
 CALL BuildTextureCube Width , Height , Depth , CubeCenterX , CubeCenterY , CubeCenterZ , Texture 
 CALL glEndList 
 
 FOR a = 1 TO 360 
 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ 
 CALL glRotatef a , 1 , 0 , 0 
 'CALL glRotatef a , 0 , 1 , 0 
 CALL glRotatef a , 0 , 0 , 1 
 CALL glCallList 1 
 
 CALL RefreshView 
 CALL Pause 15 
 NEXT a 
 
 WAIT 
 
 
 
 SUB BuildTextureCube W , H , D , cX , cY , cZ , tex 
 GL.QUADS=7 
 GL.TEXTURE.2D = 3553 
 CALL glEnable GL.TEXTURE.2D 
 CALL glBindTexture GL.TEXTURE.2D , tex 
 tc=1 ' change to a value greater than 1 to have the texture tiled 
 'front 
 CALL glBegin GL.QUADS 
 CALL glNormal cX-(W/2) , cY-(H/2) , cZ+(D/2) , cX-(W/2) , cY+(H/2) , cZ+(D/2) , cX+(W/2) , cY+(H/2) , cZ+(D/2) cX-(W/2),cY-(H/2),cZ+(D/2) , cX-(W/2),cY+(H/2),cZ+(D/2) , cX+(W/2),cY+(H/2),cZ+(D/2) 
 CALL TextureVertex 0 , tc , cX-(W/2) , cY+(H/2) , cZ+(D/2) 
 CALL TextureVertex tc , tc , cX+(W/2) , cY+(H/2) , cZ+(D/2) 
 CALL TextureVertex tc , 0 , cX+(W/2) , cY-(H/2) , cZ+(D/2) 
 CALL TextureVertex 0 , 0 , cX-(W/2) , cY-(H/2) , cZ+(D/2) 
 CALL glEnd 
 'back 
 CALL glBegin GL.QUADS 
 CALL glNormal cX+(W/2) , cY+(H/2) , cZ-(D/2) , cX-(W/2) , cY+(H/2) , cZ-(D/2) , cX-(W/2) , cY-(H/2) , cZ-(D/2) cX+(W/2),cY+(H/2),cZ-(D/2) , cX-(W/2),cY+(H/2),cZ-(D/2) , cX-(W/2),cY-(H/2),cZ-(D/2) 
 CALL TextureVertex 0 , tc , cX+(W/2) , cY+(H/2) , cZ-(D/2) 
 CALL TextureVertex tc , tc , cX-(W/2) , cY+(H/2) , cZ-(D/2) 
 CALL TextureVertex tc , 0 , cX-(W/2) , cY-(H/2) , cZ-(D/2) 
 CALL TextureVertex 0 , 0 , cX+(W/2) , cY-(H/2) , cZ-(D/2) 
 CALL glEnd 
 'left 
 CALL glBegin GL.QUADS 
 CALL glNormal cX-(W/2) , cY+(H/2) , cZ-(D/2) , cX-(W/2) , cY+(H/2) , cZ+(D/2) , cX-(W/2) , cY-(H/2) , cZ+(D/2) cX-(W/2),cY+(H/2),cZ-(D/2) , cX-(W/2),cY+(H/2),cZ+(D/2) , cX-(W/2),cY-(H/2),cZ+(D/2) 
 CALL TextureVertex 0 , tc , cX-(W/2) , cY+(H/2) , cZ-(D/2) 
 CALL TextureVertex tc , tc , cX-(W/2) , cY+(H/2) , cZ+(D/2) 
 CALL TextureVertex tc , 0 , cX-(W/2) , cY-(H/2) , cZ+(D/2) 
 CALL TextureVertex 0 , 0 , cX-(W/2) , cY-(H/2) , cZ-(D/2) 
 CALL glEnd 
 'right 
 CALL glBegin GL.QUADS 
 CALL glNormal cX+(W/2) , cY+(H/2) , cZ+(D/2) , cX+(W/2) , cY+(H/2) , cZ-(D/2) , cX+(W/2) , cY-(H/2) , cZ-(D/2) cX+(W/2),cY+(H/2),cZ+(D/2) , cX+(W/2),cY+(H/2),cZ-(D/2) , cX+(W/2),cY-(H/2),cZ-(D/2) 
 CALL TextureVertex 0 , tc , cX+(W/2) , cY+(H/2) , cZ+(D/2) 
 CALL TextureVertex tc , tc , cX+(W/2) , cY+(H/2) , cZ-(D/2) 
 CALL TextureVertex tc , 0 , cX+(W/2) , cY-(H/2) , cZ-(D/2) 
 CALL TextureVertex 0 , 0 , cX+(W/2) , cY-(H/2) , cZ+(D/2) 
 CALL glEnd 
 'top 
 CALL glBegin GL.QUADS 
 CALL glNormal cX-(W/2) , cY+(H/2) , cZ+(D/2) , cX-(W/2) , cY+(H/2) , cZ-(D/2) , cX+(W/2) , cY+(H/2) , cZ-(D/2) cX-(W/2),cY+(H/2),cZ+(D/2) , cX-(W/2),cY+(H/2),cZ-(D/2) , cX+(W/2),cY+(H/2),cZ-(D/2) 
 CALL TextureVertex 0 , tc , cX-(W/2) , cY+(H/2) , cZ+(D/2) 
 CALL TextureVertex tc , tc , cX-(W/2) , cY+(H/2) , cZ-(D/2) 
 CALL TextureVertex tc , 0 , cX+(W/2) , cY+(H/2) , cZ-(D/2) 
 CALL TextureVertex 0 , 0 , cX+(W/2) , cY+(H/2) , cZ+(D/2) 
 CALL glEnd 
 'bottom 
 CALL glBegin GL.QUADS 
 CALL glNormal cX-(W/2) , cY-(H/2) , cZ+(D/2) , cX+(W/2) , cY-(H/2) , cZ+(D/2) , cX+(W/2) , cY-(H/2) , cZ-(D/2) cX-(W/2),cY-(H/2),cZ+(D/2) , cX+(W/2),cY-(H/2),cZ+(D/2) , cX+(W/2),cY-(H/2),cZ-(D/2) 
 CALL TextureVertex 0 , tc , cX-(W/2) , cY-(H/2) , cZ+(D/2) 
 CALL TextureVertex tc , tc , cX+(W/2) , cY-(H/2) , cZ+(D/2) 
 CALL TextureVertex tc , 0 , cX+(W/2) , cY-(H/2) , cZ-(D/2) 
 CALL TextureVertex 0 , 0 , cX-(W/2) , cY-(H/2) , cZ-(D/2) 
 CALL glEnd 
 CALL glDisable GL.TEXTURE.2D 
 
