I've been experimenting with Shaders and have learned a lot playing
with OpenGL Shader Builder. This is a really cool utility (please
add more docs though!).
I've graduated to applying shaders in an app that in the past has
used the standard OpenGL pipeline to render 3D models with some
basic lighting. The basics are working nicely, but there is an
issue with the lighting direction that I can't seem to get past.
I'll try to describe what we're trying to do and the unexpected
result here:
Basically we draw our scene based on a 3D object stored in a vertex
array. Until now it's been lit using the lighting functions of the
standard pipeline and that worked ok. My experiment was to load a
vertex shader that is a verbatim copy of James McCombe's
BetterPerVertexLighting example that comes with Shader builder.
The shading is applied, but the lighting direction seems locked to
the model (not the viewing direction) so that when the object is
rotated to certain angles it becomes black. In the Shader Builder
view the lighting seems locked to the viewer's coordinate system,
so the object is always illuminated from a source above and to the
right of the viewing axis.
I expect that this difference is due to how I've set up the
modelview and possibly projection matrices - it must be different
from the code setting up the views in shader builder. This will
likely be a good exercise to get a better grasp of the viewing
transformations - below I've included our matrix setup and below
that a copy of the shader. I'm starting to suspect that the matrix
setup I'm using might be a bit unorthodox...
Thanks for any tips on this!
Rick
/////////////////////////////// Here is the matrix
setup: //////////////////////////////////////////
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
// Set aspect ratio, using smallest dimention
float w = visibleRect.size.width;
float h = visibleRect.size.height;
float fov = 128.0;
float aspect = 0.0;
if (w<h) {
aspect = h/w;
glOrtho( (-fov / zoom), (fov / zoom), (aspect * -fov / zoom),
(aspect * fov / zoom), -fov, fov);
} else {
aspect = w/h;
glOrtho((aspect * -fov / zoom), (aspect * fov / zoom), (-fov /
zoom), (fov / zoom), -fov, fov);
}
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
// Rotate the object based on an interactively set viewing angle
glRotatef(orientation.x,1.0f,0.0f,0.0f);
glRotatef(orientation.y,0.0f,1.0f,0.0f);
glRotatef(orientation.z,0.0f,0.0f,1.0f);
gluLookAt(1.0,0.0,0.0, // Set camera position
0.0,0.0,0.0,
0.0,0.0,1.0);
/////////////////////////////// End of matrix
setup ///////////////////////////////////////////////////////////
############### Here is the shader ##################################
!!ARBvp1.0
# Better Per-Vertex Lighting by James A. McCombe, Tue Jun 17 2003
#
# Implements a basic lighting model where the shadow/highlights are
determined
# based on the dot product of a light direction vector and the
vertex normal.
# After that, some simple math is done to accent the highlight
based on a set of
# parameters. Finally, its modulated with the vertex color.
#
# Parameters:
# program.env[0].xyz - Light position
# program.env[1].x - Light softness
# program.env[1].y - Highlight size
# program.env[1].z - Highlight intensity
ATTRIB vertexPosition = vertex.position;
ATTRIB vertexNormal = vertex.normal;
PARAM lightDirection = program.env[0];
PARAM lightParameters = program.env[1];
PARAM zero = { 0.0, 0.0, 0.0, 0.0 };
TEMP temp1, temp2, transPosition;
# Transform the vertex position
DP4 transPosition.x, state.matrix.mvp.row[0], vertexPosition;
DP4 transPosition.y, state.matrix.mvp.row[1], vertexPosition;
DP4 transPosition.z, state.matrix.mvp.row[2], vertexPosition;
DP4 transPosition.w, state.matrix.mvp.row[3], vertexPosition;
# Transform the vertex normal into eye space
DP3 temp1.x, state.matrix.modelview.invtrans.row[0], vertexNormal;
DP3 temp1.y, state.matrix.modelview.invtrans.row[1], vertexNormal;
DP3 temp1.z, state.matrix.modelview.invtrans.row[2], vertexNormal;
DP3 temp1, lightDirection, temp1;
MAX temp1, zero, temp1;
POW temp1, temp1.x, lightParameters.x;
ADD temp2, temp1, -lightParameters.y;
MAX temp2, zero, temp2;
MUL temp2, temp2, lightParameters.z;
MUL temp1, temp1, vertex.color;
ADD temp1, temp1, temp2;
SWZ result.color, temp1, x,y,z,1;
MOV result.position, transPosition;
END
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Mac-opengl mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/mac-opengl/rhoge%
40nmr.mgh.harvard.edu
This email sent to email@hidden
