Extracting key information from TrueType font files

   This article extracts head Revision in module properties fontRevision For example .
TrueType Font file related attributes reference article ：TrueType
Font interpreter opentype.js Source code download address ：opentype.js Source code

Figure 1 ：TrueType

Figure 2 ：OpenType

  TrueType Fonts are generally contained in a single TrueType In the font file , The suffix is .TTF.
  OpenType Font is a kind of similar to TrueType Font file , Its format code is POSTSCRIPT typeface , The suffix is .OTF.

1、 Demand analysis

   We need to extract fixed parameters among many parameters ： Revision , And it is clear that the revised version is in head Properties of the , At this point, it depends on Font Inspector How to split each attribute module , And find our head attribute , And then in head The revision is extracted from the attribute module fontRevision Parameters .
   First, we create classes for various attribute fields , Each class contains many parameters , image head、name And so on are class objects , And like version、fontRevision And so on are variable attributes in the class .

2、 Read the file

   Read the file , Read the binary file , Load into the array , And pass it as a parameter to the subsequent interface .
   Before reading the file x position , According to the characteristics of different file types , Read the identification bit as the file type , To get the current file type .

   Different file types may have different attribute modules and field lists , We use ttf File as an example , It contains the following class attribute fields , Then there are various variables under these different class attributes .

3、 see head Related properties

   We use head For example , There are many variable parameters under this class , These parameters have default data types and default values .

4、 see Font Inspector How to split each attribute module

   our TrueType Is a font file , The font file has a feature , All attribute modules in its font table are 4 Characters , As shown in the figure ： For what we are looking for head, We mark it with a solid red line .

   Now how can we split each attribute field ？ How to head Field attributes are separated from these font tables ？ adopt getTag() Interface , This interface is from DataView Retrieve in view 4 A mark of characters . That is, corresponding to the field attribute 4 Characters to return .



   So far we have found head attribute field ,head The starting offset of the attribute field in the whole binary file 300 The location of , The length is 54, We got effective information . The next thing to do is start from head Extract from fontRevision attribute , At this time, offset directly from the actual amount 300 Start reading from the location of , A fixed number of bytes are read each time , Find the target to read .
   We are head The related view structure is built in , This view contains multiple attribute parameters , The parameters we want to extract are also marked with red line of sight .

   We are headTable Extract from the structure fontRevision Parameters , We put the data data and head The starting offset from the binary file start Pass it to the interface as a parameter , then new Parser object , Pass the data in , Save as an object .


   In fact, it is equivalent to creating an attempt , There is data in this view data, Starting offset offset, Current offset relativeOffset.HeadTable The structure is as follows: ：

   Next, we use the current offset to read one by one , except head Parameter fields in , It also specifies the type of the parameter field , Determines its offset in the current field , We'll use it parseFixed() Interface , According to the type of each parameter field , Keep the current offset relativeOffset Continue to increase , Until I read fontRevision Parameters , And then use it Math In the library round The function is rounded , Perform data truncation .