 END SUB 

 ' build a textured cylinder 
 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ 
 CALL glClearColor .8 , .8 , .8 , 1 
 Texture = 1 
 CALL CreateBMPTexture "OpenGL_Logo.bmp" , Texture ' load texture and put it in texture #1 
 
 Angle = 110 
 Width = .5 
 Depth = .5 
 Height = 2 
 cX = 0 
 cY = 0 
 cZ = 0 
 Red = .5 
 Green = 0 
 Blue = 0 
 Sides = 200 
 
 CALL glGenLists 1 
 CALL glNewList 1 , 4865 
 CALL BuildTextureCylinder Angle , Width , Depth , Height , cX , cY , cZ , Texture , Sides 
 CALL glEndList 
 
 FOR a = 1 TO 360 
 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ 
 CALL glRotatef a , 1 , 0 , 0 
 CALL glRotatef a , 0 , 1 , 0 
 'CALL glRotatef a , 0 , 0 , 1 
 CALL glCallList 1 
 
 CALL RefreshView 
 CALL Pause 15 
 NEXT a 
 
 
 WAIT 
 
 
 SUB BuildTextureCylinder Angle , W , D , H , cX , cY , cZ , tex , Sides 
 GL.QUAD.STRIP = 8 
 GL.TEXTURE.2D = 3553 
 CALL glEnable GL.TEXTURE.2D 
 CALL glBindTexture GL.TEXTURE.2D , tex 
 PI = 3.14159265 
 
 sin.angle = Sin(Angle * PI / 180) 
 cos.angle = Cos(Angle * PI / 180) 
 
 theta = 0 
 dtheta = 2 * PI / Sides 
 
 TexCoordinate=1 
 
 XOval = W * Cos(theta) 
 YOval = D * Sin(theta) 
 X1 = cX + XOval * cos.angle - YOval * sin.angle 
 Z1 = cZ - XOval * sin.angle - YOval * cos.angle 
 
 ' X2 , Z2 values for the first glNormal call 
 XOval = W * Cos(dtheta) 
 YOval = D * Sin(dtheta) 
 X2 = cX + XOval * cos.angle - YOval * sin.angle 
 Z2 = cZ - XOval * sin.angle - YOval * cos.angle 
 
 CALL glBegin GL.QUAD.STRIP 
 
 WHILE theta < 2 * PI 
 
 CALL glNormal X1 , cY+(H/2) , Z1 , X2 , cY+(H/2) , Z2 , X1 , cY-(H/2) , Z1 
 CALL TextureVertex TexCoordinate , 1 , X1 , cY+(H/2) , Z1 
 CALL TextureVertex TexCoordinate , 0 , X1 , cY-(H/2) , Z1 
 
 theta = theta + dtheta 
 XOval = W * Cos(theta) 
 YOval = D * Sin(theta) 
 X2 = X1 
 Z2 = Z1 
 X1 = cX + XOval * cos.angle + YOval * sin.angle 
 Z1 = cZ - XOval * sin.angle + YOval * cos.angle 
 
 TexCoordinate = TexCoordinate - (1/Sides) 
 
 WEND 
 
 CALL glNormal X1 , cY+(H/2) , Z1 , X2 , cY+(H/2) , Z2 , X1 , cY-(H/2) , Z1 
 CALL TextureVertex TexC , 1 , X1 , cY+(H/2) , Z1 
 CALL TextureVertex TexC , 0 , X1 , cY-(H/2) , Z1 
 
 CALL glEnd 
 END SUB 

Next we will be creating "Transparent surfaces and fog"