Pango and ink rectangles

[Sven Neumann <sven gimp org>]

Hi,

while writing the new text tool for The GIMP, I encountered some
problems related to ink rectangles. I suspect the PangoFT2 backend
does not return correct values here. Before trying to fix this, 
I'd like to ask a few questions to check that I understood the 
terminolgy correctly.

Pango supports two rectangles, the logical and the ink rectangle.
Am I right to assume that the ink rectangle is supposed to be the
bounding box of the drawn glyph or layout and that I should be 
able to rely on the fact that a buffer created with the dimensions
taken from the ink rectangle should be large enough but not too
large to hold the rendered string (assuming I choose a correct 
offset to position the layout in my buffer)? According to the 
documentation, the origins of both the ink and the logical 
rectangle are located on the baseline as illustrated below. I'm 
using an italic "g" in a serif font here to show how the ink 
rectangle can exceed the logical rectangle.



      |
      |
      |       __aaaaaas,,                 
      |    awQ ?^   ~?WQQQQQQQQW`         
      |  aQWWF        )WQQQT???^          
      |.mQQQF          QQQQ[              
      |jQQQ@          <QQQQ[              
      |QQQQf          dQQQ@               
      |4QQQL         jQQQP`               
      | 4QQQc      _yQQV^                 
      |  -?9WmwaaadT?"`                   
  ----+---------------------------
      | _mQQ;                             
      | QQQQQgaa,,,                       
      | )QQQQQQQQQQQmya,.                 
    .<|mT^ --"?TT$QQQQQQQa                
   jQQ|'           -"?$QQQr               
  dQQW|                4QQ(               
  $QQQ|                jQF                
  -9QQ|a.           _sdT'                 
     "|YVGwaaaaa%%%?!`                    
      |

For this example, the ink and logical rectangles are
in arbitrary units (characters for this ASCII example):

ink rectangle:  x      = -4
                y      = -9
                width  = 31
                height = 19

 ---------------------------------
 |            __aaaaaas,,        |
 |         awQ ?^   ~?WQQQQQQQQW`|
 |       aQWWF        )WQQQT???^ |
 |     .mQQQF          QQQQ[     |          
 |     jQQQ@          <QQQQ[     |
 |     QQQQf          dQQQ@      |         
 |     4QQQL         jQQQP`      |
 |      4QQQc      _yQQV^        |
 |    |  -?9WmwaaadT?"`          |
 -----+---------------------------
 |    | _mQQ;                    |
 |      QQQQQgaa,,,              |         
 |      )QQQQQQQQQQQmya,.        |         
 |  .<wmT^ --"?TT$QQQQQQQa       |         
 | jQQD'           -"?$QQQr      |         
 |dQQW`                4QQ(      |         
 |$QQQ,                jQF       |         
 |-9QQQa.           _sdT'        |         
 |   "?YVGwaaaaa%%%?!`           |         
 ---------------------------------


logical rectangle:  x      = 0  (always zero)
                    y      = -15
                    width  = 20
                    height = 23

      ------------------------
      |                      |
      |                      |
      |                      |
      |                      |
      |                      |
      |                      |
      |       __aaaaaas,,    |            
      |    awQ ?^   ~?WQQQQQQ|QW`         
      |  aQWWF        )WQQQT?|?^          
      |.mQQQF          QQQQ[ |             
      |jQQQ@          <QQQQ[ |             
      |QQQQf          dQQQ@  |             
      |4QQQL         jQQQP`  |             
      | 4QQQc      _yQQV^    |             
      |  -?9WmwaaadT?"`      |             
     -+------------------------
      | _mQQ;                |             
      | QQQQQgaa,,,          |             
      | )QQQQQQQQQQQmya,.    |             
    .<|mT^ --"?TT$QQQQQQQa   |             
   jQQ|'           -"?$QQQr  |             
  dQQW|                4QQ(  |             
  $QQQ|                jQF   |             
  -9QQ|a.           _sdT'    | 
     "------------------------
      

Is this correct? If so, the PangoFT2 implementation fails to 
provide this data correctly. I can provide a patch that corrects 
this behaviour.


Salut, Sven