gstvideooverlaycomposition

## gstvideooverlaycomposition gstvideooverlaycomposition — Video Buffer Overlay Compositions (Subtitles, Logos) |

## Synopsis

    #include <gst/video/video-overlay-composition.h>

                        GstVideoOverlayComposition;
    GstVideoOverlayComposition * gst_video_overlay_composition_new
                                                            (GstVideoOverlayRectangle *rectangle);
    GstVideoOverlayComposition * gst_video_overlay_composition_ref
                                                            (GstVideoOverlayComposition *comp);
    void                gst_video_overlay_composition_unref (GstVideoOverlayComposition *comp);
    void                gst_video_overlay_composition_add_rectangle
                                                            (GstVideoOverlayComposition *comp,
                                                             GstVideoOverlayRectangle *rectangle);
    guint               gst_video_overlay_composition_n_rectangles
                                                            (GstVideoOverlayComposition *comp);
    GstVideoOverlayRectangle * gst_video_overlay_composition_get_rectangle
                                                            (GstVideoOverlayComposition *comp,
                                                             guint n);
    guint               gst_video_overlay_composition_get_seqnum
                                                            (GstVideoOverlayComposition *comp);
    GstVideoOverlayComposition * gst_video_overlay_composition_copy
                                                            (GstVideoOverlayComposition *comp);
    GstVideoOverlayComposition * gst_video_overlay_composition_make_writable
                                                            (GstVideoOverlayComposition *comp);
    gboolean            gst_video_overlay_composition_blend (GstVideoOverlayComposition *comp,
                                                             GstVideoFrame *video_buf);

    const GstMetaInfo * gst_video_overlay_composition_meta_get_info
                                                            (void);
    GstVideoOverlayCompositionMeta * gst_buffer_add_video_overlay_composition_meta
                                                            (GstBuffer *buf,
                                                             GstVideoOverlayComposition *comp);
    #define             gst_buffer_get_video_overlay_composition_meta(b)
    #define             gst_buffer_remove_video_overlay_composition_meta(b,
                                                             m)

    enum                GstVideoOverlayFormatFlags;
                        GstVideoOverlayRectangle;
    GstVideoOverlayRectangle * gst_video_overlay_rectangle_new_raw
                                                            (GstBuffer *pixels,
                                                             gint render_x,
                                                             gint render_y,
                                                             guint render_width,
                                                             guint render_height,
                                                             GstVideoOverlayFormatFlags flags);
    GstVideoOverlayRectangle * gst_video_overlay_rectangle_ref
                                                            (GstVideoOverlayRectangle *comp);
    void                gst_video_overlay_rectangle_unref   (GstVideoOverlayRectangle *comp);
    GstBuffer *         gst_video_overlay_rectangle_get_pixels_raw
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);
    GstBuffer *         gst_video_overlay_rectangle_get_pixels_argb
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);
    GstBuffer *         gst_video_overlay_rectangle_get_pixels_ayuv
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);
    GstBuffer *         gst_video_overlay_rectangle_get_pixels_unscaled_raw
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);
    GstBuffer *         gst_video_overlay_rectangle_get_pixels_unscaled_argb
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);
    GstBuffer *         gst_video_overlay_rectangle_get_pixels_unscaled_ayuv
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);
    gboolean            gst_video_overlay_rectangle_get_render_rectangle
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             gint *render_x,
                                                             gint *render_y,
                                                             guint *render_width,
                                                             guint *render_height);
    guint               gst_video_overlay_rectangle_get_seqnum
                                                            (GstVideoOverlayRectangle *rectangle);
    void                gst_video_overlay_rectangle_set_render_rectangle
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             gint render_x,
                                                             gint render_y,
                                                             guint render_width,
                                                             guint render_height);
    GstVideoOverlayRectangle * gst_video_overlay_rectangle_copy
                                                            (GstVideoOverlayRectangle *rectangle);
    GstVideoOverlayFormatFlags gst_video_overlay_rectangle_get_flags
                                                            (GstVideoOverlayRectangle *rectangle);
    gfloat              gst_video_overlay_rectangle_get_global_alpha
                                                            (GstVideoOverlayRectangle *rectangle);
    void                gst_video_overlay_rectangle_set_global_alpha
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             gfloat global_alpha);

## Description

Functions to create and handle overlay compositions on video buffers.

