this is the first I'm working with PDFs on this level. So please be patient with
my noob question. I understand the logical and physical structure of an PDF file
on a basic level.
I have an PDF that contains a dummy ID that needs to be replaced. To check, if there
is way to do this, I used qpdf to expand the PDF using
qpdf --qdf --object-streams=disable orig.pdf expanded.pdf
Using a hex editor I located the dummy ID in expanded.pdf and changed the value by
simply swapping two digits
<001800180017> Tj => <001700170018> Tj
and saved it. Opening expanded.pdf in Acrobat didn't show the modification. The original
ID 443 is still rendered, but searching for "443" doesn't find it. When searching for
"334", the modified content, I get the rendered original ID 443 highlighted.
The PDF consist of text and vector graphic. When I insert additional digits (which obviously
invalidates the offsets in the xref), I get an error message regarding a missing font and
all digits are shown as dots but the vector graphic is still in place. This seems to indicate
that the ID is not part of the graphic.
What did I miss?
EDIT 1:
After mkl's comment, I did a deeper analysis of my PDF and found, that beside the obvious graphic content, all text was rendered by a series of m/l/c commands follwoed by a BT/ET section. Color for stroke and non-stroke was 0,0,0 for both in the BT/ET section.
Is this because of the used embedded non-standard font?
Are PDFs with embedded fonts usually done this way? A graphics part for the visual representation and a transparent (hidden) text part just to get searching and highlighting capabilities?
Looking back I wonder what I did to get the dots when I first modified the
content. I seems impossible and I can't reproduce it either.
Thanks
Tom
First off, the following is merely guesswork as you could not share the pdf in question. Educated guesswork but guesswork nonetheless.
You report that you changed the value by simply swapping two digits in the text drawing instruction argument and now can successfully search for the value with swapped digits but that Acrobat didn't show the modification.
Furthermore you observed that all text was rendered by a series of m/l/c commands followed by a BT/ET section.
The main situation in which one observes text being rendered as arbitrary vector graphics (a series of m/l/c commands), is in pdfs in which the producer didn't want text extraction to be possible and replaced text drawing instructions by arbitrary vector graphics instructions.
This apparently is not the case in your pdf as the text drawing instructions are not replaced but merely supplemented by the vector graphics ones.
Supposing that this construct is used for a reason and not by accident, I can only assume that the pdf producer was not willing or allowed to embed the font in question but wanted the specific font appearance to be displayed without having to count on the font being installed on the computer the pdf is viewed on.
Thus, the text appearance is drawn using arbitrary vector graphics instructions and the following text drawing instructions actually draw nothing but merely make the text searchable and extractable. This way there is no need to embed the apparent font face as font program. (Text drawing instructions can be made to draw nothing either by using a font with all blank glyphs or by using the text rendering mode "invisible".)
If this assumption turns out to be correct, your task to replace the dummy id requires not merely editing the arguments of the text drawing instructions but also replacing the arbitrary vector graphics instructions showing the dummy id appearance by other instructions showing the actual id.
If you happen to have the font in question and are willing and able to embed it, you can actually replace the arbitrary vector graphics instructions by text drawing instructions using the font. Otherwise be prepared to also draw the actual id as arbitrary vector graphics.
I have a huge PDF file with 300+ pages on which a big 10+ column table is spread. I am using Linux and would like to have a simple command line command which would convert this table to a text importable to a spreadsheet.
Currently I am using pdftotext -layout, and gives quite good results, other than every page is considered independently and column widths and positions change from page to page (due to different maximum column content width on each page), so I cannot simply import the resulting text file to a spreadsheet application and split it to columns by a fixed column width.
I have tried to crop every column on every page (their position is identical across the whole PDF file), but in the result the empty rows are merged together, so the rows with content will be shifted with respect to each other.
If pdftotext had an option to convert the file with a STRICT LAYOUT (not by column content width), that would help. Or if I could stack all pages in PDF file to a single page, that could also solve it.
What are the options to solve this problem?
You are misunderstanding the nature of the content of a PDF file. There are no tables in PDf, there is no metadata (generally) to describe the content as a table. The text you see on the page may not be laid out in the reading order.
For example the PDF file might contain a line of text drawn at the top of the page, then one at the bottom, then a paragraph in the middle before jumping back up to the top for a headline.
In addition there may be no spaces between two text fragments. Text is drawn at an absolute position on the page, so you can draw (for example ) cell A, then move the current point by say 1 cm, rthen draw cell B and so on. Since there's no 'space' characters between the two cells, a naive text extraction will, naturally, assume the two lines of text are coninuous.
The STRICT LAYOUT you want isn't impossible, but you can't do it with a simple text file, because the original layout isn't made up of simple text characters, sometimes the space between two characters, or two fragments of text is done by moving the current point before drawing the text.
Ghostscript's txtwrite device in its simplest mode attempts to replicate the layout by replacing the white space with actual space characters in a fixed pitch font. This 'might' be good enough for you, but it equally well might not. That's because it operates by defining the smallest distance used on the page as being one space character. All distances between text is then replaced by a number of space characters, as many as are required to make up the space. This can (and often does) result in very wide output files with a lot of white space.
Essentially what you seem to want isn't really possible, you can't take a rich format like PDF and replicate it, including the layout, with nothing more than text characters.
I have to merge multiple PDF documents into a single PDF document. Besides this, I have to generate TOC. The original documents will contain text with a specific style (say H1). This special text becomes part of TOC.
Have used iText for merging multiple PDF files. I am unable to find example/API on parsing the document to find all the contents having style H1.
Generating TOC is next challenge.
You don't. PDFs don't have styles. They have "current Graphic State", which includes:
current transformation matrix (CTM).
stroke & fill colors
clipping path
font & size
gobs of other text state stuff (char spacing, word spacing, leading, text render mode...)
Including a separate text transformation matrix which is combined with the CTM.
So first you have to track all this stuff (which iText can mostly do for you). Then you have to determine how big "H1" text is, and latch on to all the text that is in that size screen size, taking the CTM, text matrix, and font size into account (which iText will do for you again, IIRC).
And just to make life more exciting for folks like yourself, it's entirely possible that the text you're looking at isn't text at all. It could be paths, or a bitmap... at which point you need OCR, and I don't think you'll get much in the way of size info with OCR.
You'll need to write a TextRenderListener that determines the final size of a given piece of text (and whether or not its a part of the last piece) and filter out all the stuff that's too small. You'll then build your TOC based on the text you find.
I am trying to use true type fonts, but finding that when I load different ones, some do not display at various font sizes while others do. It seems, although I am not certain, that some simply do not show below a certain font size. Is there any way to easily tell what sizes are valid for a ttf by examining the file and not simply trying it out in my app?
UPDATE: Considering smilingthax's answer, it would seem like these must be bitmap glyphs since the smaller sizes are not showing up (but using some system font instead I think). Does someone know how I can determine the valid sizes I can use with them using the iOS sdk (iPad)?
TTFs can contain Bitmap glyphs and/or Outline glyphs. Outline glyphs are scalable to any size (e.g. "31.4592 pt") although they are often optimized (hinted) for certain sizes (8,10,12,16,....72). You can get these sizes via your Font API (Win32, Freetype, ...). Bitmap glyphs are only available in certain sizes.
For example Freetype's FTFace object contains a .face_flags member which can have FT_FACE_FLAG_FIXED_SIZES set. Also the sizes are accessible via .num_fixed_sizes and the .available_sizes array.
Is there a library/tool which would list all colours used in a PDF document ?
I'm sure Acrobat itself would do this but I would like an alternative (ideally something that could be scripted).
So the idea is if you have a very simple PDF document with four colours in it the output might say :
RGB(100,0,0)
RGB(105,0,0)
CMYK(0,0,0,1)
CMYK(1,1,1,1)
You could explore the insides with pdfbox, but you would have to write some code to find and catalog all those colors.
Most PDF tools have access to this information but no api to access it. You could take any tool and add it in
Apago PDFspy generates an XML file containing all kinds of metadata extracted from PDF files. It reports color usage including spot colors.
We recently added a function called GetPageColorSpaces(0) to the Quick PDF Library - www.quickpdflibrary.com to retrieve much of the ColorSpace info used in the document.
Here is some sample output.
Resource,\"QuickPDFCS2eb0f578\",Separation,\"HKS 52 E\",DeviceCMYK,0.95,0,0.55,0
Resource,\"QuickPDFCSb7b05308\",Separation,\"Black\",DeviceCMYK,0,0,0,1
Resource,\"QuickPDFCSd9f10810\",Separation,\"Pantone 117 C\",DeviceCMYK,0,0.18,1,0.15
Resource,\"QuickPDFCS9314518c\",Separation,\"All\",DeviceCMYK,0,1,0,0.5
Resource,\"QuickPDFCS333d463d\",Separation,\"noplate\",DeviceCMYK,1,0,0,0
Resource,\"QuickPDFCSb41cafc4\",Separation,\"noprint\",DeviceCMYK,0,1,0,0
Resource,\"Cs10\",DeviceN,Black,Colorant,-1,-1,-1,-1
Resource,\"Cs10\",DeviceN,P1495,Colorant,-1,-1,-1,-1
Resource,\"Cs10\",DeviceN,CalRGB,Colorant,-1,-1,-1,-1
Resource,\"Cs10\",Separation,\"P1495\",DeviceCMYK,0,0.31,0.69,0
XObject,\"R29\",Image,,DeviceRGB,-1,-1,-1,-1
Disclaimer: I work at Atalasoft.
Our product, DotImage with the PDF Reader add-on, can do this. The easiest way is to rasterize the page and then just use any of our image analysis tools to get the colors.
This example shows how to do it if you want to group similar colors -- the deployed example will only work for PNG and JPEG, but if you download the code, it's trivial to include the add-on and get PDF as well (let me know if you need help)
Source here:
http://www.atalasoft.com/cs/blogs/31appsin31days/archive/2008/05/30/color-scheme-generator.aspx
Run it here:
http://www.atalasoft.com/31apps/ColorSchemeGenerator
If you are working with specific and simple PDF documents from a constrained source then you may be able to find the colors by reading through the content stream. However this cannot be a generic solution.
For example PDF documents can contain gradients or transparency. If your document contains this type of construct then you are likely to end up with a wide range of colors rather than a specific set.
Similarly many PDF documents contain bitmapped images. Given that these will need to be interpolated to be displayed at different resolutions, the set of colors in a displayed PDF may be bigger or different to (though obviously broadly similar to) the embedded bitmap.
Similarly many PDF documents contain constructs in multiple color spaces that are rendered into different color spaces. For example a PDF might contain a DeviceRGB bitmap, a line in an ICC based CMYK color and a Lab based rectangle. The displayed version might be in sRGB for display or CMYK for print. Each of these will influence the precise set of colors.
So the only 100% valid answer is going to be related to a particular render of a PDF at a particular resolution to a particular color space. From the resultant bitmap you can determine the colors that have been used.
There are a variety of PDF libraries that will do this type of render including DotImage (referenced in another answer) and ABCpdf .NET (on which I work).