Hacker Guide/Video Output

The video output layer

Data structures and main loop

Important data structures are defined in include/video.h and include/video_output.h. The main data structure is picture_t, which describes everything a video decoder thread needs. Please refer to this file for more information. Typically, p_data will be a pointer to YUV planar picture.

Note also the subpicture_t structure. In fact the VLC SPU decoder only parses the SPU header, and converts the SPU graphical data to an internal format which can be rendered much faster. So a part of the "real" SPU decoder lies in src/video_output/video_spu.c.

The vout_thread_t structure is much more complex, but you needn't understand everything. Basically the video output thread manages a heap of pictures and subpictures (5 by default). Every picture has a status (displayed, destroyed, empty...) and eventually a presentation time. The main job of the video output is an infinite loop to : [this is subject to change in the near future]

1. Find the next picture to display in the heap.
2. Find the current subpicture to display.
3. Render the picture (if the video output plug-in doesn't support YUV overlay). Rendering will call an optimized YUV plug-in, which will also do the scaling, add subtitles and an optional picture information field.
4. Sleep until the specified date.
5. Display the picture (plug-in function). For outputs which display RGB data, it is often accomplished with a buffer switching. p_vout->p_buffer is an array of two buffers where the YUV transform takes place, and p_vout->i_buffer_index indicates the currently displayed buffer.
6. Manage events.

Methods used by video decoders

The video output exports a bunch of functions so that decoders can send their decoded data. The most important function is vout_CreatePicture which allocates the picture buffer to the size indicated by the video decoder. It then just needs to feed (void *) p_picture->p_data with the decoded data, and call vout_DisplayPicture and vout_DatePicture upon necessary.

• picture_t * vout_CreatePicture ( vout_thread_t *p_vout, int i_type, int i_width, int i_height ) : Returns an allocated picture buffer. i_type will be for instance YUV_420_PICTURE, and i_width and i_height are in pixels.

Warning

If no picture is available in the heap, vout_CreatePicture will return NULL.

• vout_LinkPicture ( vout_thread_t *p_vout, picture_t *p_pic ) : Increases the refcount of the picture, so that it doesn't get accidently freed while the decoder still needs it. For instance, an I or P picture can still be needed after displaying to decode interleaved B pictures.
• vout_UnlinkPicture ( vout_thread_t *p_vout, picture_t *p_pic ) : Decreases the refcount of the picture. An unlink must be done for every link previously made.
• vout_DatePicture ( vout_thread_t *p_vout, picture_t *p_pic ) : Gives the picture a presentation date. You can start working on a picture before knowing precisely at what time it will be displayed. For instance to date an I or P picture, you must wait until you have decoded all previous B pictures (which are indeed placed after - decoding order != presentation order).
• vout_DisplayPicture ( vout_thread_t *p_vout, picture_t *p_pic ) : Tells the video output that a picture has been completely decoded and is ready to be rendered. It can be called before or after vout_DatePicture.
• vout_DestroyPicture ( vout_thread_t *p_vout, picture_t *p_pic ) : Marks the picture as empty (useful in case of a stream parsing error).
• subpicture_t * vout_CreateSubPicture ( vout_thread_t *p_vout, int i_channel, int i_type ) : Returns an allocated subpicture buffer. i_channel is the ID of the subpicture channel, i_type is DVD_SUBPICTURE or TEXT_SUBPICTURE, i_size is the length in bytes of the packet.
• vout_DisplaySubPicture ( vout_thread_t *p_vout, subpicture_t *p_subpic ) : Tells the video output that a subpicture has been completely decoded. It obsoletes the previous subpicture.
• vout_DestroySubPicture ( vout_thread_t *p_vout, subpicture_t *p_subpic ) : Marks the subpicture as empty.

How to write a video output plug-in

A video output takes care of the system calls to display the pictures and manage the output window. Have a look at plugins/x11/vout_x11.c. You must write the following functions :

1. int vout_Probe ( probedata_t *p_data ) : Returns a score between 0 and 999 to indicate whether it can run on the architecture. 999 is the best. p_data is currently unused.
2. int vout_Create ( vout_thread_t *p_vout ) : Basically, initializes and opens a new window. Returns TRUE if it failed.
3. int vout_Init ( vout_thread_t *p_vout ) : Creates optional picture buffers (for instance ximages or xvimages). Returns TRUE if it failed.
4. vout_End ( vout_thread_t *p_vout ) : Frees optional picture buffers.
5. vout_Destroy ( vout_thread_t *p_vout ) : Unmaps the window and frees all allocated resources.
6. int vout_Manage ( vout_thread_t *p_vout ) : Manages events (including for instance resize events).
7. vout_Display ( vout_thread_t *p_vout ) : Displays a previously rendered buffer.
8. vout_SetPalette ( vout_thread_t *p_vout, u16 *red, u16 *green, u16 *blue, u16 *transp ) : Sets the 8 bpp palette. red, green and blue are arrays of 256 unsigned shorts.

How to write a YUV plug-in

Look at the C source plugins/yuv/transforms_yuv.c. You need to redefine just the same transformations. Basically, it is a matrix multiply operation. Good luck.