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gltut / Documents / Texturing / Tutorial 17.xml

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    xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0">
    <?dbhtml filename="Tutorial 17.html" ?>
    <title>Variable Lighting</title>
    <para>Previously, we have seen textures used to vary surface parameters. But we can use textures
        to vary something else: light intensity. In this way, we can simulate light sources who's
        intensity changes with something more than just distance from the light.</para>
    <para>Our first effort in varying light intensity with textures will be to build an incandescent
        flashlight. The light beam from a flashlight is not a single solid intensity, due to the way
        the mirrors focus the light. A texture is the simplest way to define this pattern of light
        intensity.</para>
    <section>
        <?dbhtml filename="Tut17 Post Projection Space.html" ?>
        <title>Post-Projection Space</title>
        <para>Before we can look at how to use a texture to make a flashlight, we need to understand
            a special piece of mathematics. We need to revisit perspective projection. Specifically,
            we need to look at what happens when transforming after a projection operation.</para>
        <para/>
        <para/>
    </section>
    <section>
        <?dbhtml filename="Tut17 Projective Texture.html" ?>
        <title>Projective Texture</title>
        <para/>
    </section>
    <section>
        <?dbhtml filename="Tut17 Variable Point Light.html" ?>
        <title>Variable Point Light</title>
        <para/>
    </section>
    <section>
        <?dbhtml filename="Tut17 In Review.html" ?>
        <title>In Review</title>
        <para>In this tutorial, you have learned the following:</para>
        <itemizedlist>
            <listitem>
                <para>Vertex positions can be further manipulated after a perspective projection.
                    Thus the perspective transform is not special. The shape of objects in
                    post-projective space can be unusual and unexpected.</para>
            </listitem>
            <listitem>
                <para>Textures can be projected onto meshes. This is done by transforming those
                    meshes into the space of the texture, which is equivalent to transforming the
                    texture into the space of the meshes. The transform is governed by its own
                    camera matrix, as well as a projection matrix and a post-projective
                    transform.</para>
            </listitem>
            <listitem>
                <para>Cube maps are textures that have 6 face images for every mipmap level. The 6
                    faces are arranged in a cube. Texture coordinates are effectively directions of
                    a vector centered within the cube. Thus a cube map can provide a varying value
                    based on a direction in space.</para>
            </listitem>
        </itemizedlist>
        <section>
            <title>Further Study</title>
            <para>Try doing these things with the given programs.</para>
            <itemizedlist>
                <listitem>
                    <para>Instead of using a projective texture, build a lighting system for spot
                        lights entirely within the shader. It should have a maximum angle; the
                        larger the angle, the wider the spotlight. It should also have an inner
                        angle that is smaller than the maximum angle. This the the point where the
                        light starts falling off. At the maximum angle, the light intensity goes to
                        zero; at the minimum angle, the light intensity is full. The key here is
                        remembering that the dot product between the spotlight's direction and the
                        direction from the surface to the light is the cosine of the angle between
                        the two vectors. The <function>acos</function> function can be used to
                        compute the angle (in radians) from the cosine.</para>
                </listitem>
            </itemizedlist>
        </section>
        <section>
            <title>OpenGL Functions of Note</title>
            <para/>
        </section>
        <section>
            <title>GLSL Functions of Note</title>
            <funcsynopsis>
                <funcprototype>
                    <funcdef>vec4 <function>textureProj</function></funcdef>
                    <paramdef>sampler <parameter>texSampler</parameter></paramdef>
                    <paramdef>vec texCoord</paramdef>
                </funcprototype>
            </funcsynopsis>
            <para>Accesses the texture associated with <parameter>texSampler</parameter>, using
                post-projective texture coordinates specified by <parameter>texCoord</parameter>.
                The <type>sampler</type> type can be many of the sampler types, but not
                    <type>samplerCube</type>, among a few others. The texture coordinates are in
                homogeneous space, so they have one more components than the number of dimensions of
                the texture. Thus, the number of components in <parameter>texCoord</parameter> for a
                sampler of type <type>sampler1D</type> is <type>vec2</type>. For
                    <type>sampler2D</type>, it is <type>vec3</type>.</para>
        </section>
        
    </section>
    <section>
        <?dbhtml filename="Tut17 Glossary.html" ?>
        <title>Glossary</title>
        <glosslist>
            <glossentry>
                <glossterm/>
                <glossdef>
                    <para/>
                </glossdef>
            </glossentry>
        </glosslist>
        
    </section>
</chapter>