[Gambas-user] gb3: OpenGL Rotate and Translate logic

[Kevin Fishburne]

On 05/25/2012 10:58 AM, tommyline at ...2340... wrote:
> Hi Kevin,
>
> I attach simple project that uses glu.lookat and rotates around a point on the floor. Try it and tell if it's what you're looking for.
>
> Tomek

Yes, that's what I'm looking for. I boiled that program down to the up 
vector for Glu.LookAt() being computed by using the Sin() and Cos() of 
the angle for two of the vector components while leaving the third 
static. After finding the right values for the up vector I was able to 
make the program render the scene with zoom and rotation, however the 
camera is in the upper-left corner of the screen instead of at its 
center. The code and video that does that is here:

   ' Calculate offsets to center of screen.
   ScreenOffsetX = sWidth / 2 / 128 / Zoom.Current
   ScreenOffsetY = sHeight / 2 / 128 / Zoom.Current
   ' Increment test rotation.
   test += 0.01
   ' Assign values to up vector.
   UpVec1 = Sin(test)
   UpVec2 = Cos(test)
   UpVec3 = 0
   ' Scale the matrix.
   Gl.Scalef(128 * Zoom.Current, 128 * Zoom.Current, 1)
   ' Set the camera position and orientation.
   'Glu.LookAt(Camera.WorldX + ScreenOffsetX, Camera.WorldY + 
ScreenOffsetY, 1, Camera.WorldX + ScreenOffsetX, Camera.WorldY + 
ScreenOffsetY, 0, UpVec1, UpVec2, UpVec3)
   Glu.LookAt(Camera.WorldX, Camera.WorldY, 1, Camera.WorldX, 
Camera.WorldY, 0, UpVec1, UpVec2, UpVec3)

http://eightvirtues.com/sanctimonia/misc/X%20Marks%20the%20Wrong%20Spot.ogv

Notice in the commented out Glu.LookAt() I have the screen offsets which 
center the scene on my camera (the X rendered on the map is the camera's 
world coordinates). When I include the screen offsets the rotation 
provided by Glu.LookAt() is using some other origin and not my camera. 
It appears as though the origin is the upper-left corner of the screen, 
and looks like this:

http://eightvirtues.com/sanctimonia/misc/Screen%20Offset%20Fail.ogv

I saw several posts stating that using Gl.Ortho() would allow the center 
of the screen to be used as the origin, but my implementations of it had 
no effect other than rendering nothing when the near and far clipping 
values were set to unequal values. Hopefully these details will inspire 
some sort of epiphany or insight, as I'm about to go crazy, haha.

-- 
Kevin Fishburne
Eight Virtues
www: http://sales.eightvirtues.com
e-mail: sales at ...1887...
phone: (770) 853-6271