Added an expanation of the text columns code to README.md.

extractor/README.md

@@ -1,3 +1,11 @@
+TEXT EXTRACTION CODE
+====================
+The code is currently split accross the text_*.go files to make it easier to navigate. Once you
+understand the code you may wish to recombine this in the orginal text.go
+\
+
+BASIC IDEAS
+-----------
 There are two directions
 
 - *reading*
@@ -9,3 +17,28 @@ In English text,
 
 We define *depth* as distance from the bottom of a word's bounding box from the top of the page.
 depth := pageSize.Ury - r.Lly
+
+* Pages are divided into rectangular regions called `textPara`s.
+* The `textPara`s in a page are sorted in reading ouder (the order they are read, not the
+*reading* direction above).
+* Each `textPara` contains `textLine`s, lines with the `textPara`'s bounding box.
+* Each `textLine` has a text reprentation.
+
+Page text is extracted by iterating over `textPara`s and within each `textPara` iterating over its
+`textLine`s.
+
+
+WHERE TO START
+--------------
+
+`text_page.go` *makeTextPage* is the top level function that builds the `textPara`s.
+
+* A page's `textMark`s are obtained from its contentstream.
+* The `textMark`s are divided into `textWord`s.
+* The `textWord`s are grouped into depth bins with each the contents of each bin sorted by reading direction.
+* The page area is into rectangular regions for each paragraph.
+* The words in of each rectangular region are aranged inot`textLine`s. Each rectangular region and
+its constituent lines is a `textPara`.
+* The `textPara`s are sorted into reading order.
+
+