docs(book): add docs for setting the encoder options

Signed-off-by: k4yt3x <i@k4yt3x.com>

Author: k4yt3x

docs/book/src/running/command-line.md

@@ -29,7 +29,7 @@ video2x -i input.mp4 -o output.mp4 -f libplacebo -s path/to/custom/shader.glsl -
 List the available GPUs with `--list-gpus, -l`:
 
 ```bash
-$video2x --list-gpus
+$ video2x --list-gpus
 0. NVIDIA RTX A6000
         Type: Discrete GPU
         Vulkan API Version: 1.3.289
@@ -42,8 +42,63 @@ Select which GPU to use with the `--gpu, -g` argument:
 video2x -i input.mp4 -o output.mp4 -f realesrgan -r 4 -m realesr-animevideov3 -g 1
 ```
 
-Specify arbitrary extra FFmepg encoder options with the `--extra-encoder-options, -e` argument:
+Specify arbitrary extra FFmpeg encoder options with the `--extra-encoder-options, -e` argument:
 
 ```bash
 video2x -i input.mkv -o output.mkv -f realesrgan -m realesrgan-plus -r 4 -c libx264rgb -e crf=17 -e preset=veryslow -e tune=film
 ```
+
+## Encoder Options
+
+Video2X uses FFmpeg's C libraries to encode videos. Encoder options are specified in two ways:
+
+- **Common options** shared by all encoders are stored in a [`AVCodecContext`](https://ffmpeg.org/doxygen/trunk/structAVCodecContext.html) struct. Below are some options set through `AVCodecContext`:
+  - Codec
+  - Pixel format
+  - Bitrate
+  - Keyframe interval
+  - Minimum and maximum quantizer
+  - GOP size
+- **Encoder-specific** options are stored in [`AVOption`](https://ffmpeg.org/doxygen/trunk/structAVOption.html) structs and set with the [`av_opt_set`](https://ffmpeg.org/doxygen/trunk/group__opt__set__funcs.html#ga5fd4b92bdf4f392a2847f711676a7537) function. Below are some encoder-specific options for `libx264`:
+  - CRF
+  - Preset
+  - Tune
+  - Profile
+
+Common options can only be set through Video2X's command line arguments. You can run `video2x --help` and see the `Encoder options` section to see the supported options.
+
+You can specify encoder-specific options in Video2X using the `--extra-encoder-option` (`-e`) argument. To view the available options for a particular codec, run:
+
+```bash
+ffmpeg -h encoder=$ENCODER
+```
+
+For example, to view the available options for `libx264`, run:
+
+```console
+$ ffmpeg -h encoder=libx264
+Encoder libx264 [libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10]:
+    General capabilities: dr1 delay threads
+    Threading capabilities: other
+    Supported pixel formats: yuv420p yuvj420p yuv422p yuvj422p yuv444p yuvj444p nv12 nv16 nv21 yuv420p10le yuv422p10le yuv444p10le nv20le gray gray10le
+libx264 AVOptions:
+  -preset            <string>     E..V....... Set the encoding preset (cf. x264 --fullhelp) (default "medium")
+  -tune              <string>     E..V....... Tune the encoding params (cf. x264 --fullhelp)
+  -profile           <string>     E..V....... Set profile restrictions (cf. x264 --fullhelp)
+  -fastfirstpass     <boolean>    E..V....... Use fast settings when encoding first pass (default true)
+  -level             <string>     E..V....... Specify level (as defined by Annex A)
+  -passlogfile       <string>     E..V....... Filename for 2 pass stats
+  -wpredp            <string>     E..V....... Weighted prediction for P-frames
+  -a53cc             <boolean>    E..V....... Use A53 Closed Captions (if available) (default true)
+  -x264opts          <string>     E..V....... x264 options
+  -crf               <float>      E..V....... Select the quality for constant quality mode (from -1 to FLT_MAX) (default -1)
+  -crf_max           <float>      E..V....... In CRF mode, prevents VBV from lowering quality beyond this point. (from -1 to FLT_MAX) (default -1)
+  -qp                <int>        E..V....... Constant quantization parameter rate control method (from -1 to INT_MAX) (default -1)
+...
+```
+
+You can then set the encoder-specific options with the `-e` argument. The `-e` argument can be used multiple times to set multiple options. For example, the following arguments set the CRF to 17, the preset to `veryslow`, and the tune to `film` for `libx264`:
+
+```console
+-e crf=17 -e preset=veryslow -e tune=film
+```