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.

3 files changed, 34 insertions, 5 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`:
diff --git a/transcode.sh b/transcode.sh
index 0d2e841..1067bc0 100755
--- a/transcode.sh
+++ b/transcode.sh
@@ -15,6 +15,12 @@
 #     for example:
 #         ffargs=( -c:v:0 libsvtav1 -crf 30 -preset 6 -pix_fmt "$output_pixel_format" )
 #
+#     The following variables are available to presets at source time:
+#         input_codec          - video codec of the input file (e.g. h264, hevc)
+#         input_pixel_format   - pixel format after yuvj* normalisation (e.g. yuv420p)
+#         output_pixel_format  - recommended output pixel format, derived from the
+#                                input to preserve chroma subsampling and bit depth
+#
 #     Note: Presets are sourced as shell code. Only use trusted presets.
 #
 #     A per-user list of video-codecs to skip can be provided in:
diff --git a/transcode.sh.1 b/transcode.sh.1
index fe38c21..a22b802 100644
--- a/transcode.sh.1
+++ b/transcode.sh.1
@@ -153,10 +153,28 @@ array variable named
 .B ffargs
 containing additional arguments for
 .BR ffmpeg (1).
-The variable
+The following variables are available to presets at source time:
+.RS
+.TP
+.B input_codec
+The video codec of the input file as reported by
+.BR ffprobe (1)
+(e.g.\&
+.IR h264 ,
+.IR hevc ).
+.TP
+.B input_pixel_format
+The pixel format of the input file after
+.I yuvj*
+normalisation (e.g.\&
+.IR yuv420p ,
+.IR yuv444p10le ).
+.TP
 .B output_pixel_format
-is available to presets and contains the automatically selected output
-pixel format string.
+The recommended output pixel format, automatically derived from the
+input to preserve chroma subsampling (4:2:0, 4:2:2, 4:4:4) and bit
+depth (8, 10, 12, 16\-bit).
+.RE
 .IP
 Before the preset's
 .B ffargs