## Offscreen Buffers

Offscreen Buffers — Fuctions for creating and manipulating offscreen framebuffers.

## Synopsis

    CoglHandle          cogl_offscreen_new_to_texture       (CoglHandle handle);
    gboolean            cogl_is_offscreen                   (CoglHandle handle);
    void                cogl_set_framebuffer                (CoglFramebuffer *buffer);
    void                cogl_push_framebuffer               (CoglFramebuffer *buffer);
    void                cogl_pop_framebuffer                (void);
    void                cogl_set_draw_buffer                (CoglBufferTarget target,
                                                             CoglHandle offscreen);
    void                cogl_pop_draw_buffer                (void);
    void                cogl_push_draw_buffer               (void);

## Description

Cogl allows creating and operating on offscreen framebuffers.

## Details

### cogl_offscreen_new_to_texture ()

    CoglHandle          cogl_offscreen_new_to_texture       (CoglHandle handle);

This creates an offscreen buffer object using the given texture as the primary color buffer. It doesn't just initialize the contents of the offscreen buffer with the texture; they are tightly bound so that drawing to the offscreen buffer effectivly updates the contents of the given texture. You don't need to destroy the offscreen buffer before you can use the texture again.

Note: This does not work with sliced Cogl textures.

`handle` : | A CoglHandle for a Cogl texture
Returns : | a CoglHandle for the new offscreen buffer or `COGL_INVALID_HANDLE` if it wasn't possible to create the buffer. [transfer full]

### cogl_is_offscreen ()

    gboolean            cogl_is_offscreen                   (CoglHandle handle);

Determines whether the given CoglHandle references an offscreen buffer object.

`handle` : | A CoglHandle for an offscreen buffer
Returns : | `TRUE` if the handle references an offscreen buffer, `FALSE` otherwise

### cogl_set_framebuffer ()

    void                cogl_set_framebuffer                (CoglFramebuffer *buffer);

This redirects all subsequent drawing to the specified framebuffer. This can either be an offscreen buffer created with `cogl_offscreen_new_to_texture()` or in the future it may be an onscreen framebuffers too.

`buffer` : | A CoglFramebuffer object, either onscreen or offscreen.

Since 1.2

### cogl_push_framebuffer ()

    void                cogl_push_framebuffer               (CoglFramebuffer *buffer);

Redirects all subsequent drawing to the specified framebuffer. This can either be an offscreen buffer created with `cogl_offscreen_new_to_texture()` or in the future it may be an onscreen framebuffer too.

You should understand that a framebuffer owns the following state:

- The projection matrix
- The modelview matrix stack
- The viewport
- The clip stack

So these items will automatically be saved and restored when you push and pop between different framebuffers.

Also remember a newly allocated framebuffer will have an identity matrix for the projection and modelview matrices which gives you a coordinate space like OpenGL with (-1, -1) corresponding to the top left of the viewport, (1, 1) corresponding to the bottom right and +z coming out towards the viewer.

If you want to set up a coordinate space like Clutter does with (0, 0) corresponding to the top left and (framebuffer_width, framebuffer_height) corresponding to the bottom right you can do so like this:

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    static void
    setup_viewport (unsigned int width,
                    unsigned int height,
                    float fovy,
                    float aspect,
                    float z_near,
                    float z_far)
    {
      float z_camera;
      CoglMatrix projection_matrix;
      CoglMatrix mv_matrix;

      cogl_set_viewport (0, 0, width, height);
      cogl_perspective (fovy, aspect, z_near, z_far);

      cogl_get_projection_matrix (&projection_matrix);
      z_camera = 0.5 * projection_matrix.xx;

      cogl_matrix_init_identity (&mv_matrix);
      cogl_matrix_translate (&mv_matrix, -0.5f, -0.5f, -z_camera);
      cogl_matrix_scale (&mv_matrix, 1.0f / width, -1.0f / height, 1.0f / width);
      cogl_matrix_translate (&mv_matrix, 0.0f, -1.0 * height, 0.0f);
      cogl_set_modelview_matrix (&mv_matrix);
    }

    static void
    my_init_framebuffer (ClutterStage *stage,
                         CoglFramebuffer *framebuffer,
                         unsigned int framebuffer_width,
                         unsigned int framebuffer_height)
    {
      ClutterPerspective perspective;

      clutter_stage_get_perspective (stage, &perspective);

      cogl_push_framebuffer (framebuffer);
      setup_viewport (framebuffer_width,
                      framebuffer_height,
                      perspective.fovy,
                      perspective.aspect,
                      perspective.z_near,
                      perspective.z_far);
    }

The previous framebuffer can be restored by calling `cogl_pop_framebuffer()`

`buffer` : | A CoglFramebuffer object, either onscreen or offscreen.

Since 1.2

### cogl_pop_framebuffer ()

    void                cogl_pop_framebuffer                (void);

Restores the framebuffer that was previously at the top of the stack. All subsequent drawing will be redirected to this framebuffer.

Since 1.2

### cogl_set_draw_buffer ()

    void                cogl_set_draw_buffer                (CoglBufferTarget target,
                                                             CoglHandle offscreen);

### Warning

`cogl_set_draw_buffer` has been deprecated since version 1.2 and should not be used in newly-written code. The target argument was redundant since we could look at the type of CoglHandle given instead.

Redirects all subsequent drawing to the specified framebuffer. This can either be an offscreen buffer created with `cogl_offscreen_new_to_texture()` or you can revert to your original on screen window buffer.

`target` : | A CoglBufferTarget that specifies what kind of framebuffer you are setting as the render target.
`offscreen` : | If you are setting a framebuffer of type COGL_OFFSCREEN_BUFFER then this is a CoglHandle for the offscreen buffer.

### cogl_pop_draw_buffer ()

    void                cogl_pop_draw_buffer                (void);

### Warning

`cogl_pop_draw_buffer` has been deprecated since version 1.2 and should not be used in newly-written code. The draw buffer API was replaced with a framebuffer API

Restore `cogl_set_draw_buffer()` state.

### cogl_push_draw_buffer ()

    void                cogl_push_draw_buffer               (void);

### Warning

`cogl_push_draw_buffer` has been deprecated since version 1.2 and should not be used in newly-written code. The draw buffer API was replaced with a framebuffer API

Save `cogl_set_draw_buffer()` state.