docs: document all variables available to presets

Previously only output_pixel_format was mentioned. All three variables
(input_codec, input_pixel_format, output_pixel_format) are now listed
with descriptions in the script header comment, the README, and the
man page.

1 files changed, 7 insertions, 2 deletions

diff --git a/README.md b/README.md
index 4001672..64dd6c0 100644
--- a/README.md
+++ b/README.md
@@ -77,8 +77,13 @@ ${XDG_CONFIG_HOME:-$HOME/.config}/transcode.sh/presets/<name>.sh
 ```
 
 Each preset is sourced as Bash and must define an array named `ffargs`.
-The variable `output_pixel_format` is available and contains the
-automatically selected pixel format (derived from the input file).
+The following variables are available to presets at source time:
+
+| Variable | Description |
+|----------|-------------|
+| `input_codec` | Video codec of the input file (e.g. `h264`, `hevc`) |
+| `input_pixel_format` | Pixel format of the input after `yuvj*` normalisation (e.g. `yuv420p`) |
+| `output_pixel_format` | Recommended output pixel format, derived from the input to preserve chroma subsampling and bit depth |
 
 Before the preset's `ffargs` are appended, the script always passes
 these base arguments to `ffmpeg`: