![]() ![]() ![]() Multi-columnar text may present a difficulty, text may be drawn line by line, e.g. The string drawing instructions may occur in an arbitrary order, e.g "Hello" might be drawn "lo" first, then after moving back "el", then after again moving back "H" to extract the text one cannot ignore text positioning instructions and simply concatenate text strings, you always have to take the current position into account (some simple text extractors ignore this and, therefore, can fail to return something sensible) The next problem is to make sense out of the order of the strings: the first glyph in a given font used on a page is given the starting value as code, the next, different glyph is given the starting value plus one, the next, different one the starting value plus two, etc "Hello World" and a starting value of 48 (ASCII value of '0') would result in "01223453627" these fonts may contain a mapping to Unicode but they are not required to. Add-RKSJ-H these encodings may use a constant number of bytes per glyph or they may be mixed-multibyte so a text extractor must support very many encodings to start with Įncodings also may be completely ad-hoc and arbitrary in particular in case of embedded subset fonts one often sees ad-hoc encodings generated by dealing out character codes from some starting value whenever one is needed i.e. WinAnsiEncoding, many you likely don't know, e.g. ![]() There are a large number of predefined encodings, some reminding of encodings you know, e.g. The first problem is to understand the encoding of the string arguments of those text drawing instructions:Įach font can have its own encoding to extract the text one cannot simply ignore everything but the instructions drawing text and concatenate their string contents, you always have to take the current font into account (some extremely simple text extractors ignore this and, therefore, fail pretty often to return something sensible) Text extraction is the task of taking the sequence of instructions from a content stream and instead of drawing the text as indicated by the font and position setting instructions, to export it in a sensible order using a standard encoding, usually the encoding of the character type of the used programming language / platform. "Text is drawn" on a page means that among those instructions there are some setting the font to use by the instructions to come, some setting the text position and direction to use by the instructions to come, and some actually drawing text given by "string arguments". What is drawn on a PDF page is determined by a sequence of instructions in the content stream of that page. Let's instead assume that text is drawn in the PDF at hand. Let's not assume you are talking about PDFs which merely wrap some bitmap image because it should be clear that in that case you can only resort to OCR with all its restrictions. ![]()
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