mpv/DOCS/tech/libvo.txt

libvo --- the library to handle video output    by A'rpi, 2002.04
============================================

Note: before start on this, read colorspaces.txt !

The constants for different pixelformats are defined in img_format.h,
their usage is mandatory.

WARNING: Please keep in mind that some of this information may be out-dated,
so if you are working on a new vo, consider submitting preliminary patches
very early on. Currently vo_gl is one of the more up-to-date VOs to use
as reference if you are unsure about something and do not want to ask on the
list.
vo_vdpau and vo_direct3d may be a good choice, too, they use different
approaches closer to the sometimes convoluted way DirectX works.

Each vo driver _has_ to implement these:

    preinit():
	init the video system (to support querying for supported formats)

    uninit():
      Uninit the whole system, this is on the same "level" as preinit.

    control():
      Current controls (VOCTRL_QUERY_FORMAT must be implemented,
      VOCTRL_DRAW_IMAGE, VOCTRL_FULLSCREEN, VOCTRL_UPDATE_SCREENINFO
      should be implemented):
	 VOCTRL_QUERY_FORMAT  -  queries if a given pixelformat is supported.
	        It also returns various flags decsirbing the capabilities
		of the driver with teh given mode. for the flags, see
		file vfcaps.h !
		the most important flags, every driver must properly report
		these:
		    0x1	 -  supported (with or without conversion)
		    0x2  -  supported without conversion (define 0x1 too!)
		    0x100  -  driver/hardware handles timing (blocking)
		also SET sw/hw scaling and osd support flags, and flip,
		and accept_stride if you implement VOCTRL_DRAW_IMAGE (see bellow)
		NOTE: VOCTRL_QUERY_FORMAT may be called _before_ first config()
		but is always called between preinit() and uninit()
	 VOCTRL_GET_IMAGE
		libmpcodecs Direct Rendering interface
		You need to update mpi (mp_image.h) structure, for example,
		look at vo_x11, vo_sdl, vo_xv or mga_common.
	 VOCTRL_DRAW_IMAGE
		replacement for the current draw_slice/draw_frame way of
		passing video frames. by implementing SET_IMAGE, you'll get
		image in mp_image struct instead of by calling draw_*.
		unless you return VO_TRUE for VOCTRL_DRAW_IMAGE call, the
		old-style draw_* functils will be called!
		Note: draw_slice is still mandatory, for per-slice rendering!
	 VOCTRL_RESET  -  reset the video device
		This is sent on seeking and similar and is useful if you are
		using a device which prebuffers frames that need to flush them
		before refilling audio/video buffers.
	 VOCTRL_PAUSE
	 VOCTRL_RESUME
	 VOCTRL_GUISUPPORT
		return true only if driver supports co-operation with
		MPlayer's GUI (not yet used by GUI)
	 VOCTRL_SET_EQUALIZER
		set the video equalizer to the given values
		two arguments are provided: item and value
		item is a string, the possible values are (currently):
		    brightness, contrast, saturation, hue
	 VOCTRL_GET_EQUALIZER
		get the current video equalizer values
		two arguments are provided: item and value
		item is a string, the possible values are (currently):
		    brightness, contrast, saturation, hue
	 VOCTRL_ONTOP
		Makes the player window stay-on-top. Only supported (currently)
		by drivers which use X11, except SDL, as well as directx and
		gl2 under Windows.
	 VOCTRL_BORDER
		Makes the player window borderless.
	 VOCTRL_FULLSCREEN
		Switch from and to fullscreen mode
	 VOCTRL_GET_PANSCAN
	 VOCTRL_SET_PANSCAN
                Needed to implement pan-scan support ('w' and 'e' keys during
		playback in fullscreen mode)
	 VOCTRL_START_SLICE
	 	Called before the first draw_slice of each frame, useful if
		you need to do some set-up work.
	 VOCTRL_DRAW_EOSD
		Required for EOSD (ASS subtitle) support. Provides all
		information necessary to draw the EOSD for the current video
		frame.
	 VOCTRL_GET_EOSD_RES
		Required for EOSD (ASS subtitle) support. Informs the ASS
		renderer about the properties of the drawing area (size,
		borders).
	 VOCTRL_SET_DEINTERLACE
	 VOCTRL_GET_DEINTERLACE
		Get or set deinterlacing status for VOs that support some kind
		of deinterlacing.
	 VOCTRL_UPDATE_SCREENINFO
	 	Should set the xinerama_x, xinerama_y, vo_screenwidth and
		vo_screenheight appropriately for the currently used
		monitor and -xineramascreen option.
		Usually should simply call the w32_update_xinerama_info or
		update_xinerama_info function.
		By supporting this, the VO also requests the newer API
		that sets vo_dx, vo_dy etc. appropriately before config()
		is called.

