[PDFdev] Text placement inside a /Tx field

[Peter Persits]

Dear all,

 

I want to take this opportunity to thank you all for the help you have given me in the last few months. Now to my new question:

 

My app generates interactive form fields including text (/Tx) fields. In a field's appearance stream (/AP) I need to Tj my text within /Tx BMC and EMC markers. But how should I calculate the coordinates for the td operator? With Courier, Acrobat 5 generates it as follows:

 

<</BBox [ 0 0 118.95428 62.09152 ]...>>
stream
/Tx BMC
BT
/Cour 12 Tf
2 47.4677 Td
(Enter info) Tj
ET
Q EMC
endstream

 

With Helvetica, it is

 

/Helv 12 Tf
2 46.2196 Td

 

The x coordinate is always 2 regardless of font size or type, but the y coordinate does change with font size and type. I am guessing, font Descent and/or CapHeight params are in play here, but how?

 

Thanks in advance.

Peter Persits