Hi Matt, Working with vertex/fragment shaders is usually only goes one-way meaning you can upload the parameters to the shader but the shader itself usually cannot send its results back to the calling code (I believe there are extensions to do this but I'm not sure about the driver support for these and I also don't have much experience with them).
If what you want is the world coordinates inside the fragment shader, what you
can do is:
1. Create a new uniform variable in your vertex program so that your C++
code can feed the vertex program the current world matrix (e.g. uniform mat4
world_matrix)
2. Create a new varying variable for the world position in your vertex
and fragment programs so that the vertex program can give the fragment program
the world position it has calculated (e.g. varying vec4 world_pos)
3. Feed the world_matrix uniform the proper matrix in your C++ code via
the glGetUniformLocation*/glUniform* functions (I think the code I sent you
already does this inside SetTransforms())
4. Set the world_pos varying variable inside your vertex program so that
it can be properly read inside the fragment program
So in your vertex program you'll have:
varying vec4 world_pos;
uniform mat4 world_matrix;
...
void main()
{
...
world_pos = world_matrix * gl_Vertex;
...
}
And in the fragment program you can read what worldPos is just by declaring the
same varying variable and using it like an ordinary variable inside the code:
uniform vec4 light_pos; // Some other uniform that you might want to add
varying vec4 world_pos;
...
void main()
{
...
float fDist = distance(world_pos.xyz, light_pos.xyz); // Do
something with world_pos
...
}
Regards,
John
From: [email protected]
[mailto:[email protected]] On Behalf Of Matt Lind
Sent: Thursday, February 07, 2013 11:00 AM
To: [email protected]
Subject: RE: SDK: OGLLight
I'm used to writing mental ray shaders. Mental ray uses different coordinate
spaces for every scene item which means shader code contains many conversion
routines to do simple stuff like diffuse shading.
I'm still getting my feet wet with OpenGL shaders using the Phong shader you
sent me a while back. As a learning exercise I'm attempting to extend it to
support point and spot lights. I initially experienced funky lighting when the
point light was inserted into a hierarchy and moved around the scene, but then
I caught my mistake and removed the computations that consider light
orientation.
One sticking point I have right now is how to get the world coordinate of the
current shaded pixel. I see it defined in the vertex shader source (GLSL), but
how do I read that into the C++ code so I can work with it further? All the
calls I see in the C++ code push values from C++ into the vertex/fragment
shaders. I need to go the other direction.
Thanks,
Matt
From: [email protected]
[mailto:[email protected]] On Behalf Of John Voltaire
Tensuan
Sent: Wednesday, February 06, 2013 6:46 PM
To: [email protected]
Subject: RE: SDK: OGLLight
Hi Matt,
Coordinates returned by OGLLight should all be in global space. Are you seeing
any weird behavior with it?
Thanks,
John
From:
[email protected]<mailto:[email protected]>
[mailto:[email protected]] On Behalf Of Alok
Sent: Thursday, February 07, 2013 7:37 AM
To: [email protected]<mailto:[email protected]>
Subject: Re: SDK: OGLLight
Should be global, but again not sure
[cid:[email protected]]
On 06/02/2013 6:34 PM, Matt Lind wrote:
Let me rephrase - what coordinate space is returned: local? Global? Screen?
Normal? Camera?
Thanks,
Matt
From:
[email protected]<mailto:[email protected]>
[mailto:[email protected]] On Behalf Of Alok
Sent: Wednesday, February 06, 2013 3:23 PM
To: [email protected]<mailto:[email protected]>
Subject: Re: SDK: OGLLight
Not sure, but as far as I can remember, it uses homogenous coordinates so point
(x, y, z) is represented as (xw, yw, zw, w). If you are getting a return of
four scalars and want to convert to 3d cartesian , simply divide the first
three values by the fourth value.
Maybe I am wrong, but not completely.
[cid:[email protected]]
On 06/02/2013 5:58 PM, Matt Lind wrote:
Anybody know what coordinate space is used for the values returned by the
OGLLight? (e.g. OGLLight.GetLightPosition(), OGLLight.GetLightDirection(), ...)
The documentation doesn't specify.
Matt
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