You want to create a new ClutterTexture that only displays a rectangular sub-region of an existing texture.
A possible way of achieving this is to retrieve the
CoglHandle of the underlying Cogl texture of the existing
ClutterTexture, create a new handle representing the
sub-region with cogl_texture_new_from_sub_texture()
and finally populate a new ClutterTexture with that handle.
/* Create a new ClutterTexture that shows smiley.png */
image = clutter_texture_new_from_file ("smiley.png", NULL);
clutter_actor_get_size (image, &image_width, &image_height);
/* Grab the CoglHandle of the underlying Cogl texture */
texture = clutter_texture_get_cogl_texture (CLUTTER_TEXTURE (image));
/* Create a new Cogl texture from the handle above. That new texture is a
* rectangular region from image, more precisely the northwest corner
* of the image */
sub_texture = cogl_texture_new_from_sub_texture (texture,
0, 0,
image_width / 2,
image_height / 2);
/* Finally, use the newly created Cogl texture to feed a new ClutterTexture
* and thus create a new actor that displays sub_texture */
sub_image = clutter_texture_new ();
clutter_texture_set_cogl_texture (CLUTTER_TEXTURE (sub_image), sub_texture);
/*
* You could have used the more straightforward g_object_new() function that
* can create an object and set some properties on it at the same time:
* sub_image = g_object_new (CLUTTER_TYPE_TEXTURE,
* "cogl-texture", sub_texture,
* NULL);
*/
The key of this recipe is the Cogl handle that represents the underlying texture, the actual array of pixels the GPU will use when it's told to texture geometry.
From this handle, it's possible to create a new texture handle
that represents a rectangular region of the former texture. To do this
one must call cogl_texture_new_from_sub_texture()
with the position and size of the said region. The interesting bit
about this function is that, when drawing either with the original
texture or with the new one, it's still the same GPU resource (pixels)
being used, meaning that creating a sub-texture doesn't use extra GPU
memory.
Once the sub-texture handle is created, the next step is to create a new actor that will be able to draw it, namely a new ClutterTexture. You then need to tell the texture to draw from the sub-texture.
Note
The handle you can get from
clutter_texture_get_cogl_texture() is effectively
the same texture than the first layer of the material retrieved by
clutter_texture_get_cogl_material()
Example 4.1. Creating a sub-texture from an existing texture
#include <clutter/clutter.h>
int
main (int argc, char **argv)
{
ClutterActor *stage, *image, *sub_image;
CoglHandle texture, sub_texture;
gfloat image_width, image_height;
/* Initialize Clutter */
if (clutter_init (NULL, NULL) != CLUTTER_INIT_SUCCESS)
return 1;
/* Get the default stage */
stage = clutter_stage_new ();
clutter_stage_set_title (CLUTTER_STAGE (stage), "Sub-texture");
g_signal_connect (stage, "destroy", G_CALLBACK (clutter_main_quit), NULL);
/* Create a new ClutterTexture that shows smiley.png */
image = clutter_texture_new_from_file ("smiley.png", NULL);
clutter_actor_get_size (image, &image_width, &image_height);
clutter_actor_set_size (stage,
image_width * 3 / 2 + 30,
image_height + 20);
/* Grab the CoglHandle of the underlying Cogl texture */
texture = clutter_texture_get_cogl_texture (CLUTTER_TEXTURE (image));
/* Create a new Cogl texture from the handle above. That new texture is a
* rectangular region from image, more precisely the northwest corner
* of the image */
sub_texture = cogl_texture_new_from_sub_texture (texture,
0, 0,
image_width / 2,
image_height / 2);
/* Finally, use the newly created Cogl texture to feed a new ClutterTexture
* and thus create a new actor that displays sub_texture */
sub_image = clutter_texture_new ();
clutter_texture_set_cogl_texture (CLUTTER_TEXTURE (sub_image), sub_texture);
/*
* You could have used the more straightforward g_object_new() function that
* can create an object and set some properties on it at the same time:
* sub_image = g_object_new (CLUTTER_TYPE_TEXTURE,
* "cogl-texture", sub_texture,
* NULL);
*/
/* Put the original image at (10,10) and the new sub image next to it */
clutter_actor_set_position (image, 10, 10);
clutter_actor_set_position (sub_image, 20 + image_width, 10);
/* Add both ClutterTexture to the stage */
clutter_container_add (CLUTTER_CONTAINER (stage), image, sub_image, NULL);
clutter_actor_show_all (stage);
clutter_main ();
return 0;
}Now that we know how to create sub-textures, it's time to make something a bit more shiny with them. Let's animate them! In case you have not heard about implicit animations in Clutter yet, it's a good time to have a look at the animation section of this cookbook.
