Fabrice Quenneville
b8b8817cd4
- Added instructions for setting up Tesseract with language support. - Documented steps for converting PDFs to images using `pdftoppm` and alternatives like `ImageMagick`. - Included examples for single and multi-page OCR processing. - Detailed methods for merging extracted text into a single file. - Added troubleshooting tips for improving OCR results and handling selectable PDFs with `pdftotext`.
105 lines
2.9 KiB
Markdown
105 lines
2.9 KiB
Markdown
# PDF
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## Table of Contents
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- [PDF](#pdf)
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- [Table of Contents](#table-of-contents)
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- [OCR](#ocr)
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## OCR
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**Install necessary tools**
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Update the package list and install Tesseract with support for the desired language. Replace `fra` with your desired language code (e.g., `eng` for English).
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```bash
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apt update
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apt install tesseract-ocr tesseract-ocr-fra
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```
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**Verify the installation**
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Check the installed version of Tesseract and list available languages to ensure your chosen language is installed (e.g., `fra` for French).
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```bash
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tesseract --version
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tesseract --list-langs
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```
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**Install a utility to convert PDF pages into images**
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For PDFs that require OCR, you need a utility to convert PDF pages into images. Install `poppler-utils`, which includes `pdftoppm`.
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```bash
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apt install poppler-utils
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```
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**Convert PDF to images**
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Convert the PDF into JPEG images, with each page saved as a separate file. Each page will be named sequentially (e.g., `output-1.jpg`, `output-2.jpg`, etc.).
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```bash
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pdftoppm -jpeg your_file.pdf output
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```
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- **Tip**: Use a dedicated output directory to avoid overwriting existing files.
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- **Alternative tools**: If `poppler-utils` isn’t available, consider using `ImageMagick`:
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```bash
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convert -density 300 your_file.pdf output-%04d.jpg
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```
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**Perform OCR on a single image**
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Run Tesseract OCR on an image to extract text. Specify the language using the `-l` option.
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```bash
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tesseract output-1.jpg output-text -l fra
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```
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The extracted text will be saved in `output-text.txt`.
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**Perform OCR on multiple images**
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For multi-page PDFs, process all images in a loop. This extracts text from each image and saves it to a separate `.txt` file.
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```bash
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for img in output-*.jpg; do
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tesseract "$img" "${img%.jpg}" -l fra
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done
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```
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- **Note**: The `${img%.jpg}` syntax removes the `.jpg` extension, ensuring each `.txt` file matches its corresponding image.
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**Combine all text files**
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Merge the text from all processed pages into a single file. This is useful for assembling the full content of the PDF.
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```bash
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cat output-*.txt > complete_text.txt
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```
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If filenames are out of order, use a sorting approach before merging:
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```bash
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ls output-*.txt | sort -V | xargs cat > complete_text.txt
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```
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**Troubleshooting and Tips**
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**If Tesseract doesn’t recognize text**:
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- Ensure the images have sufficient quality and resolution. Use `-r` with `pdftoppm` to increase the DPI (e.g., `-r 300` for 300 DPI).
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- Try additional Tesseract language packs for better recognition of specific text styles.
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**Using `pdftotext` for simpler PDFs**:
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If the PDF contains selectable text (not just images), `pdftotext` from `poppler-utils` can extract text directly without OCR.
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```bash
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pdftotext your_file.pdf output.txt
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```
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Replace `convert` with `magick` for newer versions.
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**Verify language codes for Tesseract**:
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You can find a list of supported languages on the [Tesseract GitHub page](https://github.com/tesseract-ocr/tesseract).
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