CS405 Lab 4: Illumination

Highlights of this lab:

• Overview of colors, materials and lights
• A color cube example "COLOR"
• Lighting examples "LIGHTING 1", "LIGHTING 2", and "LIGHTING 3"

For examples mentioned in the lab notes click here

Download the lab instructor's lecture demo here.

Assignment:

• make some modifications to "smooth.c"
• make some modifications to "movelight.c"
• show instructor results and answers to questions

Lab Notes

A. Colors

Color Modes

OpenGL supports two color modes. The first is RGBA model, where you select the value of the red, green, blue and alpha(opacity) parameters. The second is color-index mode, whereby you fill a palette with the colors. We will focus on RGBA mode, because most of the interesting things you can do involve lighting and textures, which are difficult to do using a color palette. When you are using lighting or texture-mapping effects(lighting is turned on), the color of the vertex is the cumulative effect of the material color and the light that's shining on that vertex. When the light is turned off, the color of a vertex is the effect of the color setting.

Color and Shading

Color in RGBA mode is set by specifying the red, green, blue and alpha intensities. The glColor*() comes in a variety of formats. You can specify a bright red triangle using the following commands:

glColor3f(1.0f, 0.0f, 0.0f);  //no alpha value form
glBegin(GL_TRIANGLES);
glVertex3f(-1, 0, 0);
glVertex3f(1, 0, 0);
glVertex3f(0, 1, 0);
glEnd();

Notice that the glColor*() function can be placed inside a glBegin()/glEnd() pair. Thus you can specify individual colors for each individual vertex if you desire. If the colors of two vertices are different, what is the color between the two vertices? For example, what is the color of the center of the triangle in the following code?

glBegin(GL_TRIANGLES);
glColor3f(1.0f, 0.0f, 0.0f);  //red 
glVertex3f(-1, 0, 0);
glColor3f(0.0f, 1.0f, 0.0f);  //green
glVertex3f(1, 0, 0);
glColor3f(0.0f, 0.0f, 1.0f);  //blue
glVertex3f(0, 1, 0);
glEnd();

The answer is that it depends on the shading model specified. If smooth shading(default) is specified, the color values are interpolated between vertices. In this case the color at the center would be gray. If flat shading is specified, the one vertex is selected as being representative of all the vertices; thus the entire primitive is displayed using one single color. For a single polygon, the first vertex is used to specified to color; for all the other primitives it the last specified vertex in each polygon or line segment.

The following is a cube model that the top is left off and a small black ball is placed inside the cube.

BOOL CColorView::RenderScene( void )
{

    glColor3f( 0.0f, 0.0f, 0.0f );
    auxSolidSphere( 0.3f ); // <- slight memory leak here

    //glShadeModel( GL_FLAT ); // to show flat shading

    // define the colors
    GLfloat color1[3] = { 1.0f, 0.0f, 0.0f }; // red
    GLfloat color2[3] = { 0.0f, 1.0f, 0.0f }; // green
    GLfloat color3[3] = { 0.0f, 0.0f, 1.0f }; // blue
    GLfloat color4[3] = { 1.0f, 1.0f, 1.0f }; // white
    GLfloat color5[3] = { 0.0f, 0.0f, 0.0f }; // black
    GLfloat color6[3] = { 1.0f, 0.0f, 1.0f }; // magenta
    GLfloat color7[3] = { 0.0f, 1.0f, 1.0f }; // cyan
    GLfloat color8[3] = { 1.0f, 1.0f, 0.0f }; // yellow

    // Connect the four sides
    glBegin(GL_QUAD_STRIP);
	glColor3fv( color6 );
	glVertex3f(-1.0f, 1.0f, 1.0f);

	glColor3fv( color1 );
	glVertex3f(-1.0f, -1.0f, 1.0f);

	glColor3fv( color4 );
	glVertex3f(1.0f, 1.0f, 1.0f);

	glColor3fv( color8 );
	glVertex3f(1.0f, -1.0f, 1.0f);

	glColor3fv( color7 );
	glVertex3f(1.0f, 1.0f, -1.0f);

	glColor3fv( color2 );
	glVertex3f(1.0f, -1.0f, -1.0f);

	glColor3fv( color3 );
	glVertex3f(-1.0f, 1.0f, -1.0f);

	glColor3fv( color5 );
	glVertex3f(-1.0f, -1.0f,  -1.0f);

	glColor3fv( color6 );
	glVertex3f(-1.0f, 1.0f, 1.0f);

	glColor3fv( color1 );
	glVertex3f(-1.0f, -1.0f, 1.0f);

    glEnd();

