Introduction
If you have been involved in
the development of a document management application or process, you have
probably run across a requirement to add, or interface with, forms
recognition. It is also likely you have been frustrated with the accuracy of
the results. Forms recognition and processing is a complicated procedure with
an infinite number of variables and factors contributing to the accuracy of the
extracted fields. Every document is different, but most share similar issues
contributing to inaccuracies including scan quality, user input legibility,
recognition engine quality, etc. However, few variables carry as much weight
or significance as form alignment.
Alignment is critical to the
accuracy of forms recognition because the form fields are confined to very
specific locations on the master form. Therefore, seemingly minute differences
of a few pixels or degrees can completely throw off what is presented to the
optical text recognition (OCR) or optical mark recognition (OMR) engine, making
it impossible to get correct results without scanning the document again or
fixing the document in some way.
Figure 1 from left to right:
(1) Master form with field areas defined in green
(2) Filled and scanned form with alignment imperfections
(3) Combined overlay of first two images
Using LEADTOOLS to Automatically Correct Alignment Issues
Forms
alignment in and of itself is a multi-faceted issue consisting of three major
obstacles: skew angle, offset margins, scan resolution and non-linear
deformations. LEADTOOLS uses its decades of experience and research in the
document imaging industry to make it easy for developers to correct alignment
problems in their scanned documents. Best of all, the corrections are done
automatically with only a few very simple lines of code.
ocrEngines = new List<IOcrEngine>();
for (int i = 0; i < Environment.ProcessorCount; i++)
{
ocrEngines.Add(OcrEngineManager.CreateEngine(OcrEngineType.Advantage, false));
ocrEngines[i].Startup(formsCodec, null, String.Empty, String.Empty);
}
formsRepository = new DiskMasterFormsRepository(formsCodec, _MasterFormFolder);
autoEngine = new AutoFormsEngine(formsRepository, ocrEngines, null,
AutoFormsRecognitionManager.Default | AutoFormsRecognitionManager.Ocr, 30, 70, true);
AutoFormsRunResult runResult = autoEngine.Run(document, null);
if (runResult != null)
{
foreach (FormPage formPage in runResult.FormFields)
{
foreach (FormField field in formPage)
{
}
}
}
LEADTOOLS
believes that form alignment is so crucial and integral to the success of forms
recognition and processing that it is done automatically as a part of its
classification (recognition) algorithm. This is a huge benefit to developers
since more time can be devoted to other areas of their application, rather than
researching and debugging problems with inaccurate results that disappear with
properly aligned forms.
Don’t
just take our word for it though. Let’s dive deeper into each of the four
forms alignment stumbling blocks, how they adversely affect accuracy, and how
using LEADTOOLS will have a huge impact on the overall quality of your forms
recognition application.
Skew Angle
If you have spent any amount
of time scanning documents, you know very well that the paper isn’t always laid
down or fed in perfectly straight. Since the zones defining the form fields
are rectangular, it will be very hard for the filled-in text to fit within the
restrictive area. With a very modest skew angle of only one degree, a few
fields might still land in their boxes, depending on their proximity to the
origin of rotation. Most fields, however, will have tops and bottoms of
characters cut off, or potentially even show up in a completely different
field’s box for larger skew angles.
Figure 2: Example of image skewed by only 1 degree
Offset Margins
Offset margins are similar to the skew angle in that it is a
problem with how the document is oriented when scanned. Extra margins are
added or subtracted to the scanned image when the top-left corner is not in the
correct location, shifting everything up or down and left or right in relation
to the original document. Flatbed scanners are more prone to this than those
with an automatic document feeder (ADF), but if the guides are poorly chosen or
if the paper slips, the same imperfections can appear.
For full page OCR, offset margins present few problems as
long as the document has been de-skewed and the rows of text line up
horizontally. When recognizing specific fields with a small rectangular region
of interest, an offset margin of only five to ten pixels can cause characters to go
missing from either side, or for the entire row of characters to be
unrecognizable due to being cut off at the top or bottom.
Figure 3: Example of image offset by only (5,10) pixels
Scan Resolution
The scan resolution plays another major role in aligning
forms. The DPI (Dots / Pixels Per Inch) of a scanned document sets the
coordinate system and therefore can throw off where the fields are physically
located in the bitmap. For example, the point located at (300,300) on a 300
DPI image will actually be at (200,200) on a 200 DPI image.
Figure 4: Example of how coordinate systems will differ
with different scan resolutions
Most SDKs and forms recognition solutions don’t bother with
this and leave it up to either the programmer to convert the coordinate spaces
on the fly, or force the user into scanning the form at the same DPI as the
original master form. LEADTOOLS sees the value in giving customers and
programmers the flexibility and peace of mind in knowing that as long as the
scan quality is high enough for OCR, OMR or barcode recognition to be accurate
(typically 150-200 DPI is a good baseline), LEADTOOLS can recognize the form
and process the fields. All of the coordinates are relative so that once a
filled form is scanned and classified, it contains properties that help map the
fields to the master form with impeccable accuracy.
Non-Linear Deformations
Finally, the most difficult alignment imperfection to
overcome is non-linear deformation. Every scanner is different, and can cause
stretching and shrinking in some areas of the document while others appear
normal. This means that a document scanned at the same resolution on one
scanner might not line up with the exact same paper document fed through a
different scanner.
The most common cause of this is the differences in ADF
speed. When a document is fed into the scanner, the feeder usually starts
slow, speeds up to full speed, and some scanners even slow down again at the
end of a document. This would cause the middle of the document to line up
correctly, but causing the top and/or bottom to be stretched and in turn
misalign the fields. Variations in lenses and even the age of the scanner can
also cause non-linear deformations. This poses a huge problem for customers since their
forms recognition will usually only work as expected when the filled forms are
scanned with the exact same scanner as the master forms.
Figure 5: Example of non-linear deformation. Note how the
top of the image aligns but the bottom starts to stretch
Very few forms recognition SDKs are capable of this advanced
image processing, but LEADTOOLS is proud to be far ahead of the competition
with its full-scale forms recognition document alignment correction. In the
final figure below, you will see a form that has been fully corrected with
LEADTOOLS forms alignment.
Figure 6: Properly Aligned Form. Note how the data to be
extracted fits perfectly within the fields
Conclusion
As evidenced above, there are huge benefits when choosing
LEADTOOLS as your forms recognition engine. Its automated forms alignment
features can correct the skew angle, offset margins, calculate the relative
coordinates for different scan resolutions, and account for non-linear
transformations; allowing your filled forms to be in perfect unity with your
master forms. When this happens, you will get better accuracy, faster
processing speeds, more peace of mind and a faster time to market for your
application.
Download the Full Forms Recognition Example
You can download the fully functional demo which includes
the features discussed above. To run this example you will need the following:
-
LEADTOOLS
free 60 day evaluation
- Visual Studio 2008 or later
-
Browse to the LEADTOOLS Examples folder (e.g. C:\LEADTOOLS
18\Examples\) where you can find example projects for this and many more
technologies in LEADTOOLS
Support
Need help getting this sample up and going? Contact
our support team for free technical support! For pricing or licensing
questions, you can contact our sales team (sales@leadtools.com)
or call us at 704-332-5532.
