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Amit D edited this page Jan 20, 2019 · 20 revisions

How to install Ocropus on a Mac?

Starting from version 1.3.3 the default installation should exactly the same. Previously, there was some native C code which was needed to compile dynamically with a compiler with a different flag for Mac.

How to install Ocropus on Windows?

https://github.com/tmbdev/ocropy/issues/73 and https://github.com/tmbdev/ocropy/issues/263#issuecomment-347008893 and https://github.com/tmbdev/ocropy/issues/12#issuecomment-390774124

How to install Ocropus on Fedora?

https://github.com/tmbdev/ocropy/issues/48

How to install Ocropus on Linux Mint?

https://github.com/tmbdev/ocropy/issues/191

How to install Ocropus on CentOS?

https://github.com/tmbdev/ocropy/issues/226

Is there a docker image for Ocropus?

Yes, e.g. https://hub.docker.com/r/kbai/ocropy/

When can colorful, grayscale or black-and-white images used?

  • Binarization ocropus-nlbin: You can input colorful pictures and the output are a binarized black-white version .bin.png as well as a grayscale version .nrm.png.
  • Page Segmentation ocropus-gpageseg: You can input a (normalized) grayscale or black-and-white picture (any colorful picture will be automatically transformed to a grayscale picture). The output are by default black-and-white lines, but with the option --gray it is possible to output also normalized grayscale lines.
  • Text recognition ocropus-rpred: You can use either the binarized lines .bin.png or the (normalized) grayscale lines .nrm.png.
  • Training ocropus-rtrain: You can use either the binarized lines .bin.png or the (normalized) grayscale lines .nrm.png.

Tips on the usage: ocropus-rtrain was intended to be used with grayscale, but ocropus-rpred was not. Grayscale images can be especially useful in cases where binarization leads to a loss of connectedness in glyph shapes. Ocropus preprocessing will generate normalized grayscale images, that's what you should use. If you just cut out grayscale images yourself, it probably won't work as well.

Generally, in order to get good performance out of the 1D LSTM, you need line normalization. The line normalizer requires binary input. So, there are different options: you can run both recognition and normalization with binary input (which is used for both normalization and recognition), you can supply a grayscale image and the library will generate a binary image for normalization, or you can supply both a grayscale and a binary input, in which case the binary input is used for normalization and the grayscale for recognition. At least that was the original idea; most of the applications have used binary only, so the other codepaths haven't been tested much. Furthermore, "grayscale" usually means "normalized grayscale", that is, something that looks nearly binary but may have gray pixels around the edges

Should I use scans of single pages or can I also use scans of two pages in the same image?

It is possible to use scans of two pages in the same image, which will then be handled equivalent as one page with two columns. This can work okay if the scan is good and equivalent (no different skewing, enough space around the joint/fold). However, in most cases it is better to split the double-page into two single page images before continue with the OCR. For doing this task one can use for example:

for i in *.tif; do echo $i; convert $i -crop 2x1+0+0@ +repage +adjoin [insert_output_folder_name_here]/output$i; done

Can ocropus be used for handwritten text recognition?

Well, in theory this should work, but probably need a lot of training data (images + ground truth) and training steps. I don't know of anyone has done much into this direction with ocropus. There might be other software/projects more tailored for handwritten text, e.g. https://transkribus.eu/Transkribus/ .

What exactly is meant by 300 dpi for digital images?

The default parameters and settings of OCRopus assume 300dpi scanned images with black text on white pages and some standard font size (9pt to 14pt). This is normally the first thing you should check when the recognition quality is not good. When using a document/book scanner you normally can adjust this parameter, with other devices like a fax or smartphone camera you might not have this information.

As an indication: An A4-papier (210mm × 297mm) scanned with 300dpi results in an image with 2504 x 3540 pixels.

Pictures with low resolution or very hight resolution (e.g. 1200dpi) will usually not work well (there are some tests which might alerting the user in these cases). Resizing/rescaling the image might help in cases you can influence the scan quality on the device itself. Possible algorithms for that are:

What is the "ALN" line during training?

ALN is the result of decoding the alignment of the raw output with the ground truth. That is, ALN uses the ground truth. You can't use it for predictions, but it helps follow the progress of training.

How to avoid the UserWarning: Matplotlib is building the font cache?

If you see in the beginning of the commands the following warning

/usr/lib/python2.7/dist-packages/matplotlib/font_manager.py:273: UserWarning: Matplotlib is building the font cache using fc-list. This may take a moment.
  warnings.warn('Matplotlib is building the font cache using fc-list. This may take a moment.')

then one of or several of the following calls should help

rm -rf ~/.matplotlib
rm -rf ~/.cache/matplotlib
rm -rf ~/.cache/fontconfig/

See https://github.com/matplotlib/matplotlib/issues/5836

How can Ocropus be run on a command line only without graphical output?

Try to set export MPLBACKEND="agg".

How can the training been speed up? Is it possible to parallelize the training or to use GPU power for training?

The training cannot be parallelized or otherwise easily speed up. However, a faster training is available with CLSTM which can be connected to ocropus. Moreover, some work on a GPU-based version of ocropy is available at https://github.com/tmbdev/ocropy2.

What is the preferred name of this project?

The project was started as OCRopus, then it was mostly used in the lowercase form ocropus and now this repository is simply named ocropy. In the earlier days there used to be a high level repository (OCRopus) that included other subrepos and contained a top-level driver and build script. A subset of the tools implemented in Python is the content of this repository which is called ocropy. Nowadays, the names OCRopus, ocropus, and ocropy can be used interchangeable. See also https://github.com/tmbdev/ocropy/issues/97.

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