The problem with the PDF format is that it was never meant for CAD. When Adobe co-founder John Warnock conceived the portable document format, his dream was that publishing issues like spot colors, fonts, and half-tones be reproduced perfectly, no matter the medium -- screen or printed page.
PDF became pervasive in offices once Adobe made Acrobat Reader free, and it became familiar to everyone, including for sending CAD drawings. Autodesk was a reluctant convert, as its DWF format is far better suited to the accuracy required by CAD drawings; PDF export was not added to AutoCAD until release 2011 -- 20 years after PDF first launched -- and as of last year's release the program also imports PDF files.
PDF was never meant for CAD, and that's is why exported .pdf files are imperfect, and importing them back into a CAD system is even more problematic. There is, for instance, insufficient accuracy for curves, which end up looking segmented or jittery. The accuracy of intersections is degraded compared to our use of object snaps to ensure precision. And don't get me started on text.
"Once saved as a PDF, there is no longer much information about the original CAD drawing, other than layers and TrueType fonts," explained Jean Haney. She owns Visual Integrity, a Dutch-based firm that has 21 years of experience in translating files to and from PDF. Her tip: after exporting PDF from the CAD program, examine the result in Acrobat Reader by zooming to 1000% or more and then looking at curved areas. See if the curves are smooth (see figure 1) or jagged (see figure 2.). If jagged, then the resolution set by the CAD program is too low; aim for 2400dpi.
Figure 1 High-resolution PDF file of a drawing exported by CAD program
Figure 2 Text and diagonal lines affected by low resolution export by CAD program
If it looks blocky instead, this indicates that it is a raster PDF file. A raster PDF (like a scanned drawing) will need to be traced or redrawn. (See figure 2.)
Figure 3 CAD drawing exported to PDF in raster format
So, we can export 2D drawings to 2D PDFs, because it's expected of us, not because it's an ideal neutral format. What about 3D? Well, 3D PDF is a kludge: the 3D model is inserted into a PDF page as an object. To view in 3D, we click the object, after which we can rotate and zoom. Our 3D models must be translated to U3D [universal 3D] format, a format that otherwise is not universal.
"People also got disappointed of PDF when they discovered that it is a very open standard. Although the PDF standard was usable in a prepress environment, there were simply too many ways in which a perfectly valid but unusable PDF-file could be created," writes Laurens Leurs at presspressure.com.
The good news is that PDF is now an ISO standard, meaning it is open for anyone to use for free, frozen at version 1.7, and Adobe in theory cannot make proprietary updates. In practice, it does through "extension levels" that add functions useful to Adobe's line of software.
About Visual Integrity
Jean Haney and Jan Homan co-founded Visual Integrity in the early 1990s. She does the commercial side, he is the primary developer and CTO. Their first program called PDF FLY, ran on several dialects of Unix to convert PS to EPS [encapsulated PostScript] and MIF [maker interchange format], which technical publishing programs like FrameMaker used. As the market shifted, they added Linux, Windows and then Mac OS X. (See figure 4.)
Figure 4 File formats supported by PDF FLY server
"But then we saw market for technical publishing programs going away, and being replaced by software like Quark and even Word. We saw that there was a market in CAD, and so sold created pdf2cad (a subset of PDF Fly), initially exporting in DXF and HPGL formats, and then eight years ago added DWG in cooperation with Open Design Alliance." When ODA members license "PDF underlay," they actually are getting Visual Integrity technology. There is a separate plug-in for BricsCAD that converts PDFs to DWG.
Visual Integrity also offers Conversion SDK [software development kit], for Windows, Linux and Mac OS X, which allows applications or Web services to input EPS, AI, and PS files and output to DXF, WMF/EMF [Windows and enhanced meta format], SVG [scalable vector graphics] and all standard raster formats. "SVG is a great vector Web-format,” she said, “and I wish it had taken off sooner. It's over 20 years old but now only slowly gaining traction."
