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.

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  -  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 (currenlty):
		    brightness, contrast, saturation, hue

    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)
      
      IMPORTAMT 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!

    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.

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

    flip_page(): this is called after each frame, this diplays the buffer for
	 real. This is 'swapbuffers' when double-buffering.