Re: Light Direction relative to World Matrix

From: Kevin Spencer (kspencer_at_takempis.com)
Date: 09/16/04 

Date: Thu, 16 Sep 2004 10:04:21 -0400

Thanks Mark! I kind of suspected that might be the track to move forward on.
Your explanation is exactly what I needed! 

-- 
Thanks again,
Kevin Spencer
.Net Developer
Microsoft MVP
I get paid good money to
solve puzzles for a living
"Mark Davis [MSFT]" <markda@online.microsoft.com> wrote in message
news:u#CAFf4mEHA.740@cpmsftngxa06.phx.gbl...
> Hi Kevin...
>
> If you rotate the world and keep the light in a fixed position, the shadow
> movement would be expected.
>
> I believe that your world matrix would need to be an identity matrix.
>
> It's usual practice to move the camera and derive the view matrix
transform
> from the camera position, orientation and lookat vectors. The world stays
> fixed and the camera (and view) move around in the world space.
>
> You can then move the light in an arc (always looking at 0,0,0) and get
the
> effect you are looking for. On each frame, calculate the new position of
> the light (x=x*cos(angle)-y*sin(angle),y=y*cos(angle)-x*sin(angle)) and
the
> new direction vector (lookat-pos). Don't trust my math, though:O) Of
> course, you probably also want to vary the intensity/color of the light to
> achieve a more realistic sunset/sunrise.
>
> Mark Davis (MSFT)
> --
> This posting is provided "AS IS" with no warranties, and confers no
rights.
> Use of included information and code is subject to the terms specified at
> http://www.microsoft.com/info/cpyright.htm.
> --------------------
> >Reply-To: "Kevin Spencer" <kspencer@takempis.com>
> >From: "Kevin Spencer" <kspencer@takempis.com>
> >Subject: Light Direction relative to World Matrix
> >Date: Wed, 15 Sep 2004 09:24:34 -0400
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> >
> >Still working on my terrain-modelling app, and am now working on
> positioning
> >and moving the camera, world matix, and lights. The problem I'm having:
> >
> >I position the camera directly above the 0,0,0 location in the center of
my
> >triangle strip, and point it down, making my "up" direction in the Z
> >direction. I can then rotate the world matrix to rotate my camera. I use
an
> >Altitude field to set the vertical (Y) position (LookFrom) of the camera,
> >and a CameraRotation field to rotate the world matrix, thus rotating my
> >camera. I use a Tilt field to change the Z-component of my LookAt vector,
> >and thus enable the camera to both rotate and tilt up and down. However,
I
> >find that when I change the X or Z component of the LookAt vector, it
> >changes the direction of the light (directional) relative to the world
> >matrix. This means that when I rotate the camera, the shadows on the
> terrain
> >change, which they shouldn't. The light direction should remain constant
> >relative to the world matrix. I can set X and Z to (0,0) to position the
> >camera exactly in the center of the triangle strip (it's a spiralling
> >square), and of course, the shadows don't change. However, I want to be
> able
> >to simulate different times of day by setting the angle of the light. I
> >spent a long time last night trying to find the solution.
> >
> >I believe my math skills are not well-developed enough. I know that the
> >directional light direction is a vector, but I dont' know how to rotate
it
> >relative to the world matrix. I can rotate a matrix, but how do you
rotate
> a
> >vector? Or am I barking up the wrong tree here? Should I be rotating my
> >camera instead of the world matrix?
> >
> >Any help?
> >
> >-- 
> >TIA,
> >Kevin Spencer
> >.Net Developer
> >Microsoft MVP
> >I get paid good money to
> >solve puzzles for a living
> >
> >
> >
>

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