There are many sources and many types of PDFs, and that affects the quality of drawings. "We get a number of PDFs files a day from people who are wondering how to convert them perfectly,” she said. “If the PDF files are computer-generated, we can usually create an accurate, editable CAD drawing. If they send scanned PDF files, we recommend they look for raster-to-vector tracing software, which will get them part of the way there and then require some manual clean-up."
Q & A
Q: How do you deal with text in drawings? A: If it is TrueType text, we read it directly. If it is CAD text, such as AutoCAD's SHX, it is “plotted text” [individual strokes, like lines]. We plan to add OCR [optical character recognize] of text to our next release. It can be any kind of plotted text, and we will convert it into a text. We support Unicode for international character support and can even “convert to curves”, which takes care of “strange” characters.
Q: PDF files from CAD systems can contain raster images, such as corporate logos and background images. What do you do with them? A: Raster images are converted to referenced DWG image objects.
Q: What if the drawing itself is a raster image in the PDF? A: We only do vector-to-vector and vector-to-image conversion. There are specialized raster-to-vector programs out there that make assumptions about CAD drawings, as to what their content means. We recommend customer contact Trix Systems as a starting point.
Q: What do you do to educate users about the problems that can happen in conversion? A: We have a Web site, and we instruct those who contact us about problems. We also offer a free test conversion where we explain what we did to create the file when returning the resulting CAD drawing.
Q: Autodesk added PDF import to AutoCAD 2017. How does this affect you? A: It will be interesting to see if they can handle all the support calls that the conversion results will generate. We still sell our software to customers of earlier releases of AutoCAD, as well as most other CAD systems.
Q: Do you have software that converts DWG to PDF? A: We do not, but we could think about that.
Q: What are some of the things your pdf2cad software does to import PDF files more accurately? A: We don’t use a print driver, as some programs do, because every extra step dilutes the conversion accuracy. We don’t depend on a specific CAD program, either. When converting to vector formats, we have our own powerful proprietary format-mapping engine which compensates for deficient features in the PDF file. This is unique.
Our software imports characters as strings [of text] or curves. We look at the characters next to other characters to determine which are words and to figure out the alignment. Our "Characters as Curves" option is useful for recreating symbols. Since curves are not ideal for text (because it makes characters too large), we have the "Unknown Fonts" option that handles them as symbols.
If fonts used in the PDF are missing from your computer, then you can specify which similar-looking one to use through font mapping.
The "PDF cropping" determines if, say, a fill pattern, should not extend beyond the clipping boundary. "Crop Picture" removes white margins from the edges of the PDF page, while "Page Mapping" converts multi-page PDF into a single one, because many CAD systems cannot handle multi-page PDFs.
We can force nearly horizontal or vertical lines to be orthogonal, and combine compound objects into blocks. Layers can be manipulated so that entities are grouped onto layers by color, by lineweight, or other layer definitions in the PDF file.
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Pdf2cad is $199 with 30-day support, and there is an annual subscription of $40/year for updates and support.
The SDK starts at $1,999 for fewer than 10,000 conversions a year, with max price of $9,999, plus a separate version for OEMs and ODA members. The SDK takes just two calls and one day to implement: open, convert the file; create an INI file to control conversion options.
"PDF was never designed to do all this, so it is remarkable that it works at all for CAD drawings," Ms Haney concluded. http://visual-integrity.com
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And One More Thing...
Siemens has for three years been helping Land Rover develop their 15-meter entry to the 35th America’s Cup catamaran race. (See figure 5.) The boat has 1.2km of cabling connecting 190 sensors and four video cameras, whose data is fed into 85,000 hours of design and build.
Land Rover BAR (Ben Ainslie Racing) used NX software for design, Teamcenter for data management, Fibersim for composites engineering, and Femap and STAR CCM for engineering analysis and computational fluid dynamics (CFD) analysis.
Figure 5 British team posing with their catamaran and a couple of Land Rover SUVs that happened to be on the dock (image credit Lloyd Images)
The America's Cup was first raced off the waters of England in 1851., and is named after the country that won it that year. This year's race is off the coast of Bermuda, with Land Rover being the challenger against Golden Gate Yacht Club. http://www.siemens.com/plm
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