    // The Bottom
    glBegin(GL_QUADS);

	glColor3fv( color1 );
	glVertex3f(-1.0f, -1.0f, 1.0f);

	glColor3fv( color8 );
	glVertex3f(1.0f, -1.0f, 1.0f);

	glColor3fv( color2 );
	glVertex3f(1.0f, -1.0f, -1.0f);

	glColor3fv( color5 );
	glVertex3f(-1.0f, -1.0f,  -1.0f);
    glEnd();

    return TRUE;
}

Figure 1: The color cube with smooth shading selected

This model draws the color cube -- red, green and blue as one triplet; cyan, magenta and yellow as another triplet; Figure 1 shows the effect of smooth shading. With no lighting effects, it is very hard to distinguish between the faces of the polygons that makes up an object. Figure 2 shows the same scene with flat shading. The color of each face is entirely the result of the order in which the vertices were specified.

Figure 2: The color cube with flat shading selected

B. Materials

OpenGL's materials are descriptions of what things are made of -- or at least what they appear to be made of -- by describing how they reflect light.

Types of material properties:

OpenGL has five properties that describe how a surface dissipates light. These properties are typically described by RGBA values that stipulate the color of the dissipated light.

Diffuse and ambient properties:

The diffuse and ambient reflective material properties are a type of reflective effect that is independent of the viewpoint. Diffuse lighting describes how an object reflects a light that is shining on the object. That is, it is how the surface diffuses a direct light source. Ambient lighting describes how a surface reflects the ambient light available. The ambient light is the indirect lighting that is in a scene: the amount of light that is coming from all directions so that all surfaces are equally illuminated by it.

Specular and shininess properties:

The specular and the shininess properties of the surface describe the reflective effects that are affected by the position of the viewpoint. Specular light is reflected light from a surface that produces the reflective highlights in a surface. The shininess is a value that describes how focused the reflective properties are.

Emissive property:

Emissive light is the light that an object gives off by itself. A light source is typically the only object that you may give an emissive value. Lamps, fires, and lightning are all objects that give off their own light.

Specifying a Material Property:

Specifying a material property is almost the same as specifying a color. Let's specify a gray material for both the ambient and diffuse properties.

    GLfloat materialDiffuse[] = {0.2, 0.2, 0.2, 1};
    GLfloat materialAmbient[] = {0.5, 0.5, 0.5, 1};
    ::glMaterialfv(GL_FRONT, GL_DIFFUSE, materialDiffuse);
    ::glMaterialfv(GL_FRONT, GL_AMBIENT, materialAmbient);

glMaterial*() is used to specify the materials properties that make up the resultant color of a surface. The first parameter indicates which face of a polygon the property should be applied to.

Please refer to glMaterial*( ) Description.

How To Choose the Material Properties of an Object?

The steps are as follows:

1. Decide on the diffuse and ambient colors;
2. If the object has a hidden interior, maybe only the front faces are needed to be included in the calculations, thus the GL_FRONT is applied.
3. Decide on the shininess of the object.
4. Decide whether the object is giving off light. If it is, assign it the emissive properties

Creating material properties

In LIGHTING 1 project, The program creates a 4X4X4 matrix of blue spheres that varies the ambient and diffuse properties along the y-axis, the specular along the x-axis, and the shininess along the z-axis. The matrices are in the following:

// set up arrays of various properties
// placed here for convenience
GLfloat materialSpecular[4][4] = {
	{ 0.1f,  0.1f,  0.1f, 1.0f },
	{ 0.33f, 0.33f, 0.33f, 1.0f },
	{ 0.67f, 0.67f, 0.67f, 1.0f },
	{ 0.9f,  0.9f,  0.9f,  1.0f },
};

GLfloat materialAmbDiff[4][4] ={
	{ 0.0f, 0.0f, 0.12f, 1.0f },
	{ 0.0f, 0.0f, 0.25f, 1.0f },
	{ 0.0f, 0.0f, 0.50f, 1.0f },
	{ 0.0f, 0.0f, 1.00f, 1.0f },
};

GLfloat materialShininess[4][1] = {
	{ 0.0f },
	{ 5.0f },
	{ 25.0f },
	{ 125.0f }
};

These values were chosen because they give a relatively even increase in each property as the values change. Figure 3 presents the changes that these properties bring when combined. The color of the spheres are defined by the ambient and diffuse values, and you can see from both the array and the illustration that the colors start off as a very dark blue rise to a bright pure blue. The shininess effect can be observed as the highlight goes from being spread across the entire illuminated hemisphere to a point of light on the surface. If you rotate the matrix about the y-axis, you will see that the highlight disappears when the angles of the reflection no longer hits the viewpoint.

