Two sided light model

[matt_leeds_1]

hi everyone hope someone can help me

i’ve just recently started glsl as part of project and am using 3ds models which require a two sided light model. I have the book by randi rost and have put together a vertex shader but i have no idea if its right and no idea what goes into the fragment shader.

This is what i have so far (taken from chapter 9 of the orange book)

vertex shader

  
varying vec3 normal;

void DirectionalLight(in int i, in vec3 normal, inout vec4 ambient, inout vec4 diffuse, inout vec4 specular)
{
	float nDotVP;		// normal . light direction
	float nDotHV;		// normal . light half vector
	float pf;			// power factor

	nDotVP = max(0.0, dot(normal, vec3(gl_LightSource[0].position)));
	nDotHV = max(0.0, dot(normal, vec3(gl_LightSource[0].halfVector)));

	if(nDotVP == 0.0)
		pf = 0.0;
	else
		pf = pow(nDotHV, gl_FrontMaterial.shininess);

	ambient += gl_LightSource[0].ambient;
	diffuse += gl_LightSource[0].diffuse;
	specular += gl_LightSource[0].specular * pf;
}	

void main(void)
{
	// Vertex transformation
	gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;

	// Normal transformation
	normal = gl_NormalMatrix * gl_Normal;

	normal = normalize(normal);
	
	vec4 amb;
	vec4 diff;
	vec4 spec;
	vec4 color;

	// Clear light intensity accumulators
	amb = vec4(0.0);
	diff = vec4(0.0);
	spec = vec4(0.0);

	if(gl_LightSource[0].position.w == 0.0)
		DirectionalLight(0, normal, amb, diff, spec);

	color = gl_FrontLightModelProduct.sceneColor +
		amb * gl_FrontMaterial.ambient +
		diff * gl_FrontMaterial.diffuse;

	color += spec * gl_FrontMaterial.specular;

	gl_FrontColor = color;
	
	// Compute back sided surface

	normal = -normal;

	// CLear light intensity accumulators
	amb = vec4(0.0);
	diff = vec4(0.0);
	spec = vec4(0.0);

	if(gl_LightSource[0].position.w == 0.0)
		DirectionalLight(0, normal, amb, diff, spec);

	color = gl_BackLightModelProduct.sceneColor +
		amb * gl_BackMaterial.ambient +
		diff * gl_BackMaterial.diffuse;

	color += spec * gl_BackMaterial.specular;

	gl_BackColor = color;

}

fragment shader

  
varying vec3 normal;

void main(void)
{
}

any help would be much appreciated

thanks in advance

matt

[Vincent_Chau]

Wow. This is a pretty old post, and you may have since worked out how to do it. Just in case someone else stumbles across this question, you don’t need to provide a fragment shader.

Just create a program object, attach the vertex shader object, and link. Let OpenGL use the fixed functionality for the fragment stages.