Example 4.2. Creating a sub-texture from an existing texture
#include <clutter/clutter.h>
/* Context will be used to carry interesting variables between functions */
typedef struct
{
ClutterActor *sub_nw, *sub_ne, *sub_sw, *sub_se;
gfloat image_width, image_height;
} Context;
/* Here, we animate the texture to go way by giving the new coordinates
* outside of the stage. We rotate the sub-textures around their anchor
* point (set in setup_sub() as well, it looks cool. */
static gboolean
go_away (gpointer data)
{
Context *context = data;
clutter_actor_animate (context->sub_nw, CLUTTER_EASE_OUT_CUBIC, 1500,
"x", -context->image_width,
"y", -context->image_height,
"rotation-angle-z", 2000.,
NULL);
clutter_actor_animate (context->sub_ne, CLUTTER_EASE_OUT_CUBIC, 1500,
"x", +context->image_width,
"y", -context->image_height,
"rotation-angle-z", 2000.,
NULL);
clutter_actor_animate (context->sub_sw, CLUTTER_EASE_OUT_CUBIC, 1500,
"x", -context->image_width,
"y", +context->image_height,
"rotation-angle-z", 2000.,
NULL);
clutter_actor_animate (context->sub_se, CLUTTER_EASE_OUT_CUBIC, 1500,
"x", -context->image_width,
"y", +context->image_height,
"rotation-angle-z", 2000.,
NULL);
return G_SOURCE_REMOVE; /* remove the timeout source */
}
/* We split the four sub-textures faking to be the big texture, moving them
* away by 10 pixels in each direction */
static gboolean
split (gpointer data)
{
Context *context = data;
gfloat x, y;
clutter_actor_get_position (context->sub_nw, &x, &y);
clutter_actor_animate (context->sub_nw, CLUTTER_EASE_OUT_CUBIC, 300,
"x", x - 10,
"y", y - 10,
NULL);
clutter_actor_get_position (context->sub_ne, &x, &y);
clutter_actor_animate (context->sub_ne, CLUTTER_EASE_OUT_CUBIC, 300,
"x", x + 10,
"y", y - 10,
NULL);
clutter_actor_get_position (context->sub_sw, &x, &y);
clutter_actor_animate (context->sub_sw, CLUTTER_EASE_OUT_CUBIC, 300,
"x", x - 10,
"y", y + 10,
NULL);
clutter_actor_get_position (context->sub_se, &x, &y);
clutter_actor_animate (context->sub_se, CLUTTER_EASE_OUT_CUBIC, 300,
"x", x + 10,
"y", y + 10,
NULL);
/* In 500ms the textures will flee! */
clutter_threads_add_timeout (500, go_away, context);
return G_SOURCE_REMOVE; /* remove the timeout source */
}
static ClutterActor *
setup_sub (CoglHandle texture,
gint image_width,
gint image_height,
gint t_x,
gint t_y,
gint t_width,
gint t_height)
{
CoglHandle sub_texture;
ClutterActor *sub_image;
/* Create a new sub-texture from textures */
sub_texture = cogl_texture_new_from_sub_texture (texture,
t_x, t_y,
t_width, t_height);
/* Create the corresponding ClutterTexture */
sub_image = g_object_new (CLUTTER_TYPE_TEXTURE,
"cogl-texture", sub_texture,
NULL);
/* Set the anchor point in the middle of each sub_image so the position and
* rotation of the textures are relative to that point */
clutter_actor_set_anchor_point (sub_image, image_width / 4, image_height / 4);
return sub_image;
}
#define IMAGE "smiley.png"
int
main (int argc,
char **argv)
{
gfloat image_width, image_height, stage_width, stage_height;
ClutterActor *stage, *image;
GError *error = NULL;
CoglHandle texture;
Context context;
if (clutter_init (NULL, NULL) != CLUTTER_INIT_SUCCESS)
return 1;
stage = clutter_stage_new ();
clutter_actor_get_size (stage, &stage_width, &stage_height);
clutter_stage_set_title (CLUTTER_STAGE (stage), "Animate sub-textures");
g_signal_connect (stage, "destroy", G_CALLBACK (clutter_main_quit), NULL);
/* Load smiley.png, creating a new ClutterTexture, get its size and the
* Cogl texture handle */
image = clutter_texture_new_from_file (IMAGE, &error);
if (error != NULL)
{
g_warning ("Could not load " IMAGE ": %s", error->message);
g_clear_error (&error);
return 1;
}
clutter_actor_get_size (image, &image_width, &image_height);
texture = clutter_texture_get_cogl_texture (CLUTTER_TEXTURE (image));
/* Create four sub-textures from image, actually splitting the image in
* four */
context.sub_nw = setup_sub (texture, image_width, image_height,
0, 0, image_width / 2 , image_height / 2);
context.sub_ne = setup_sub (texture, image_width, image_height,
image_width / 2 , 0,
image_width / 2, image_height / 2);
context.sub_sw = setup_sub (texture, image_width, image_height,
0.f, image_height / 2,
image_width / 2, image_height / 2);
context.sub_se = setup_sub (texture, image_width, image_height,
image_width / 2, image_height / 2,
image_width / 2, image_height / 2);
/* We don't need the image anymore as we won't display it and as
* cogl_texture_new_from_sub_texture() keeps a reference to the underlying
* texture ressource */
g_object_unref (image);
/* Position the sub-texures in the middle of the screen, recreating the
* original texture */
clutter_actor_set_position (context.sub_nw,
stage_width / 2 - image_width / 4,
stage_height / 2 - image_height / 4);
clutter_actor_set_position (context.sub_ne,
stage_width / 2 + image_width / 4,
stage_height / 2 - image_height / 4);
clutter_actor_set_position (context.sub_sw,
stage_width / 2 - image_width / 4,
stage_height / 2 + image_height / 4);
clutter_actor_set_position (context.sub_se,
stage_width / 2 + image_width / 4,
stage_height / 2 + image_height / 4);
/* Add the four sub-textures to the stage */
clutter_container_add (CLUTTER_CONTAINER (stage), context.sub_nw,
context.sub_ne, context.sub_sw, context.sub_se, NULL);
clutter_actor_show_all (stage);
context.image_width = image_width;
context.image_height = image_height;
/* In two seconds, we'll split the texture! */
clutter_threads_add_timeout (2000, split, &context);
clutter_main ();
return 0;
}