    config():
      Set up the video system. You get the dimensions and flags.
      width, height: size of the source image
      d_width, d_height: wanted scaled/display size (it's a hint)
      Flags:
        0x01	- force fullscreen (-fs)
	0x02	- allow mode switching (-vm)
	0x04	- allow software scaling (-zoom)
	0x08	- flipping (-flip)
      They're defined as VOFLAG_* (see libvo/video_out.h)

      IMPORTANT NOTE: config() may be called 0 (zero), 1 or more (2,3...)
      times between preinit() and uninit() calls. You MUST handle it, and
      you shouldn't crash at second config() call or at uninit() without
      any config() call! To make your life easier, vo_config_count is
      set to the number of previous config() call, counted from preinit().
      It's set by the caller (vf_vo.c), you don't have to increase it!
      So, you can check for vo_config_count>0 in uninit() when freeing
      resources allocated in config() to avoid crash!

      You should implement VOCTRL_UPDATE_SCREENINFO so that vo_dx, vo_dy,
      vo_dwidth and vo_dheight are already pre-set to values that take
      aspect and -geometry into account. It is also necessary to properly
      support multi-monitor setups (if based on x11_common, w32_common).

    draw_slice(): this displays YV12 pictures (3 planes, one full sized that
	 contains brightness (Y), and 2 quarter-sized which the colour-info
	 (U,V). MPEG codecs (libmpeg2, opendivx) use this. This doesn't have
	 to display the whole frame, only update small parts of it.
	 If this is not supported, it must be signaled in QUERY_FORMAT with
	 VOCAP_NOSLICES.

    draw_frame(): this is the older interface, this displays only complete
	 frames, and can do only packed format (YUY2, RGB/BGR).
	 Win32 codecs use this (DivX, Indeo, etc).
	 If you implement VOCTRL_DRAW_IMAGE, you do not need to implement draw_frame.

    draw_osd(): this displays subtitles and OSD.
	 It's a bit tricky to use it, since it's a callback-style stuff.
	 It should call vo_draw_text() with screen dimension and your
	 draw_alpha implementation for the pixelformat (function pointer).
	 The vo_draw_text() checks the characters to draw, and calls
	 draw_alpha() for each. As a help, osd.c contains draw_alpha for
	 each pixelformats, use this if possible!
	 Note that if you do not draw directly onto the video you should
	 use vo_draw_text_ext() which allows you to specify the border
	 values etc. needed to draw DVD menu highlights at the correct place.
	 If you do not want to implement this, you can still use -vf
	 expand=osd=1 to draw the OSD, or even implement code to insert
	 this filter automatically.
	 Make sure you set VFCAP_OSD depending on whether you implemented it
	 or not.

	 NOTE: This one will be obsolete soon! But it's still useful when
	 you want to do tricks, like rendering osd _after_ hardware scaling
	 (tdfxfb) or render subtitles under of the image (vo_mpegpes, sdl)

	 NOTE2: above NOTE is probably wrong, there are currently no plans to
	 obsolete draw_osd, though there is the more advanced EOSD support for
	 ASS subtitles.

    flip_page(): this is called after each frame, this displays the buffer for
	 real. This is 'swapbuffers' when doublebuffering.
	 Try to do as little work here as possible, since that affect jitter/
	 A-V sync.