An overlay composition describes one or more overlay rectangles to be blended on top of a video buffer.

This API serves two main purposes:

GstOverlayComposition

- it can be used to attach overlay information (subtitles or logos) to non-raw video buffers such as GL/VAAPI/VDPAU surfaces. The actual blending of the overlay can then be done by e.g. the video sink that processes these non-raw buffers.
- it can also be used to blend overlay rectangles on top of raw video buffers, thus consolidating blending functionality for raw video in one place.

## Details

### GstVideoOverlayComposition

    typedef struct _GstVideoOverlayComposition GstVideoOverlayComposition;

An opaque video overlay composition object. A composition contains multiple overlay rectangles.

### gst_video_overlay_composition_new ()

    GstVideoOverlayComposition * gst_video_overlay_composition_new
                                                            (GstVideoOverlayRectangle *rectangle);

Creates a new video overlay composition object to hold one or more overlay rectangles.

`rectangle` : | a GstVideoOverlayRectangle to add to the composition. [transfer none]
Returns : | a new GstVideoOverlayComposition. Unref with `gst_video_overlay_composition_unref()` when no longer needed. [transfer full]

### gst_video_overlay_composition_ref ()

    GstVideoOverlayComposition * gst_video_overlay_composition_ref
                                                            (GstVideoOverlayComposition *comp);

Increases the refcount of the given composition by one.

Note that the refcount affects the writeability of `comp`, use `gst_video_overlay_composition_make_writable()` to ensure a composition and its rectangles can be modified.

`comp` : | a a GstVideoOverlayComposition.
Returns : | `comp`. [transfer full]

### gst_video_overlay_composition_unref ()

    void                gst_video_overlay_composition_unref (GstVideoOverlayComposition *comp);

Decreases the refcount of the composition. If the refcount reaches 0, the composition will be freed.

`comp` : | a GstVideoOverlayComposition. [transfer full]

### gst_video_overlay_composition_add_rectangle ()

    void                gst_video_overlay_composition_add_rectangle
                                                            (GstVideoOverlayComposition *comp,
                                                             GstVideoOverlayRectangle *rectangle);

Adds an overlay rectangle to an existing overlay composition object. This must be done right after creating the overlay composition.

`comp` : | a GstVideoOverlayComposition
`rectangle` : | a GstVideoOverlayRectangle to add to the composition. [transfer none]

### gst_video_overlay_composition_n_rectangles ()

    guint               gst_video_overlay_composition_n_rectangles
                                                            (GstVideoOverlayComposition *comp);

Returns the number of GstVideoOverlayRectangles contained in `comp`.

`comp` : | a GstVideoOverlayComposition
Returns : | the number of rectangles

### gst_video_overlay_composition_get_rectangle ()

    GstVideoOverlayRectangle * gst_video_overlay_composition_get_rectangle
                                                            (GstVideoOverlayComposition *comp,
                                                             guint n);

Returns the `n`-th GstVideoOverlayRectangle contained in `comp`.

`comp` : | a GstVideoOverlayComposition
`n` : | number of the rectangle to get
Returns : | the `n`-th rectangle, or NULL if `n` is out of bounds. Will not return a new reference, the caller will need to obtain her own reference using `gst_video_overlay_rectangle_ref()` if needed. [transfer none]

### gst_video_overlay_composition_get_seqnum ()

    guint               gst_video_overlay_composition_get_seqnum
                                                            (GstVideoOverlayComposition *comp);

Returns the sequence number of this composition. Sequence numbers are monotonically increasing and unique for overlay compositions and rectangles (meaning there will never be a rectangle with the same sequence number as a composition).

