<?xml version="1.0" encoding="UTF-8"?>
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<refentry xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:svg="http://www.w3.org/2000/svg" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:db="http://docbook.org/ns/docbook" xmlns:scilab="http://www.scilab.org"  xml:lang="en" scilab:needs-examples="no" xml:id="lighting">
    <refnamediv>
        <refname>lighting</refname>
        <refpurpose>Lighting overview</refpurpose>
    </refnamediv>
    <refsection>
        <title>Description</title>
        <para>
            Lighting in scilab approximates real world lighting where the appearance of
            a surface is given by interactions between light rays and the material properties of the
            surface.
        </para>
        <para>Because simulate real world lighting is complex
            and computationally expensive, one uses a simplified model.
        </para>
        <para>
            A light is represented mainly by its colors: ambient, diffuse and specular. It can
            be created using <link linkend="light">light</link> function,
            and deleted using <link linkend="delete">delete</link> function.
        </para>
        <para>
            The surface material properties is represented also by its colors
            (ambient, diffuse and specular) and it shininess level.
        </para>
        <para>
            The surface contains the <literal>color_material</literal> property that enables
            the use of the surface color as the diffuse color.
        </para>
        <para>
            The final surface color is given by the  light model equation applied to the product
            of the surface and light colors. Following there is the light model equation:
        </para>
        <scilab:image>
            set(gca(), "font_style", 1, "font_size", 4);
            xstring(0.0, 0.4, '$Color = Ambient_{term} + Diffuse_{term} + Specular_{term} \\ Ambient_{term} = L_{ambient}*S_{ambient} \\ Diffuse_{term} = L_{diffuse}*S_{diffuse}*(L_{direction} \cdot S_{normal}) \\ Specular_{term} = L_{specular}*S_{specular}*(S_{normal} \cdot (L_{direction}+View_{direction}) ))^{S_{shininess}}$');
            f=gcf();
            f.figure_size = [750,280];
        </scilab:image>
        <listitem>
            <para>
                Ambient color models the light received indirect by the surface and
                is independent of the surface normal or light direction.
            </para>
        </listitem>
        <listitem>
            <para>
                Diffuse color models the light received directly, and thus it's dependent
                of the light ray direction and the surface normal.
            </para>
        </listitem>
        <listitem>
            <para>
                Specular color defines the specular highlight color.
            </para>
        </listitem>
        <para>
            Following one can compare the surface with and without lighting:
        </para>
        <para>
            <scilab:image>
                plot3d();
                l = light();
            </scilab:image>
        </para>
        <para>
            <scilab:image>
                plot3d();
            </scilab:image>
        </para>
    </refsection>
    <refsection>
        <title>Lighting properties list</title>
        <variablelist>
            <varlistentry>
                <term>Light properties </term>
                <listitem>
                    <variablelist>
                        <varlistentry>
                            <term>visible: </term>
                            <listitem>
                                <para>
                                    This field defines if the light is enabled <literal>"on"</literal>
                                    or disabled <literal>"off"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                        <varlistentry>
                            <term>light_type: </term>
                            <listitem>
                                <para>This field defines the type of light used. There are two
                                    types available:<literal>"directional"</literal> and <literal>"point"</literal>.
                                    Directional lights are positioned infinitely far from the surface
                                    thus emitting parallel rays.
                                </para>
                                <para>
                                    Point lights are defined as a point in space thus emitting rays in all
                                    directions.
                                    The default value is <literal>"point"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                        <varlistentry>
                            <term>direction: </term>
                            <listitem>
                                <para>This field defines the ray direction for directional light.
                                    The direction can be changed using a 3 element vector <literal>"[x, y, z]"</literal>
                                    representing a vector starting from the origin.
                                </para>
                                <para>
                                    The default value is <literal>"[0 0 1]"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                        <varlistentry>
                            <term>position: </term>
                            <listitem>
                                <para>This field defines the light position for point light.
                                    The position can be changed using a 3 element vector <literal>"[x, y, z]"</literal>.
                                </para>
                                <para>
                                    The default value is <literal>"[0 0 1]"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                        <varlistentry>
                            <term>ambient_color: </term>
                            <listitem>
                                <para>This field defines the ambient color of the light.
