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.