`comp` : | a GstVideoOverlayComposition
Returns : | the sequence number of `comp`

### gst_video_overlay_composition_copy ()

    GstVideoOverlayComposition * gst_video_overlay_composition_copy
                                                            (GstVideoOverlayComposition *comp);

Makes a copy of `comp` and all contained rectangles, so that it is possible to modify the composition and contained rectangles (e.g. add additional rectangles or change the render co-ordinates or render dimension). The actual overlay pixel data buffers contained in the rectangles are not copied.

`comp` : | a GstVideoOverlayComposition to copy. [transfer none]
Returns : | a new GstVideoOverlayComposition equivalent to `comp`. [transfer full]

### gst_video_overlay_composition_make_writable ()

    GstVideoOverlayComposition * gst_video_overlay_composition_make_writable
                                                            (GstVideoOverlayComposition *comp);

Takes ownership of `comp` and returns a version of `comp` that is writable (i.e. can be modified). Will either return `comp` right away, or create a new writable copy of `comp` and unref `comp` itself. All the contained rectangles will also be copied, but the actual overlay pixel data buffers contained in the rectangles are not copied.

`comp` : | a GstVideoOverlayComposition to copy. [transfer full]
Returns : | a writable GstVideoOverlayComposition equivalent to `comp`. [transfer full]

### gst_video_overlay_composition_blend ()

    gboolean            gst_video_overlay_composition_blend (GstVideoOverlayComposition *comp,
                                                             GstVideoFrame *video_buf);

Blends the overlay rectangles in `comp` on top of the raw video data contained in `video_buf`. The data in `video_buf` must be writable and mapped appropriately.

`comp` : | a GstVideoOverlayComposition
`video_buf` : | a GstVideoFrame containing raw video data in a supported format

### gst_video_overlay_composition_meta_get_info ()

    const GstMetaInfo * gst_video_overlay_composition_meta_get_info
                                                            (void);

### gst_buffer_add_video_overlay_composition_meta ()

    GstVideoOverlayCompositionMeta * gst_buffer_add_video_overlay_composition_meta
                                                            (GstBuffer *buf,
                                                             GstVideoOverlayComposition *comp);

Sets an overlay composition on a buffer. The buffer will obtain its own reference to the composition, meaning this function does not take ownership of `comp`.

`buf` : | a GstBuffer
`comp` : | a GstVideoOverlayComposition. [allow-none]

### gst_buffer_get_video_overlay_composition_meta()

    #define             gst_buffer_get_video_overlay_composition_meta(b)

### gst_buffer_remove_video_overlay_composition_meta()

    #define             gst_buffer_remove_video_overlay_composition_meta(b,m)

### enum GstVideoOverlayFormatFlags

    typedef enum {
      GST_VIDEO_OVERLAY_FORMAT_FLAG_NONE = 0,
      GST_VIDEO_OVERLAY_FORMAT_FLAG_PREMULTIPLIED_ALPHA = 1,
      GST_VIDEO_OVERLAY_FORMAT_FLAG_GLOBAL_ALPHA = 2
    } GstVideoOverlayFormatFlags;

Overlay format flags.

`GST_VIDEO_OVERLAY_FORMAT_FLAG_NONE` | no flags
`GST_VIDEO_OVERLAY_FORMAT_FLAG_PREMULTIPLIED_ALPHA` | RGB are premultiplied by A/255.
`GST_VIDEO_OVERLAY_FORMAT_FLAG_GLOBAL_ALPHA` | a global-alpha value != 1 is set.

### GstVideoOverlayRectangle

    typedef struct _GstVideoOverlayRectangle GstVideoOverlayRectangle;

An opaque video overlay rectangle object. A rectangle contains a single overlay rectangle which can be added to a composition.

