RE: [Devel] Freetype & Ascent

[Ken Hancock]

No, no one ever responded.  I ended up reading the OS/2 table directly and
doing my own calculations.


> -----Original Message-----
> From: address@hidden 
> [mailto:address@hidden On Behalf Of Keith Trummel
> Sent: Monday, June 28, 2004 4:35 PM
> To: 'address@hidden'
> Subject: RE: [Devel] Freetype & Ascent
> 
> I was searching my old email to see if anyone responded to 
> this, but I couldn't find a response.  
> 
> I am encountering a similar problem.  I have a font where the 
> ascender and descender are 0, but in the OS/2 metric table 
> they seem to be correct.  So I also was wondering why despite 
> the comments the code to use the OS/2 info is commented out.
> 
> I also noticed that that code appears to be wrong in a couple of ways.
> First, I would think the check should be:
> 
>         if ( face->os2.version != 0xFFFF && root->ascender == 0 )
> 
> instead of:
> 
>         if ( face->os2.version != 0xFFFFU && root->ascender )
> 
> Also I think the root->descender should be set as follows:
> 
>         root->descender =  face->os2.sTypoDescender;
> 
> instead of:
> 
>           root->descender = -face->os2.sTypoDescender;
> 
> and then the height computed using the negative of the descender.
> 
> Keith Trummel
> 
> -----Original Message-----
> From: Ken Hancock [mailto:address@hidden
> Sent: Wednesday, May 19, 2004 3:17 PM
> To: address@hidden
> Subject: [Devel] Freetype & Ascent
> 
> 
> I noticed the following code in sfobjs.c is commented out:
> 
>         /* XXX: Computing the ascender/descender/height is 
> very different */
>         /*      from what the specification tells you.  
> Apparently, we    */
>         /*      must be careful because                       
>             */
>         /*                                                    
>             */
>         /*      - not all fonts have an OS/2 table; in this 
> case, we take */
>         /*        the values in the horizontal header.  
> However, these    */
>         /*        values very often are not reliable.         
>             */
>         /*                                                    
>             */
>         /*      - otherwise, the correct typographic values 
> are in the    */
>         /*        sTypoAscender, sTypoDescender & 
> sTypoLineGap fields.    */
> 
> Is sTypoAscender (caps height) exposed anywhere?  I would 
> think that it's required for proper center justification 
> since as far as I can tell diacriticals and decenders are 
> normally not taken into account when determining the center 
> position of text in a bounding box.
> 
> penPos.y = (bounds.bottom - bounds.top) / 2 +         // 
> center of bounding box
>               (pix_height * sTypoAscender / 2) -      // move 
> down by caps
> height
>               (pix_height * face->ascent)             // move 
> back up to
> top of glyph bounding box
> 
> Arial, for example sets the following values:
> 
>   Ascent: 1854
>  Descent:  434
> Em Squre: 2048
>  Leading:   67
> 
> OS/2 metric table:
>  Typographic
>      Ascent: 1491 (caps height of 'A')
>     Descent:  431
>    Line Gap:  307
> 
>      Windows
>      Ascent: 1854
>     Descent:  434
> 
> 
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