RE: [cgmo-webcgm] draft clarification for getObjectExtent & text

["Galt, Stuart A" <stuart.a.galt@boeing.com>]

Looks fine to me

Stuart Galt.

> -----Original Message-----
> From: Lofton Henderson [mailto:lofton@rockynet.com] 
> Sent: Saturday, March 07, 2009 11:38 AM
> To: cgmo-webcgm@lists.oasis-open.org
> Subject: [cgmo-webcgm] draft clarification for getObjectExtent & text
> 
> All --
> 
> Please review and comment...
> 
> Per the 4-mar telecon agreement about the principles, here is 
> draft language to clarify the contribution of text elements 
> to the object extent calculation.
> 
> Reference:
> http://www.w3.org/Graphics/WebCGM/drafts/current-editor-21/Web
CGM21-DOM.html#getObjectExtent
> 
> Replace the 3rd paragraph:
> 
> [[[
> >The contribution of text elements to the object extent is 
> conceptually 
> >calculated from the containing parallellogram of the displayed text, 
> >defined as follows. The length of the side in the text-up 
> direction is 
> >the bottomline-to-topline distance of the font, after computation of 
> >the effective font size that reflects all text attributes, 
> the height 
> >of the Restricted Text box, the Restricted Text Type, and the Style 
> >Properties text-size and text-font. The length of the side in the 
> >text-baseline direction is the length of the restricted text 
> box if the 
> >entire Restricted Text element is contained within the 
> object, or the 
> >sum of the glyph widths if only an Append Text element is 
> within the object.
> ]]]
> 
> with:
> 
> ===== start replacement text =====
> The contribution of text elements to the object extent of a 
> Restricted Text element is calculated as follows.  First, an 
> effective font size is derived that reflects all applicable 
> CGM text attributes for the text string in question, as well 
> as the Style Properties text-size and text-font, plus the 
> "effective Restricted Text parallelogram" and the Restricted 
> Text Type.  The "effective Restricted Text parallelogram" is 
> defined to be the Restricted Text parallelogram as possibly 
> altered by the @@text-size Style Property@@.  Then, the text 
> string can be laid out glyph-by-glyph at this effective font 
> size according to all applicable CGM attributes and 
> applicable Style Properties, as defined in CGM:1999 section 6.7.3.
> 
> For the purposes of the object extent (bounding box) 
> calculation, each glyph is treated as a separate graphics 
> element.  The calculations assume that all glyphs occupy 
> their full glyph cell.  In particular, the calculations 
> assume that each glyph extends vertically from the 
> bottom-line to the top-line of the font (see Figure 11, 
> CGM:1999 section 6.7.3.2) -- i.e., the vertical extent of 
> each glyph includes the full ascent and descent values for 
> the font, regardless of whether the particular glyph actually 
> has ascenders and/or descenders.
> 
> The object extent of the text string is he minimal 
> axis-aligned bounding rectangle that contains all the glyph cells.
> 
> In practice, this calculation can usually be simplified 
> considerably.  By far the most commonly occurring 
> WebCGM-compliant use cases involve fonts with left-to-right 
> text progression and Restricted Text Type values 'box-cap' or 
> 'box-all'.  For the 'boxed-all' case, the sum of the glyph 
> boxes is simply the Restricted Text parallelogram, and the 
> object extent is simply the minimal axis-aligned rectangle 
> that contains said parallelogram.  'Boxed-cap' is equally 
> straightforward, involving a simple arithmetic adjustment to 
> the Restricted Text parallelogram.
> 
> Note:  Although WebCGM requires the use of RESTRICTED TEXT 
> and prohibits use of TEXT, viewers may encounter legacy CGM 
> content that uses TEXT.  It is recommended that DOM 
> implementations of getObjectExtent should be able to 
> correctly handle such metafiles, according to the 
> calculations specified here.
> 
> Note 2:  An informative alternative perspective about the 
> text contribution to the object extent might be helpful:  the 
> sum of the glyph boxes is equivalent to the Restricted Text 
> parallelogram that would result if a WebCGM generator were 
> writing the text string with 'boxed-all' Restricted Text Type.
> ===== end replacement text =====
> 
> Regards,
> -Lofton.
> 
> 
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