Thanks very much for the idea! I've given a go at implementing it, using two functions from the archive by RaGR:
*edit* Well .. it is usable, but it seems somewhat slow. If someone has a faster and/or better method for this, I would still like to know.
Open in online IDEGraphics3D 800, 600, 0, 2 SetBuffer BackBuffer() ;create plane plane = CreatePlane() TurnEntity plane, -90, 0, 0 EntityPickMode plane, 2 EntityAlpha plane, 0.2 ;create camera cam = CreateCamera() MoveEntity cam, 0, 0, -15 ;create red dot sp = CreateSphere() ScaleEntity sp, 0.1, 0.1, 0.1 EntityColor sp, 255, 0, 0 EntityFX sp, 1 ;create mesh with 1 triangle mesh = CreateMesh() surf = CreateSurface(mesh) ;generate triangle For i = 2 To 0 Step -1 h = i * 120 x# = Cos(h)*(5+i) y# = Sin(h)*(5+i) TFormPoint x, 0, y, plane, 0 AddVertex surf, TFormedX(), TFormedY(), TFormedZ() Next AddTriangle surf, 0, 1, 2 ;read triangle coords x0# = VertexX(surf, 0) y0# = VertexY(surf, 0) z0# = VertexZ(surf, 0) x1# = VertexX(surf, 1) y1# = VertexY(surf, 1) z1# = VertexZ(surf, 1) x2# = VertexX(surf, 2) y2# = VertexY(surf, 2) z2# = VertexZ(surf, 2) ;main loop Repeat ;keys If KeyHit(200) Then TranslateEntity plane, 0, 0, -1 If KeyHit(208) Then TranslateEntity plane, 0, 0, 1 If KeyHit(57) Then PositionEntity plane, 0, 0, 0 RenderWorld() Text 0, 0, "use cursor up/dn and space to move plane along z axis" Text 0, 20, "triangle will turn red if picked dot is inside triangle" ;draw outlines CameraProject cam, x0, y0, z0 gx0# = ProjectedX() gy0# = ProjectedY() CameraProject cam, x1, y1, z1 gx1# = ProjectedX() gy1# = ProjectedY() CameraProject cam, x2, y2, z2 gx2# = ProjectedX() gy2# = ProjectedY() Line gx0,gy0,gx1,gy1 Line gx1,gy1,gx2,gy2 Line gx2,gy2,gx0,gy0 ;pick 3d mouse position CameraPick cam, MouseX(), MouseY() PositionEntity sp, PickedX(), PickedY(), PickedZ() ;determine if point is in triangle test = pointintriangle(PickedX(),PickedY(),PickedZ(), x0,y0,z0,x1,y1,z1,x2,y2,z2) If test Then EntityColor mesh,255,0,0 Else EntityColor mesh,0,255,0 End If Flip Until KeyHit(1) End Function PointInTriangle(Px#, Py#, Pz#, x0#,y0#,z0#, x1#,y1#,z1#, x2#,y2#,z2#) ;determine min/max x/y/z values nx1# = findmin(x0,x1,x2) nx2# = findmax(x0,x1,x2) ny1# = findmin(y0,y1,y2) ny2# = findmax(y0,y1,y2) nz1# = findmin(z0,z1,z2) nz2# = findmax(z0,z1,z2) ;with/height/depth nxx# = nx2 - nx1 nyy# = ny2 - ny1 nzz# = nz2 - nz1 ;determine which angle to drop ;;;If (nzz < nxx) And (nzz < nyy) Then axis = 1 ;xy ->drop z If (nyy < nzz) And (nyy < nxx) Then axis = 2 ;xz ->drop y If (nxx < nyy) And (nxx < nzz) Then axis = 3 ;yz ->drop x Select axis Case 1 test = PointInTriangle2D(px,py,x0,y0,x1,y1,x2,y2) ;inside 2d tri tstz# = PointHeightOnTri(px,py,x0,z0,y0,x1,z1,y1,x2,z2,y2) ;z value of triangle tstc# = pz# ;z value of point Case 2 test = PointInTriangle2D(px,pz,x0,z0,x1,z1,x2,z2) ;inside 2d tri tstz# = PointHeightOnTri(px,pz,x0,y0,z0,x1,y1,z1,x2,y2,z2) ;z value of triangle tstc# = py# ;z value of point Case 3 test = PointInTriangle2D(py,pz,y0,z0,y1,z1,y2,z2) ;inside 2d tri tstz# = PointHeightOnTri(py,pz,y0,x0,z0,y1,x1,z1,y2,x2,z2) ;z value of triangle tstc# = px# ;z value of point End Select ;calculate difference between z triangle and z point dif# = Abs(tstz#-tstc#) ;if below epsilon If dif# < 0.001 Then Return test ;else no intersection Return False End Function ;find min. val of abc Function findmin#(a#,b#,c#) If b < a Then a = b If c < a Then a = c Return a End Function ;find max. val of abc Function findmax#(a#,b#,c#) If b > a Then a = b If c > a Then a = c Return a End Function ; by RaGR (Ralph G. Roeske). Function PointHeightOnTri#(px#,pz#, x1#,y1#,z1#,x2#,y2#,z2#,x3#,y3#,z3#) e#=(x2-x1)*(z3-z1)-(x3-x1)*(z2-z1) e=1.0/e e1#=e*((x2-px)*(z3-pz)-(x3-px)*(z2-pz)) e2#=e*((x3-px)*(z1-pz)-(x1-px)*(z3-pz)) e3#=e*((x1-px)*(z2-pz)-(x2-px)*(z1-pz)) Return e1*y1+e2*y2+e3*y3 End Function ; by RaGR (Ralph G. Roeske). Function PointInTriangle2D(px#,pz#, x1#,z1#,x2#,z2#,x3#,z3#) Local bc#,ca#,ab#,ap#,bp#,cp#,abc# bc# = x2*z3 - z2*x3 ca# = x3*z1 - z3*x1 ab# = x1*z2 - z1*x2 ap# = x1*pz - z1*px bp# = x2*pz - z2*px cp# = x3*pz - z3*px abc# = Sgn(bc + ca + ab) If (abc*(bc-bp+cp)=>0) And (abc*(ca-cp+ap)=>0) And (abc*(ab-ap+bp)=>0) Return True End Function
*edit* Well .. it is usable, but it seems somewhat slow. If someone has a faster and/or better method for this, I would still like to know.