Shaders 103 – Lighting

Hello!

By now you should know what shaders are, and how they work.  You should also know how to integrate them into your code.  Since I have spent a lot of time putting lighting and what not into our game, I have become a bit of an expert with it.  So today I am going to go over how to do some fragment based lighting.

Changes from OpenGL and Movement Matrices

While I didn’t do the lighting in our game last semester, you can’t take old OpenGL code with lighting and a whole bunch of glTranslate and glRotate calls and expect it to work.

The first thing we are going to have to do is build a whole bunch of matrix functions that build a perspective, look at, rotation, translation, multiplication, invert and transform matrices.  When you download the CG API some of the sample code does have these functions build in, but they expect you to know what they do and how they work.

Here is how we will now be rendering objects instead of using the ‘gl’ draw calls.

/*** Render brass solid sphere ***/

setBrassMaterial();

/* modelView = rotateMatrix * translateMatrix */
makeRotateMatrix(70, 1, 1, 1, rotateMatrix);
makeTranslateMatrix(2, 0, 0, translateMatrix);
multMatrix(modelMatrix, translateMatrix, rotateMatrix);

/* invModelMatrix = inverse(modelMatrix) */
invertMatrix(invModelMatrix, modelMatrix);

/* Transform world-space eye and light positions to sphere's object-space. */
transform(objSpaceEyePosition, invModelMatrix, eyePosition);
cgSetParameter3fv(myCgFragmentParam_eyePosition, objSpaceEyePosition);
transform(objSpaceLightPosition, invModelMatrix, lightPosition);
cgSetParameter3fv(myCgFragmentParam_lightPosition, objSpaceLightPosition);

/* modelViewMatrix = viewMatrix * modelMatrix */
multMatrix(modelViewMatrix, viewMatrix, modelMatrix);

/* modelViewProj = projectionMatrix * modelViewMatrix */
multMatrix(modelViewProjMatrix, myProjectionMatrix, modelViewMatrix);

/* Set matrix parameter with row-major matrix. */
cgSetMatrixParameterfr(myCgVertexParam_modelViewProj, modelViewProjMatrix);
cgUpdateProgramParameters(myCgVertexProgram);
cgUpdateProgramParameters(myCgFragmentProgram);
glutSolidSphere(2.0, 40, 40);

Now this may seem like a lot, but it is necessary for working with shaders.

The beginning where we call the setBrassMaterial() function is where we set the objects parameters.   We will get to that a bit later.  For now think of it as your glColor call.

The first part where we create the matrix using a simple rotation and translation matrix is fairly simple.  You would just pass on those parameters as if you were doing a normal glRotate or glTranslate call.  You can replace these with variables so you can move these.  For now this object is stationary so we do not need it to move

However the next part is where you  multiply them to get your modelMatrix and invert it to get your final matrix.  This is so we can calculate lighting with respect to the sphere object.  We then update our eye and light Cg parameters that we will see later.

The last bit of code creates the modelView matrix and actually draws the sphere.

Using Materials

The book uses this method of creating functions that set the emissive, ambient, diffuse, specular and shininess values.  Like this:

static void setBrassMaterial(void)
{

const float brassEmissive[3] = {0.0, 0.0, 0.0},
brassAmbient[3] = {0.33, 0.22, 0.03},
brassDiffuse[3] = {0.78, 0.57, 0.11},
brassSpecular[3] = {0.99, 0.91, 0.81},
brassShininess = 27.8;

cgSetParameter3fv(myCgFragmentParam_Ke, brassEmissive);
checkForCgError("setting Ke parameter");
cgSetParameter3fv(myCgFragmentParam_Ka, brassAmbient);
checkForCgError("setting Ka parameter");
cgSetParameter3fv(myCgFragmentParam_Kd, brassDiffuse);
checkForCgError("setting Kd parameter");
cgSetParameter3fv(myCgFragmentParam_Ks, brassSpecular);
checkForCgError("setting Ks parameter");
cgSetParameter1f(myCgFragmentParam_shininess, brassShininess);
checkForCgError("setting shininess parameter");

}

So this function just sets the colour of each of the light parameters that we want.  Using this we can make several material functions for different objects and control them independently in whatever way we want.  You can make a character, enemy and level material.  Right before you load your character, you can make their lighting bright so that they stand out.  For enemies, you can give them a bit of a red highlight to show the player that they pose a threat.

What to Initialise

Now we are in our initCg() function let us break it down into a vertex and fragment area.

Vertex Initialisation

myCgVertexProfile = cgGLGetLatestProfile(CG_GL_VERTEX);
cgGLSetOptimalOptions(myCgVertexProfile);
checkForCgError("selecting vertex profile");

myCgVertexProgram =
cgCreateProgramFromFile(
myCgContext,              /* Cg runtime context */
CG_SOURCE,                /* Program in human-readable form */
myVertexProgramFileName,  /* Name of file containing program */
myCgVertexProfile,        /* Profile: OpenGL ARB vertex program */
myVertexProgramName,      /* Entry function name */
NULL);                    /* No extra compiler options */
checkForCgError("creating vertex program from file");
cgGLLoadProgram(myCgVertexProgram);
checkForCgError("loading vertex program");

#define GET_VERTEX_PARAM(name) \
myCgVertexParam_##name = \
cgGetNamedParameter(myCgVertexProgram, #name); \
checkForCgError("could not get " #name " parameter");

GET_VERTEX_PARAM(modelViewProj);

This is a fairly simple vertex initialisation.  The main point is to see that we are passing the modelViewProj matrix.  If you go back up to our draw code you can see where we update myCgVertexParam_modelViewProj parameter.

