how to get a spotlight in opengl? - c++

Question

reference
http://www.chai3d.org/doc/classc_light.html
code
light2->setPos(cVector3d( 0, 0,0.0)); // position the light source
light2->m_ambient.set(0.8, 0.8, 0.8);
light2->m_diffuse.set(0.8, 0.8, 0.8);
light2->m_specular.set(0.8, 0.8, 0.8);
light2->setDirectionalLight(false);//make a positional light
the rendering code which uses opengl is
void cLight::renderLightSource()
{
// check if light source enabled
if (m_enabled == false)
{
// disable OpenGL light source
glDisable(m_glLightNumber);
return;
}
computeGlobalCurrentObjectOnly();
// enable this light in OpenGL
glEnable(m_glLightNumber);
// set lighting components
glLightfv(m_glLightNumber, GL_AMBIENT, m_ambient.pColor());
glLightfv(m_glLightNumber, GL_DIFFUSE, m_diffuse.pColor() );
glLightfv(m_glLightNumber, GL_SPECULAR, m_specular.pColor());
// position the light source in (global) space (because we're not
// _rendered_ as part of the scene graph)
float position[4];
position[0] = (float)m_globalPos.x;
position[1] = (float)m_globalPos.y;
position[2] = (float)m_globalPos.z;
//position[0] = (float)m_localPos.x;
//position[1] = (float)m_localPos.y;
//position[2] = (float)m_localPos.z;
// Directional light source...
if (m_directionalLight) position[3] = 0.0f;
// Positional light source...
else position[3] = 1.0f;
glLightfv(m_glLightNumber, GL_POSITION, (const float *)&position);
// set cutoff angle
glLightf(m_glLightNumber, GL_SPOT_CUTOFF, m_cutOffAngle);
// set the direction of my light beam, if I'm a _positional_ spotlight
if (m_directionalLight == false)
{
cVector3d dir = m_globalRot.getCol0();
float direction[4];
direction[0] = (float)dir.x;
direction[1] = (float)dir.y;
direction[2] = (float)dir.z;
direction[3] = 0.0f;
glLightfv(m_glLightNumber, GL_SPOT_DIRECTION, (const float *)&direction);
}
// set attenuation factors
glLightf(m_glLightNumber, GL_CONSTANT_ATTENUATION, m_attConstant);
glLightf(m_glLightNumber, GL_LINEAR_ATTENUATION, m_attLinear);
glLightf(m_glLightNumber, GL_QUADRATIC_ATTENUATION, m_attQuadratic);
// set exponent factor
glLightf(m_glLightNumber, GL_SPOT_EXPONENT, m_spotExponent);
}
why do I get my whole environment uniformly lighted?
how do I get a concetrated light around the origin 0,0,0, which fades away after 1 or 2 unit distance? My origin is the middle cube in the grid.

Answer 1

caveat emptor: This is "from the top of my head" from way-back-when i used to fumble with OpenGL.
I think the OpenGL concept of a "directional light" is sort of like a point-lightsource at infinity, meaning the light-vector is invariant across the entire scene.
In order to do the spotlight effect, you need to form the dot-product of the light-direction and the light-vector (vector from vertex to light) and attenuate the light exponentially as the angle increases.
I remember reading a tutorial about this once, will search...
Right, its described here
Scroll down to the description of "spotlights".
In the code you've listed, I believe you need to ensure that m_spotExponent is greater-than zero to get a "cone" effect. Higher values yield a "sharper" transition from "lit" to "dark" parts of the cone (I think).
Hope that helps