RE: TT and subtitling

I concur that we probably would not define the form of a
presentation unit as such. However, it is useful to have
this term defined in an abstract system model.
 
G.

	-----Original Message-----
	From: Johnb@screen.subtitling.com [mailto:Johnb@screen.subtitling.com] 
	Sent: Monday, February 03, 2003 9:25 AM
	To: Glenn A. Adams
	Cc: public-tt@w3.org
	Subject: RE: TT and subtitling
	
	

	GAA wrote: 

		Re: difference in access and presentation units, it would depend 
		upon how the latter is defined. For example, if defined as the 
		bitmap to be BLTed into a graphics plane, then if the access 
		unit were that bitmap, there would not be a difference; however, 
		if the access unit were a compressed image, e.g., JPEG, PNG, 
		etc., then there would be a non-zero decoding time. Further, 
		but if the access unit were text which required layout and 
		glyph rasterization, then there would be a fair difference, 
		and non-zero decoding time as well. 
		  

	So do you see the form of the presentation unit as always being a bitmap? I would favour not defining the presentation unit at all - since its form will be largely dependent upon the qualities of the display surface. I see that there is a useful distinction to be made between the two however, not least to illustrate any overheads and implications in converting from access units to presentation units.

	regards John Birch 

	The views and opinions expressed are the author's own and do not necessarily reflect the views and opinions of the Screen Subtitling Systems Limited.