Figure 3: A matrix of spheres showing the range of material properties

Simpified Materials

You can simplify the specification of material properties by enabling GL_COLOR_MATERIAL. This mode uses glColor*() calls to set one or more material properties. It is often easier and faster to use color material mode to switch properties than to specify the whole material.

By default the properties modified by glColor*() calls in color material mode are diffuse and ambient. These can be changed by using glColorMaterial().

Please refer to the glColorMaterial() description.

C. Lighting

OpenGL has two types of lighting: global shading, or the ambient light and its associated parameters, and individual light sources, which have position and direction.

Enabling Lighting

Global Lighting

The glLightModel*() function is used for setting global lighting parameters. You can select the RGBA value for the global ambient light that's present in the scene. Next, you can alter the calculations for specular highlights:

• Is the viewer local? (false by default)
• Should I calculate lighting for both front and back faces? (false by default)

Individual Light Sources

An individual light source will bring into sharp detail the differences between differently illuminated sides. After enabling an individual light source, you may want to set the ambient and diffuse RGBA values for the light. The ambient part of the light's values is the glLight*() function. The parameters passed in are the particular light's enum value, the enum of the parameter you want to change, and the value of the parameter.

This function can be used to do the following:

Setting the Illuminating Parameters

The diffuse RGBA value of the color that the light contributes to the reflectance off an object and is what you can consider the "color" of the light. Shining a light with red diffuse RGBA settings on a white sphere would give a red coloring to that part of the sphere that the light illuminates. The specular component is the color of the highlights that reflect off a shiny surface. Typically you would set the specular value to be the same as the diffuse value, since the reflected highlight is usually the same as light. Figure 4 is a scene from the "LIGHTING 2" program. These are three white mildly reflective spheres. The sphere in the middle has a light with white diffuse and specular shining on it. The sphere on the left has a green diffuse and white specular light shining on it. The sphere on the right has a white diffuse and a green specular lighting on it.


Figure 4: Three identical spheres lit by different light components

Setting the Position

The position is transformed by the modelview matrix when glLight*( ) is called (just as if it were a point), and it is stored in eye coordinates. If the w component of the position is 0.0, the light is treated as a directional source. Diffuse and specular lighting calculations take the light's direction, but not its actual position, into account, and attenuation is disabled. Otherwise, it is positional source. Diffuse and specular lighting calculations are based on the actual location of the light in eye coordinates, and attenuation is then enabled. The default position is (0,0,1,0); thus, the default light source is directional, parallel to, and in the direction of the -z axis.

Setting attenuation:

Attenuation is the reduction in the intensity of the light as the distance increases. OpenGL has three parameter to calculate the attenuation factor. GL_CONSTANT_ATTENUATION is the constant, and by default its value is 1. The other parameters -- GL_LINEAR_ATTENUATION and GL_QUADRATIC_ATTENUATION -- are used in the calculation to reduce the light

Creating a Spotlight

The range of light dispersion with GL_SPOT_CUTOFF parameter specifies the angle from the centerline of the spotlight's direction. In figure 5, A value 15 degrees defines cone of 30 degrees wide. By default this is 180 degrees, yielding a full 360 degrees. The acceptable range is [0,90], except for the special value of 180. To set the direction of the spotlight, use the GL_SPOT_DIRECTION parameter. The vector is in object coordinates, and it's effected by whatever matrix transformations are effective. By default it's pointing down the negative z-axis. The final setting for spotlights controls how intense the light is at its center. The GL_SPOT_EXPONENT controls how to concentrate the light's intensity. By default the light is equally bright across the diameter of the spotlight. The exponent value of the power the exponent on a cosine of the angle from the center of the spot. The higher the value, the more concentrated the light will be at the center.


Figure 5: Components of a spotlight

Please refer to glLight*() Description.

Creating a scene with multiple light sources

We'll use multiple lights to create a scene. The source code for this can be found in the "LIGHTING 3" subdirectory. Figure 6 shows a rendering of the scene.

