document a few useful cli commmands

content/guide/converting-formats.md

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+---
+title: "How to quickly convert file formats"
+date: 2023-08-08
+categories:
+  - collection
+  - file formats
+author: Lukas Winkler
+description: This is a collection of a few useful commands for converting files between formats that I use all the time.
+cc_license: true
+toc: true
+---
+
+This is a collection of a few useful commands for converting files between formats that I use all the time.
+
+<!--more-->
+
+## Audio
+
+### Audio → FLAC
+
+lossless if converting from e.g. WAV
+
+```bash
+flac --best input.wav
+```
+
+### Audio → Opus
+
+Check [Opus Recommended Settings](https://wiki.xiph.org/Opus_Recommended_Settings) for the ideal bitrate for your use case.
+24 Kb/s is a good basis for very small files that are still high quality for voice recordings. Only use `--downmix-mono` if you don't loose information by merging stereo audio to mono.
+
+```bash
+opusenc --bitrate 24 --downmix-mono input.wav output.opus
+```
+
+## PDFs
+
+### PDF → Extracted Images
+
+```bash
+mkdir tmp
+pdfimages input.pdf -all tmp/name
+```
+
+This will extract all images that are contained in the input pdf to `tmp/name-000.png`, `tmp/name-001.jpg`, etc.
+
+### PDF → PNG
+
+```bash
+pdftoppm input.pdf slides -png -scale-to 1080 -progress
+```
+
+This will generate images like `slides-1.png` for every page of the PDF with the specified width. With e.g. `-scale-to 3840` one can quickly convert a PDF of presentation slides to images for a high quality 4K video. 
+
+### PDF → compressed PDF
+
+[source](https://askubuntu.com/questions/113544/how-can-i-reduce-the-file-size-of-a-scanned-pdf-file/256449#256449)
+
+Sometimes I have a PDF that is far too large (hundreds of MB for a simple document) because it was generated in an ineffcient way. Using ghostscript can in many cases reduce the file size dramatically while decreasing the quality only a bit.
+
+```bash
+gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dPrinted=false -dNOPAUSE -dBATCH -sOutputFile=small.pdf input.pdf 
+```
+
+Depending on the required quality, `/ebook` can be replaced with one of `/printer`, `/prepress` and `/default` (or `/screen` for a very bad quality). See the [documentation](https://ghostscript.readthedocs.io/en/latest/VectorDevices.html#controls-and-features-specific-to-postscript-and-pdf-input) for more information
+
+### PDF → PDF with OCR
+
+```bash
+ocrmypdf -cdr --force-ocr input.pdf ocr.pdf -l deu
+```
+
+Check the [documentation](https://ocrmypdf.readthedocs.io/en/latest/cookbook.html) for more information.
+

hugo_version.txt

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-0.111.3
+0.117.0

layouts/_default/single.html

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         </header>
         {{ partial "toc.html" . }}
         {{ .Content }}
+        <hr>
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