Vertex Shader Code

void v_fragmentLighting(
float4 position : POSITION,
float3 normal   : NORMAL,

out float4 oPosition : POSITION,
out float3 objectPos : TEXCOORD0,
out float3 oNormal   : TEXCOORD1,

uniform float4x4 modelViewProj)
{
oPosition = mul(modelViewProj, position);
objectPos = position.xyz;
oNormal = normal;
}

You can still see that this vertex shader is still simple.  We take our model view matrix and multiply that by our position and output both our position and our object position.

Fragment Initialisation

#define GET_FRAGMENT_PARAM(name) \
myCgFragmentParam_##name = \
cgGetNamedParameter(myCgFragmentProgram, #name); \
checkForCgError("could not get " #name " parameter");

GET_FRAGMENT_PARAM(globalAmbient);
GET_FRAGMENT_PARAM(lightColor);
GET_FRAGMENT_PARAM(lightPosition);
GET_FRAGMENT_PARAM(eyePosition);
GET_FRAGMENT_PARAM(Ke);
GET_FRAGMENT_PARAM(Ka);
GET_FRAGMENT_PARAM(Kd);
GET_FRAGMENT_PARAM(Ks);
GET_FRAGMENT_PARAM(shininess);

/* Set light source color parameters once. */
cgSetParameter3fv(myCgFragmentParam_globalAmbient, myGlobalAmbient);
cgSetParameter3fv(myCgFragmentParam_lightColor, myLightColor);

This not the full code for the initialisation.  This smidgen of code contains the new parameters that we will be passing into our fragment shader to compute our lighting.

Fragment Shader Code

void basicLight(
float4 position : TEXCOORD0,
float3 normal   : TEXCOORD1,

out float4 color : COLOR,

uniform float3 globalAmbient,
uniform float3 lightColor,
uniform float3 lightPosition,
uniform float3 eyePosition,
uniform float3 Ke,
uniform float3 Ka,
uniform float3 Kd,
uniform float3 Ks,
uniform float shininess)
{
float3 P = position.xyz;
float3 N = normalize(normal);

// Compute emissive term
float3 emissive = Ke;

// Compute ambient term
float3 ambient = Ka * globalAmbient;

// Compute the diffuse term
float3 L = normalize(lightPosition - P);
float diffuseLight = max(dot(L, N), 0);
float3 diffuse = Kd * lightColor * diffuseLight;

// Compute the specular term
float3 V = normalize(eyePosition - P);
float3 H = normalize(L + V);
float specularLight = pow(max(dot(H, N), 0), shininess);
if (diffuseLight <= 0) specularLight = 0;
float3 specular = Ks * lightColor * specularLight;

color.xyz = emissive + ambient + diffuse + specular;
color.w = 1;
}

This code takes in our parameters that we pass in our C++ code to compute emissive, ambient, diffuse and specular lighting.  Emissive and ambient are fairly easy to compute, however diffuse and specular require some more work.

Emissive Light

Emissive is the light that is emitted or given off by a surface.  This can be used to stimulate glowing
Equation: emissive = Ke
Ke is the materials emissive color

Ambient Light

Ambient or ambience is light that has bounced around from different objects.  This can be used to make your environments better.  You can have a grey ambient for smoggy cities or a nice bright yellow ambient for forests and nature environments.
Equation: ambient = Ka * globalAmbient
Ka is the material’s ambient reflectance
globalAmbient is the color of the incoming ambient light

Diffuse Light 1

Diffuse light is reflected off a surface equally in all directions.  Even if an object has small nooks and crannies, the light will bounce of its rough texture
Equation: diffuse = Kd * lightColor * max(N dot L, 0)
Kd is the material’s diffuse color
lightColor is the color of the incoming diffuse light
N is the normalised surface normal
L is the normalised vector toward the light source
P is the point being shaded

Diffuse Lighting 2
Specular Light 1

Specular lighting is light scattered from a surface around the mirror direction.  It is only seen on very shiny and metallic materials.  Unlike the above types of light, Specular depends on where the viewer is looking at for it to work.  It also takes into account how shiny a surface is.
Equation:  specular = Ks * lightColor * facing * (max(N dot H, 0))^shininess
Kd is the materials specular color
lightColor is the color of the incoming specular light
N is the normalized surface normal
V is the normalized vector toward the viewpoint
L is the normalized vector  toward the light source
H is the normalized vector that is halfway between V and L
P is the point being shaded
facing is 1 is N dot L is greater then 0 and 0 otherwise

Specular Light 2

Then you add all the lights together and that is lighting in a nutshell.

Fragment Lighting

Thank your for reading,
– Moose