### gst_video_overlay_rectangle_new_raw ()

    GstVideoOverlayRectangle * gst_video_overlay_rectangle_new_raw
                                                            (GstBuffer *pixels,
                                                             gint render_x,
                                                             gint render_y,
                                                             guint render_width,
                                                             guint render_height,
                                                             GstVideoOverlayFormatFlags flags);

Creates a new video overlay rectangle with ARGB or AYUV pixel data. The layout in case of ARGB of the components in memory is B-G-R-A on little-endian platforms (corresponding to GST_VIDEO_FORMAT_BGRA) and A-R-G-B on big-endian platforms (corresponding to GST_VIDEO_FORMAT_ARGB). In other words, pixels are treated as 32-bit words and the lowest 8 bits then contain the blue component value and the highest 8 bits contain the alpha component value. Unless specified in the flags, the RGB values are non-premultiplied. This is the format that is used by most hardware, and also many rendering libraries such as Cairo, for example. The pixel data buffer must have GstVideoMeta set.

`pixels` : | a GstBuffer pointing to the pixel memory. [transfer none]
`render_x` : | the X co-ordinate on the video where the top-left corner of this overlay rectangle should be rendered to
`render_y` : | the Y co-ordinate on the video where the top-left corner of this overlay rectangle should be rendered to
`render_width` : | the render width of this rectangle on the video
`render_height` : | the render height of this rectangle on the video
`flags` : | flags
Returns : | a new GstVideoOverlayRectangle. Unref with `gst_video_overlay_rectangle_unref()` when no longer needed. [transfer full]

### gst_video_overlay_rectangle_ref ()

    GstVideoOverlayRectangle * gst_video_overlay_rectangle_ref
                                                            (GstVideoOverlayRectangle *comp);

Increases the refcount of the given rectangle by one.

Note that the refcount affects the writeability of `comp`, use `gst_video_overlay_rectangle_copy()` to ensure a rectangle can be modified (there is no `gst_video_overlay_rectangle_make_writable()` because it is unlikely that someone will hold the single reference to the rectangle and not know that that's the case).

`comp` : | a a GstVideoOverlayRectangle.
Returns : | `comp`. [transfer full]

### gst_video_overlay_rectangle_unref ()

    void                gst_video_overlay_rectangle_unref   (GstVideoOverlayRectangle *comp);

Decreases the refcount of the rectangle. If the refcount reaches 0, the rectangle will be freed.

`comp` : | a GstVideoOverlayRectangle. [transfer full]

### gst_video_overlay_rectangle_get_pixels_raw ()

    GstBuffer *         gst_video_overlay_rectangle_get_pixels_raw
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);

`rectangle` : | a GstVideoOverlayRectangle
`flags` : | flags If a global_alpha value != 1 is set for the rectangle, the caller should set the GST_VIDEO_OVERLAY_FORMAT_FLAG_GLOBAL_ALPHA flag if he wants to apply global-alpha himself. If the flag is not set global_alpha is applied internally before returning the pixel-data.
Returns : | a GstBuffer holding the pixel data with format as originally provided and specified in video meta with width and height of the render dimensions as per `gst_video_overlay_rectangle_get_render_rectangle()`. This function does not return a reference, the caller should obtain a reference of her own with `gst_buffer_ref()` if needed. [transfer none]

### gst_video_overlay_rectangle_get_pixels_argb ()

    GstBuffer *         gst_video_overlay_rectangle_get_pixels_argb
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);

`rectangle` : | a GstVideoOverlayRectangle
`flags` : | flags If a global_alpha value != 1 is set for the rectangle, the caller should set the GST_VIDEO_OVERLAY_FORMAT_FLAG_GLOBAL_ALPHA flag if he wants to apply global-alpha himself. If the flag is not set global_alpha is applied internally before returning the pixel-data.
Returns : | a GstBuffer holding the ARGB pixel data with width and height of the render dimensions as per `gst_video_overlay_rectangle_get_render_rectangle()`. This function does not return a reference, the caller should obtain a reference of her own with `gst_buffer_ref()` if needed. [transfer none]

### gst_video_overlay_rectangle_get_pixels_ayuv ()

    GstBuffer *         gst_video_overlay_rectangle_get_pixels_ayuv
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);

