6. Creating a reflection of a texture
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## 6. Creating a reflection of a texture

### 6.1. Problem

You want to create the reflection of a texture.

The reflection is going to be positioned below the original texture, and is going to fade out as if the original was placed on a glassy surface.

### 6.2. Solution

You can use a ClutterClone actor and override its paint implementation with a custom one:

    static void
    _clone_paint_cb (ClutterActor *actor)
    {
      /* ... */

      /* get the Cogl material of the source texture */
      material = clutter_texture_get_cogl_material (CLUTTER_TEXTURE (source));

      /* get the size of the actor, which will be used to size the reflection */
      clutter_actor_get_allocation_box (actor, &box);
      clutter_actor_box_get_size (&box, &width, &height);

      /* get the composite opacity of the actor */
      opacity = clutter_actor_get_paint_opacity (actor);

      /* figure out the two colors for the reflection: the first is
       * full color and the second is the same, but at 0 opacity
       */
      cogl_color_init_from_4f (&color_1, 1.0, 1.0, 1.0, opacity / 255.);
      cogl_color_premultiply (&color_1);
      cogl_color_init_from_4f (&color_2, 1.0, 1.0, 1.0, 0.0);
      cogl_color_premultiply (&color_2);

      /* describe the four vertices of the quad; since it has
       * to be a reflection, we need to invert it as well
       */
      vertices[0].x = 0; vertices[0].y = 0; vertices[0].z = 0;
      vertices[0].tx = 0.0; vertices[0].ty = 1.0;
      vertices[0].color = color_1;

      vertices[1].x = width; vertices[1].y = 0; vertices[1].z = 0;
      vertices[1].tx = 1.0; vertices[1].ty = 1.0;
      vertices[1].color = color_1;

      vertices[2].x = width; vertices[2].y = height; vertices[2].z = 0;
      vertices[2].tx = 1.0; vertices[2].ty = 0.0;
      vertices[2].color = color_2;

      vertices[3].x = 0; vertices[3].y = height; vertices[3].z = 0;
      vertices[3].tx = 0.0; vertices[3].ty = 0.0;
      vertices[3].color = color_2;

      /* paint the same texture but with a different geometry */
      cogl_set_source (material);
      cogl_polygon (vertices, 4, TRUE);

      /* ... */
    }

    int
    main (int argc, char *argv[])
    {
      if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
        return 1;

      /* ... get stage etc. */

      ClutterActor *texture;
      GError *error = NULL;

      texture = clutter_texture_new ();

      /* load the image from a file */
      clutter_texture_set_from_file (CLUTTER_TEXTURE (texture),
                                     image_path,
                                     &error);

      ClutterActor *clone;

      clone = clutter_clone_new (texture);

      g_signal_connect (clone,
                        "paint",
                        G_CALLBACK (_clone_paint_cb),
                        NULL);

      /* ... clutter_main () etc. */
    }

### 6.3. Discussion

The essence of painting a reflection of a texture lies in reusing the same material used by the original. This not only allows painting always an up to date version of the original, but it also saves resources.

In the code example above we take the CoglMaterial out of the source ClutterTexture and we ask the Cogl pipeline to paint it by using `cogl_set_source()`. The main difference between this code and the equivalent code inside the ClutterTexture `paint()` implementation is that we also specify the texture vertices and their color by using the CoglTextureVertex structure and the `cogl_polygon()` function.

The CoglTextureVertex structure contains three fields for the position of the vertex in 3D space:

    typedef struct _CoglTextureVertex {
      float x;
      float y;
      float z;
      ...

It also contains the normalized texture coordinate (also known as texture element, or texel):

      ...
      float tx;
      float ty;
      ...

And, finally, the color of the vertex, expressed as a CoglColor:

      ...
      CoglColor color;
    } CoglTextureVertex;

The example code sets the position of the vertices in clockwise order starting from the top left corner, and sets the coordinate of the texels in counter-clockwise order, starting with the bottom left corner. This makes sure that the copy of the original texture appears as being flipped vertically.

The gradual fading out to the background color is done by setting the color of the top vertices to be fully opaque, and the color of the bottom ones to be fully transparent; GL will then automatically create a gradient that will be applied when painting the material.

### Note

The color values must be pre-multiplied with their alpha component, otherwise the bleding will not be correct. You can either multiply the values by yourself when creating the color or, better yet, use the `cogl_color_premultiply()` that Cogl provides for this operation.