Figure 6: 3D text demonstrating chrome text with colored lights

The following is the RenderScene( ) member function:

 
BOOL CLightingView::RenderScene( void )
{

    // Need to add a check for the font here before assuming it's there...
    static GLuint myFont = GenerateDisplayListForFont( "Impact", 0.2f );

    GLfloat materialSpecular[4] = { 1.0f, 1.0f, 1.0f, 1.0f };
    GLfloat materialShininess[1] = { 128.0f };
    GLfloat materialAmbient[4] = { 0.25f, 0.25f, 0.25f, 1.0f };
    GLfloat materialDiffuse[4] = { 0.4f, 0.4f, 0.4f, 1.0f };

    GLfloat local_ambient[] = { 0.0f, 0.0f, 0.0f, 1.0f };

    ::glEnable(GL_DEPTH_TEST);
    ::glDepthFunc(GL_LESS);


    ::glLightModelfv(GL_LIGHT_MODEL_AMBIENT, local_ambient);

    GLfloat ambient0[] =  { 0.0f, 0.0f, 0.0f, 1.0f };
    GLfloat diffuse0[] =  { 1.0f, 0.0f, 0.0f, 1.0f };
    GLfloat specular0[] = { 1.0f, 0.0f, 0.0f, 1.0f };
    GLfloat position0[] = { 2.0f, -1.5f, -1.5f, 1.0f };

    GLfloat ambient1[] =  { 0.0f, 0.0f, 0.0f, 1.0f };
    GLfloat diffuse1[] =  { 1.0f, 1.0f, 1.0f, 1.0f };
    GLfloat specular1[] = { 0.5f, 0.5f, 0.5f, 1.0f };
    GLfloat position1[] = { 2.0f, 1.0f, -1.0f, 0.0f };

    GLfloat ambient2[] =  { 0.0f, 0.0f, 0.0f, 1.0f };
    GLfloat diffuse2[] =  { 0.0f, 0.0f, 1.0f, 1.0f };
    GLfloat specular2[] = { 0.0f, 0.0f, 1.0f, 1.0f };
    GLfloat position2[] = { -0.5f, -0.5f, -1.0f, 1.0f };

    GLfloat ambient3[] =  { 0.0f, 0.0f, 0.0f, 1.0f };
    GLfloat diffuse3[] =  { 1.0f, 1.0f, 1.0f, 1.0f };
    GLfloat specular3[] = { 0.0f, 0.0f, 0.0f, 1.0f };
    GLfloat position3[] = { 2.0f, 0.5f, 0.5f, 0.0f };


    // Disable lighting to draw some simple spheres that
    // represent the lights
    ::glDisable( GL_LIGHTING );

    ::glPushMatrix();
	::glColor3fv( diffuse0 );
	::glTranslatef( position0[0], position0[1], position0[2] );
	::auxWireSphere( 0.15f );

    ::glPopMatrix();

    ::glPushMatrix();
	::glColor3fv( diffuse1 );
	::glTranslatef( position1[0], position1[1], position1[2] );
	//      ::auxWireSphere( 0.15f );
    ::glPopMatrix();

    ::glPushMatrix();
	::glColor3fv( diffuse2 );
	::glTranslatef( position2[0], position2[1], position2[2] );
	::auxWireSphere( 0.15f );

    ::glPopMatrix();

    ::glPushMatrix();
	::glColor3fv( diffuse3 );
	::glTranslatef( position3[0], position3[1], position3[2] );
	//      ::auxWireSphere( 0.15f );
    ::glPopMatrix();

    ::glEnable( GL_LIGHTING );

    ::glEnable(GL_LIGHT0);
    ::glLightfv(GL_LIGHT0, GL_AMBIENT, ambient0);
    ::glLightfv(GL_LIGHT0, GL_POSITION, position0);
    ::glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuse0);
    ::glLightfv(GL_LIGHT0, GL_SPECULAR, specular0);

    ::glEnable(GL_LIGHT1);
    ::glLightfv(GL_LIGHT1, GL_AMBIENT, ambient1);
    ::glLightfv(GL_LIGHT1, GL_POSITION, position1);
    ::glLightfv(GL_LIGHT1, GL_DIFFUSE, diffuse1);
    ::glLightfv(GL_LIGHT1, GL_SPECULAR, specular1);

    ::glEnable(GL_LIGHT2);
    ::glLightfv(GL_LIGHT2, GL_AMBIENT, ambient2);
    ::glLightfv(GL_LIGHT2, GL_POSITION, position2);
    ::glLightfv(GL_LIGHT2, GL_DIFFUSE, diffuse2);
    ::glLightfv(GL_LIGHT2, GL_SPECULAR, specular2);