`rectangle` : | a GstVideoOverlayRectangle
`flags` : | flags If a global_alpha value != 1 is set for the rectangle, the caller should set the GST_VIDEO_OVERLAY_FORMAT_FLAG_GLOBAL_ALPHA flag if he wants to apply global-alpha himself. If the flag is not set global_alpha is applied internally before returning the pixel-data.
Returns : | a GstBuffer holding the AYUV pixel data with width and height of the render dimensions as per `gst_video_overlay_rectangle_get_render_rectangle()`. This function does not return a reference, the caller should obtain a reference of her own with `gst_buffer_ref()` if needed. [transfer none]

### gst_video_overlay_rectangle_get_pixels_unscaled_raw ()

    GstBuffer *         gst_video_overlay_rectangle_get_pixels_unscaled_raw
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);

Retrieves the pixel data as it is. This is useful if the caller can do the scaling itself when handling the overlaying. The rectangle will need to be scaled to the render dimensions, which can be retrieved using `gst_video_overlay_rectangle_get_render_rectangle()`.

`rectangle` : | a GstVideoOverlayRectangle
`flags` : | flags. If a global_alpha value != 1 is set for the rectangle, the caller should set the GST_VIDEO_OVERLAY_FORMAT_FLAG_GLOBAL_ALPHA flag if he wants to apply global-alpha himself. If the flag is not set global_alpha is applied internally before returning the pixel-data.
Returns : | a GstBuffer holding the pixel data with GstVideoMeta set. This function does not return a reference, the caller should obtain a reference of her own with `gst_buffer_ref()` if needed. [transfer none]

### gst_video_overlay_rectangle_get_pixels_unscaled_argb ()

    GstBuffer *         gst_video_overlay_rectangle_get_pixels_unscaled_argb
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);

Retrieves the pixel data as it is. This is useful if the caller can do the scaling itself when handling the overlaying. The rectangle will need to be scaled to the render dimensions, which can be retrieved using `gst_video_overlay_rectangle_get_render_rectangle()`.

`rectangle` : | a GstVideoOverlayRectangle
`flags` : | flags. If a global_alpha value != 1 is set for the rectangle, the caller should set the GST_VIDEO_OVERLAY_FORMAT_FLAG_GLOBAL_ALPHA flag if he wants to apply global-alpha himself. If the flag is not set global_alpha is applied internally before returning the pixel-data.
Returns : | a GstBuffer holding the ARGB pixel data with GstVideoMeta set. This function does not return a reference, the caller should obtain a reference of her own with `gst_buffer_ref()` if needed. [transfer none]

### gst_video_overlay_rectangle_get_pixels_unscaled_ayuv ()

    GstBuffer *         gst_video_overlay_rectangle_get_pixels_unscaled_ayuv
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             GstVideoOverlayFormatFlags flags);

Retrieves the pixel data as it is. This is useful if the caller can do the scaling itself when handling the overlaying. The rectangle will need to be scaled to the render dimensions, which can be retrieved using `gst_video_overlay_rectangle_get_render_rectangle()`.

`rectangle` : | a GstVideoOverlayRectangle
`flags` : | flags. If a global_alpha value != 1 is set for the rectangle, the caller should set the GST_VIDEO_OVERLAY_FORMAT_FLAG_GLOBAL_ALPHA flag if he wants to apply global-alpha himself. If the flag is not set global_alpha is applied internally before returning the pixel-data.
Returns : | a GstBuffer holding the AYUV pixel data with GstVideoMeta set. This function does not return a reference, the caller should obtain a reference of her own with `gst_buffer_ref()` if needed. [transfer none]

### gst_video_overlay_rectangle_get_render_rectangle ()

    gboolean            gst_video_overlay_rectangle_get_render_rectangle
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             gint *render_x,
                                                             gint *render_y,
                                                             guint *render_width,
                                                             guint *render_height);

Retrieves the render position and render dimension of the overlay rectangle on the video.

`rectangle` : | a GstVideoOverlayRectangle
`render_x` : | address where to store the X render offset. [out][allow-none]
`render_y` : | address where to store the Y render offset. [out][allow-none]
`render_width` : | address where to store the render width. [out][allow-none]
`render_height` : | address where to store the render height. [out][allow-none]
Returns : | TRUE if valid render dimensions were retrieved.