### 6.4. Full example

Example 4.3. Creating a glassy reflection of a texture

              #include <stdlib.h>
    #include <clutter/clutter.h>

    /* pixels between the source and its reflection */
    #define V_PADDING       4

    static void
    _clone_paint_cb (ClutterActor *actor)
    {
      ClutterActor *source;
      ClutterActorBox box;
      CoglHandle material;
      gfloat width, height;
      guint8 opacity;
      CoglColor color_1, color_2;
      CoglTextureVertex vertices[4];

      /* if we don't have a source actor, don't paint */
      source = clutter_clone_get_source (CLUTTER_CLONE (actor));
      if (source == NULL)
        goto out;

      /* if the source texture does not have any content, don't paint */
      material = clutter_texture_get_cogl_material (CLUTTER_TEXTURE (source));
      if (material == NULL)
        goto out;

      /* get the size of the reflection */
      clutter_actor_get_allocation_box (actor, &box);
      clutter_actor_box_get_size (&box, &width, &height);

      /* get the composite opacity of the actor */
      opacity = clutter_actor_get_paint_opacity (actor);

      /* figure out the two colors for the reflection: the first is
       * full color and the second is the same, but at 0 opacity
       */
      cogl_color_init_from_4f (&color_1, 1.0, 1.0, 1.0, opacity / 255.);
      cogl_color_premultiply (&color_1);
      cogl_color_init_from_4f (&color_2, 1.0, 1.0, 1.0, 0.0);
      cogl_color_premultiply (&color_2);

      /* now describe the four vertices of the quad; since it has
       * to be a reflection, we need to invert it as well
       */
      vertices[0].x = 0; vertices[0].y = 0; vertices[0].z = 0;
      vertices[0].tx = 0.0; vertices[0].ty = 1.0;
      vertices[0].color = color_1;

      vertices[1].x = width; vertices[1].y = 0; vertices[1].z = 0;
      vertices[1].tx = 1.0; vertices[1].ty = 1.0;
      vertices[1].color = color_1;

      vertices[2].x = width; vertices[2].y = height; vertices[2].z = 0;
      vertices[2].tx = 1.0; vertices[2].ty = 0.0;
      vertices[2].color = color_2;

      vertices[3].x = 0; vertices[3].y = height; vertices[3].z = 0;
      vertices[3].tx = 0.0; vertices[3].ty = 0.0;
      vertices[3].color = color_2;

      /* paint the same texture but with a different geometry */
      cogl_set_source (material);
      cogl_polygon (vertices, 4, TRUE);

    out:
      /* prevent the default clone handler from running */
      g_signal_stop_emission_by_name (actor, "paint");
    }

    int
    main (int argc, char *argv[])
    {
      ClutterActor *stage;
      ClutterActor *texture;
      GError *error = NULL;
      ClutterActor *clone;
      gfloat y_offset;

      if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
        return 1;

      stage = clutter_stage_new ();
      clutter_stage_set_title (CLUTTER_STAGE (stage), "Reflection");
      g_signal_connect (stage, "destroy", G_CALLBACK (clutter_main_quit), NULL);

      texture = clutter_texture_new ();
      clutter_texture_set_from_file (CLUTTER_TEXTURE (texture),
                                     "redhand.png",
                                     &error);
      clutter_actor_add_constraint (texture, clutter_align_constraint_new (stage, CLUTTER_ALIGN_X_AXIS, 0.5));
      clutter_actor_add_constraint (texture, clutter_align_constraint_new (stage, CLUTTER_ALIGN_Y_AXIS, 0.2));

      y_offset = clutter_actor_get_height (texture) + V_PADDING;

      clone = clutter_clone_new (texture);
      clutter_actor_add_constraint (clone, clutter_bind_constraint_new (texture, CLUTTER_BIND_X, 0.0));
      clutter_actor_add_constraint (clone, clutter_bind_constraint_new (texture, CLUTTER_BIND_Y, y_offset));
      g_signal_connect (clone,
                        "paint",
                        G_CALLBACK (_clone_paint_cb),
                        NULL);

      clutter_container_add (CLUTTER_CONTAINER (stage), texture, clone, NULL);

      clutter_actor_show (stage);

      clutter_main ();

      return EXIT_SUCCESS;
    }

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