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Inter-Office Memorandum To Tim Townsend Date 7 November 1977 From Bob Metcalfe, Charles Simonyi Location Palo Alto Subject Scrolling and Panning Organization SDD/~R5~ SDD ARCHIVES I have read and understood XEROX Pages To ----- Reviewer -----' ---- Date # of Pages Ref .,11S(')D- ~" 7 This memo responds to your request for a characterization of the performance implications of horizontal scrolling, now called panning. In this memo we use the term scrolling to mean vertical movement and panning to mean horizontal movement, both of a page image on a display. The purpose of scrolling and panning is, of course, to enable viewing of a page which is too large for the display, perhaps because a larger, more legible font has been employed for user comfort. In your request you mentioned what we now call an EDP page consisting of 60 lines of 132 fixed pitch characters in a single font. Our response treats the more general Diamond page case first and then examines possible performance improvements for the EDP case. There are three page representations relevant to this discussion: (1) piece tables, (2) line descriptions, and (3) bit maps. Page edits are performed on piece tables. Piece tables are converted to line descriptions enroute to various other representations, one of which is the bit map maintained for the display. Given a new piece table or one upon which an edit has just been performed, we estimate that 1 second of DO processing will be required to compute the bit map viewed by a user on his display, just as specified in the requirements. It should be noted that a lower delay will be perceived because the bit map is displayed as it is updated. Because scrolling and panning imply no modification of the piece table and often keep much of the same bit map in view, recomputing the bit map can frequently and to great advantage be avoided using the so-called BITBLT operation to move the bit map unaltered. We estimate that the display can be updated in .1 seconds when a scroll or pan operation keeps the display's view inside the existing bit map. Because memory is expensive, however, the amount of bit map maintained is only as large as that viewed. Therefore, a scroll or pan operation commonly results in a view which requires that new bit map be computed. In the worst case an entirely new bit map