                                    The color is defined by a 3 element vector <literal>"[red, green, blue]"</literal>
                                    with each element in the range [0, 1].
                                </para>
                                <para>
                                    The default value is <literal>"[0.1 0.1 0.1]"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                        <varlistentry>
                            <term>diffuse_color: </term>
                            <listitem>
                                <para>This field defines the diffuse color of the light.
                                    The color is defined by a 3 element vector <literal>"[red, green, blue]"</literal>
                                    with each element in the range [0, 1].
                                </para>
                                <para>
                                    The default value is <literal>"[1 1 1]"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                        <varlistentry>
                            <term>specular_color: </term>
                            <listitem>
                                <para>This field defines the specular color of the light.
                                    The color is defined by a 3 element vector <literal>"[red, green, blue]"</literal>
                                    with each element in the range [0, 1].
                                </para>
                                <para>
                                    The default value is <literal>"[1 1 1]"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                    </variablelist>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>Surface properties </term>
                <listitem>
                    <variablelist>
                        <varlistentry>
                            <term>use_color_material: </term>
                            <listitem>
                                <para>This field is use to enable the use of the surface color as diffuse color.
                                    Its default value is <literal>"on"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                        <varlistentry>
                            <term>material_shininess: </term>
                            <listitem>
                                <para>This field defines the shininess level of the surface. This parameter
                                    controls the shinines color spreading in the surface. Any positive value can
                                    be used.
                                </para>
                                <para>
                                    Good values to use are in the [0 10] range, where low values generates
                                    strong highlight and high values generates barely perceptible highlights.
                                    The default value is <literal>"2"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                        <varlistentry>
                            <term>ambient_color: </term>
                            <listitem>
                                <para>This field defines the ambient color of the surface.
                                    The color is defined by a 3 element vector <literal>"[red, green, blue]"</literal>
                                    with each element in the range [0, 1].
                                </para>
                                <para>
                                    The default value is <literal>"[1 1 1]"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                        <varlistentry>
                            <term>diffuse_color: </term>
                            <listitem>
                                <para>This field defines the diffuse color of the surface.
                                    The color is defined by a 3 element vector <literal>"[red, green, blue]"</literal>
                                    with each element in the range [0, 1]. Notice that this field is
                                    only used when use_color_material is disabled.
                                </para>
                                <para>
                                    The default value is <literal>"[1 1 1]"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                        <varlistentry>
                            <term>specular_color: </term>
                            <listitem>
                                <para>This field defines the specular color of the surface.
                                    The color is defined by a 3 element vector <literal>"[red, green, blue]"</literal>
                                    with each element in the range [0, 1].
                                </para>
                                <para>
                                    The default value is <literal>"[1 1 1]"</literal>.
                                </para>
                            </listitem>
                        </varlistentry>
                    </variablelist>
                </listitem>
            </varlistentry>
        </variablelist>
    </refsection>
    <refsection>
        <title>Examples</title>
        <para>
            Point light without color_material:
        </para>
        <programlisting role="example"><![CDATA[
plot3d();
s=gce();
s.color_flag=0;
s.thickness=0;
s.hiddencolor = -1;
light();
//change diffuse color to orange
s.diffuse_color = [1, 0.6, 0];
s.use_color_material="off";
//black spots turn dark orange
s.ambient_color = [1, 0.6, 0];
         ]]></programlisting>
        <para>
            Directional light:
        </para>
        <programlisting role="example"><![CDATA[
plot3d();
s=gce();
l = light();
s.color_flag=0;
s.thickness=0;
l.light_type = "directional";
l.direction = [0 0 1];
l.diffuse_color = [1 0.9 0.9];
l.ambient_color = [0.5 0.4 0.4];
l.specular_color = [0.5 0.3 0.7];
         ]]></programlisting>
    </refsection>
    <refsection role="see also">
        <title>See also</title>
        <simplelist type="inline">
            <member>
                <link linkend="light">light</link>
            </member>
            <member>
                <link linkend="light_properties">light properties</link>
            </member>
            <member>
                <link linkend="surface_properties">surface properties</link>
            </member>
            <member>
                <link linkend="delete">delete</link>
            </member>
        </simplelist>
    </refsection>
</refentry>