### gst_video_overlay_rectangle_get_seqnum ()

    guint               gst_video_overlay_rectangle_get_seqnum
                                                            (GstVideoOverlayRectangle *rectangle);

Returns the sequence number of this rectangle. Sequence numbers are monotonically increasing and unique for overlay compositions and rectangles (meaning there will never be a rectangle with the same sequence number as a composition).

Using the sequence number of a rectangle as an indicator for changed pixel-data of a rectangle is dangereous. Some API calls, like e.g. `gst_video_overlay_rectangle_set_global_alpha()`, automatically update the per rectangle sequence number, which is misleading for renderers/ consumers, that handle global-alpha themselves. For them the pixel-data returned by gst_video_overlay_rectangle_get_pixels_*() wont be different for different global-alpha values. In this case a renderer could also use the GstBuffer pointers as a hint for changed pixel-data.

`rectangle` : | a GstVideoOverlayRectangle
Returns : | the sequence number of `rectangle`

### gst_video_overlay_rectangle_set_render_rectangle ()

    void                gst_video_overlay_rectangle_set_render_rectangle
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             gint render_x,
                                                             gint render_y,
                                                             guint render_width,
                                                             guint render_height);

Sets the render position and dimensions of the rectangle on the video. This function is mainly for elements that modify the size of the video in some way (e.g. through scaling or cropping) and need to adjust the details of any overlays to match the operation that changed the size.

`rectangle` must be writable, meaning its refcount must be 1. You can make the rectangles inside a GstVideoOverlayComposition writable using `gst_video_overlay_composition_make_writable()` or `gst_video_overlay_composition_copy()`.

`rectangle` : | a GstVideoOverlayRectangle
`render_x` : | render X position of rectangle on video
`render_y` : | render Y position of rectangle on video
`render_width` : | render width of rectangle
`render_height` : | render height of rectangle

### gst_video_overlay_rectangle_copy ()

    GstVideoOverlayRectangle * gst_video_overlay_rectangle_copy
                                                            (GstVideoOverlayRectangle *rectangle);

Makes a copy of `rectangle`, so that it is possible to modify it (e.g. to change the render co-ordinates or render dimension). The actual overlay pixel data buffers contained in the rectangle are not copied.

`rectangle` : | a GstVideoOverlayRectangle to copy. [transfer none]
Returns : | a new GstVideoOverlayRectangle equivalent to `rectangle`. [transfer full]

### gst_video_overlay_rectangle_get_flags ()

    GstVideoOverlayFormatFlags gst_video_overlay_rectangle_get_flags
                                                            (GstVideoOverlayRectangle *rectangle);

Retrieves the flags associated with a GstVideoOverlayRectangle. This is useful if the caller can handle both premultiplied alpha and non premultiplied alpha, for example. By knowing whether the rectangle uses premultiplied or not, it can request the pixel data in the format it is stored in, to avoid unnecessary conversion.

`rectangle` : | a GstVideoOverlayRectangle
Returns : | the GstVideoOverlayFormatFlags associated with the rectangle.

### gst_video_overlay_rectangle_get_global_alpha ()

    gfloat              gst_video_overlay_rectangle_get_global_alpha
                                                            (GstVideoOverlayRectangle *rectangle);

Retrieves the global-alpha value associated with a GstVideoOverlayRectangle.

`rectangle` : | a GstVideoOverlayRectangle
Returns : | the global-alpha value associated with the rectangle.

### gst_video_overlay_rectangle_set_global_alpha ()

    void                gst_video_overlay_rectangle_set_global_alpha
                                                            (GstVideoOverlayRectangle *rectangle,
                                                             gfloat global_alpha);

Sets the global alpha value associated with a GstVideoOverlayRectangle. Per- pixel alpha values are multiplied with this value. Valid values: 0 <= global_alpha <= 1; 1 to deactivate.

# `rectangle` must be writable, meaning its refcount must be 1. You can make the rectangles inside a GstVideoOverlayComposition writable using `gst_video_overlay_composition_make_writable()` or `gst_video_overlay_composition_copy()`.

`rectangle` : | a GstVideoOverlayRectangle