    ::glEnable(GL_LIGHT3);
    ::glLightfv(GL_LIGHT3, GL_AMBIENT, ambient3);
    ::glLightfv(GL_LIGHT3, GL_POSITION, position3);
    ::glLightfv(GL_LIGHT3, GL_DIFFUSE, diffuse3);
    ::glLightfv(GL_LIGHT3, GL_SPECULAR, specular3);

    ::glMaterialfv( GL_FRONT, GL_SPECULAR, materialSpecular );
    ::glMaterialfv( GL_FRONT, GL_SHININESS, materialShininess );
    ::glMaterialfv( GL_FRONT, GL_DIFFUSE, materialDiffuse );
    ::glMaterialfv( GL_FRONT, GL_AMBIENT, materialAmbient );

    ::glPushMatrix();
	::glTranslatef( -3.0f, 0.0f, 0.0f );
	GLTextOut( myFont, "OpenGL & Windows" );
    ::glPopMatrix();

    return TRUE;
}

Assignment

This exercise is broken into two parts:

1. Color
2. Light and Material

Part 1-Color

Goals:

• Learn about flat versus smooth shading
• Learn about the influence of different vertices on flat shading (See also Table 5-2 in the Redbook)
• Learn some different calls to glColor

Instructions (Record answers for 4, 5, and 6):

1. Create an empty "Win32 Console Project"
2. Add the following code to the project: smooth.c
3. Build and Run this code
4. Change the glShadeModel from GL_SMOOTH to GL_FLAT.
   What color is the triangle? Why?
5. Change the GL_TRIANGLES to GL_LINE_LOOP.
   What color are the lines? Why?
6. Change the GL_LINELOOP to GL_POLYGON.
   What color is the triangle now? Why?
7. Undo steps 4, 5, and 6.
8. Use three other versions of glColor (besides glColor3f) to create the full intensity (red, green, blue) of the original triangle. The following table outlines the suffixes you can use and provides a hint of the maximum values (See also Table 5-1 in the Redbook).
   

       Suffix   Data Type
       b        1-byte integer
       s        2-byte integer
       i        4-byte integer
       ub       unsigned 1-byte integer
       us       unsigned 2-byte integer
       ui       unsigned 4-byte integer
   

Show Instructor the results and answers to 4, 5, 6, and 8

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Part 2-Light and Material

• Learn about positional versus directional light source
• Learn more about moving the light source
• Learn more about material and lighting properties
• Experiment with attenuation parameters
• Learn about global ambient light

1. Create an empty "Win32 Console Project"
2. Add the following code to the project:
   movelight.c
3. This code has a positional light which rotates around a Torus.
   Without compiling how would you determine that it is a positional light?
4. Left mouse click to see how the light rotates around the object.
5. Modify the "position" array so that the light is directional.
   What happens? (HINT: move the light around the torus)
6. Change the light back to a positional light.
7. You will notice that there is a glShadeModel(GL_SMOOTH);
   Why do you want smooth shading instead of flat?
8. Modify the code so that a right click will reverse the rotation.
9. Replace the rotation with translation so that clicking on the right mouse will move the light infinitely along the positive x-axis (toward the right), and the left mouse click will reverse the direction moving the light toward the left.
   Hint: use glTranslated() rather than glRotated() in display(), and choose an appropriate value to use instead of spin.
10. Change the attenuation so that the light decreases in intensity as it's moved away from the object.
    Hint: Add calls to glLight*() to set the desired attenuation parameters. By default, GL_CONSTANT_ATTENUATION is 1.0, GL_LINEAR_ATTENUATION is 0, and GL_QUADRATIC_ATTENUATION is 0. Modify them to be higher values. Suggestion: try linear and quadratic values which are fractions less than 1.
11. Try to change the color of the Torus to red using glColor. Does it work?
12. Use glMaterialfv with diffuse and ambient values set to pure red.
13. Use glLightfv with diffuse and ambient values set to pure green. What would you expect to happen? What happens?
14. Even though you may have predicted otherwise, the object is viewable. This is because there is a global ambient light on the scene, which is set to (0.2,0.2,0.2,1.0).
15. Turn off the global ambient light by using the following command:

    glLightModelfv(GL_LIGHT_MODEL_AMBIENT, lmodel_ambient)

    where lmodel_ambient has been set to an array with RGBA values.

Show the results and answers to your instructor.

Be prepared to comment out the glLightModelfv() function and ambient and diffuse light settings.

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