Update page 'Extract images from OCTGN packs'

Extract-images-from-OCTGN-packs.md

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+Or "*How to not kill Ponyhead's bandwidth*"
+
+The MCGServer needs images in a ponyhead-like format for serving them to the client, this is desirable for other kind of projects as well, so here's a guide on how to extract images from OCTGN image packs for the [MLP:CCG OCTGN game definition](http://www.octgngames.com/mlpccg), which are easily available online.
+
+# Requirements
+- [Go 1.10+](https://golang.org/) (but earlier versions might work)
+- MLP:CCG OCTGN Game definition, install it in OCTGN or get it from [GitHub](https://github.com/GameMasterLuna/MLPCCG-OCTGN)
+- MLP:CCG OCTGN Image packs, look them up on the [MLP:CCG OCTGN Discord group](https://discord.gg/QkGx4FT)
+- Windows is **not** needed, all the tools are cross-platform!
+
+# Let's do the thing!
+
+## 1. Create a GUID translation map
+
+This map translates all the GUIDs from the image packs to card IDs, for example:
+
+`656...df9/Sets/c0a...e58/Cards/3c7...181.jpg` ➞ `cn41.jpg`
+
+To install it, run this command to retrieve the source code and compile it:
+```
+go get github.com/Hamcha/OCTGN2PH/buildmap
+```
+
+If you installed the game definition via OCTGN, you can just run `buildmap > cardmap.json` and it *should* work fine, if it doesn't or if you downloaded the game definition outside OCTGN's folder you need to specify the path to the Game "Sets" sub-folder using `-set-path`, like this:
+```
+buildmap -set-path path/to/MLP-OCTGN/Sets > cardmap.json
+```
+
+## 2. Extract image packs with the converted filenames
+
+This time we need a different tool:
+```
+go get git.fromouter.space/mcg/mlp-server-tools/tools/genpics
+```
+
+Make sure all the image packs (`.o8c`) files are in the same directory with nothing else (at least no other `.o8c` files), then run `genpics` like this:
+```
+genpics -packdir dir/to/imgpacks -mapfile path/to/cardmap.json -out images
+```
+This will output all files in a new directory called `images`.
+
+If the packs contain files not referenced in the translation map, they will retain their original name (but not path) and placed in a sub-folder called `unknown`.
+
+## That's it!
+
+If all went accordingly, you should have a folder full of card images, like this:
+
+![Output card folder](https://i.imgur.com/5cOM6xA.png)