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cookbook: Add a recipe for texture reflection
A common request: how to create a clone of a texture that looks like a reflection.
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Emmanuele Bassi
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3aa3893a11
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4170eacd94
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doc/cookbook/images/textures-reflection.png
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doc/cookbook/images/textures-reflection.png
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@ -500,4 +500,202 @@ main (int argc, char *argv[])
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</section>
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</section>
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<section id="textures-reflection">
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<title>Creating a reflection of a texture</title>
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<section>
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<title>Problem</title>
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<para>You want to create the reflection of a texture.</para>
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<para>The reflection is going to be positioned below the original
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texture, and is going to fade out as if the original was placed on
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a glassy surface.</para>
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</section>
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<section>
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<title>Solution</title>
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<para>You can use a ClutterClone actor and override its paint
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implementation with a custom one:</para>
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<informalexample>
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<programlisting>
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<![CDATA[
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static void
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_clone_paint_cb (ClutterActor *actor)
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{
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ClutterActor *source;
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ClutterActorBox box;
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CoglHandle material;
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gfloat width, height;
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guint8 opacity;
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CoglColor color_1, color_2;
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CoglTextureVertex vertices[4];
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/* if we don't have a source actor, don't paint */
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source = clutter_clone_get_source (CLUTTER_CLONE (actor));
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if (source == NULL)
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goto out;
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/* if the source texture does not have any content, don't paint */
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material = clutter_texture_get_cogl_material (CLUTTER_TEXTURE (source));
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if (material == NULL)
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goto out;
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/* set the size of the reflection to be the same as the source */
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clutter_actor_get_allocation_box (actor, &box);
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clutter_actor_box_get_size (&box, &width, &height);
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/* get the composite opacity of the actor */
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opacity = clutter_actor_get_paint_opacity (actor);
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/* figure out the two colors for the reflection: the first is
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* full color and the second is the same, but at 0 opacity
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*/
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cogl_color_set_from_4f (&color_1, 1.0, 1.0, 1.0, opacity / 255.);
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cogl_color_premultiply (&color_1);
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cogl_color_set_from_4f (&color_2, 1.0, 1.0, 1.0, 0.0);
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cogl_color_premultiply (&color_2);
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/* now describe the four vertices of the quad; since it has
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* to be a reflection, we need to invert it as well
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*/
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vertices[0].x = 0; vertices[0].y = 0; vertices[0].z = 0;
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vertices[0].tx = 0.0; vertices[0].ty = 1.0;
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vertices[0].color = color_1;
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vertices[1].x = width; vertices[1].y = 0; vertices[1].z = 0;
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vertices[1].tx = 1.0; vertices[1].ty = 1.0;
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vertices[1].color = color_1;
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vertices[2].x = width; vertices[2].y = height; vertices[2].z = 0;
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vertices[2].tx = 1.0; vertices[2].ty = 0.0;
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vertices[2].color = color_2;
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vertices[3].x = 0; vertices[3].y = height; vertices[3].z = 0;
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vertices[3].tx = 0.0; vertices[3].ty = 0.0;
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vertices[3].color = color_2;
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/* paint the same texture but with a different geometry */
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cogl_set_source (material);
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cogl_polygon (vertices, 4, TRUE);
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out:
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/* prevent the default clone handler from running */
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g_signal_stop_emission_by_name (actor, "paint");
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}
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int
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main (int argc, char *argv[])
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{
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clutter_init (&argc, &argv);
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/* ... get stage etc. */
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ClutterActor *texture;
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GError *error = NULL;
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texture = clutter_texture_new ();
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/* load the image from a file */
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clutter_texture_set_from_file (CLUTTER_TEXTURE (texture),
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image_path,
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&error);
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ClutterActor *clone;
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clone = clutter_clone_new (texture);
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g_signal_connect (clone,
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"paint",
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G_CALLBACK (_clone_paint_cb),
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NULL);
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/* ... clutter_main () etc. */
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}
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]]>
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</programlisting>
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</informalexample>
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<figure id="textures-reflection-image">
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<title>Reflection of a texture</title>
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<graphic fileref="images/textures-reflection.png" format="PNG"/>
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</figure>
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</section>
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<section>
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<title>Discussion</title>
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<para>The essence of painting a reflection of a texture lies in reusing
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the same material used by the original. This not only allows painting
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always an up to date version of the original, but it also saves
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resources.</para>
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<para>In the code example above we take the <type>CoglMaterial</type>
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out of the source <type>ClutterTexture</type> and we ask the Cogl
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pipeline to paint it by using <function>cogl_set_source()</function>. The
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main difference between this code and the equivalent code inside the
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<type>ClutterTexture</type> <function>paint()</function> implementation
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is that we also specify the texture vertices and their color by using the
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<type>CoglTextureVertex</type> structure and the
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<function>cogl_polygon()</function> function.</para>
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<para>The <type>CoglTextureVertex</type> structure contains three fields
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for the position of the vertex in 3D space:</para>
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<informalexample>
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<programlisting><![CDATA[
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typedef struct _CoglTextureVertex {
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float x;
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float y;
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float z;
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...
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]]></programlisting>
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</informalexample>
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<para>It also contains the normalized texture coordinate (also known as
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texture element, or <emphasis>texel</emphasis>):</para>
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<informalexample>
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<programlisting><![CDATA[
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...
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float tx;
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float ty;
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...
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]]></programlisting>
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</informalexample>
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<para>And, finally, the color of the vertex, expressed as a
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<type>CoglColor</type>:</para>
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<informalexample>
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<programlisting><![CDATA[
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...
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CoglColor color;
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} CoglTextureVertex;
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]]></programlisting>
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</informalexample>
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<para>The example code sets the position of the vertices in clockwise
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order starting from the top left corner, and sets the coordinate of the
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texels in counter-clockwise order, starting with the bottom left corner.
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This makes sure that the copy of the original texture appears as being
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flipped vertically.</para>
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<para>The gradual fading out to the background color is done by setting
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the color of the top vertices to be fully opaque, and the color of the
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bottom ones to be fully transparent; GL will then automatically create a
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gradient that will be applied when painting the material.</para>
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<note><para>The color values must be pre-multiplied with their alpha
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component, otherwise the bleding will not be correct. You can either
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multiply the values by yourself when creating the color or, better yet,
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use the <function>cogl_color_premultiply()</function> that Cogl provides
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for this operation.</para></note>
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</section>
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</section>
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</chapter>
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