[feature request] Define functions for making mpv play the curent subtitle audio at point 25%, 50% and 75% of the audio

[rodrigo-morales-1]

Let me explain my use case: When I am editing a subtitle in a *.srt file, I sometimes want to check if the audio for that segment is consistent with the subtitle, to check this I am willing to listen to the entire audio for that subtitle. When editing the end time of a subtitle, I frequently want to check if the last part of the audio is consistent with the subtitle. Currently, when the end time of a subtitle changes, the audio is played from the start time, so I unwillingly need to listen to the audio starting from the beginning. I believe it would be more convenient for some users (such as me) if there were a function that would allow the user to play the audio starting at position 75%, so that the user don't need to listen to the 0%-75% segment, which the user might not be interested in listening.

In my use case, there are some subtitles which are 15 seconds long. Sometimes when I change the end time, I want to listen the last part of the audio (i.e. the segment 75%-100% in the current subtitle audio). Having to listen to a 15 seconds audio even though I'm interested in the last part is bothersome. I wish there were a function that I could map to a key to a function that makes mpv play the current subtitle audio starting at 75%.

I believe having functions that play the current subtitle audio at positions 25%, 50% and 75% would make subed more convenient to other users, because users can map those keys to a key and instruct mpv to play starting at a point that is closer to the part they are interested in listening.

[rodrigo-morales-1]

I have written these functions to accomplish the behavior I described:

(defun my-subed-mpv-jump-to-25% ()
  (interactive)
  (let ((cur-sub-start (subed-subtitle-msecs-start))
        (cur-sub-stop (subed-subtitle-msecs-stop)))
    (subed-mpv-jump
     (+ cur-sub-start
        (* (- cur-sub-stop cur-sub-start) 0.25)))))

(defun my-subed-mpv-jump-to-50% ()
  (interactive)
  (let ((cur-sub-start (subed-subtitle-msecs-start))
        (cur-sub-stop (subed-subtitle-msecs-stop)))
    (subed-mpv-jump
     (+ cur-sub-start
        (* (- cur-sub-stop cur-sub-start) 0.5)))))

(defun my-subed-mpv-jump-to-75% ()
  (interactive)
  (let ((cur-sub-start (subed-subtitle-msecs-start))
        (cur-sub-stop (subed-subtitle-msecs-stop)))
    (subed-mpv-jump
     (+ cur-sub-start
        (* (- cur-sub-stop cur-sub-start) 0.75)))))

(defun my-subed-mpv-jump-to-90% ()
  (interactive)
  (let ((cur-sub-start (subed-subtitle-msecs-start))
        (cur-sub-stop (subed-subtitle-msecs-stop)))
    (subed-mpv-jump
     (+ cur-sub-start
        (* (- cur-sub-stop cur-sub-start) 0.90)))))

I mapped those functions to these keys:

(define-key subed-mode-map (kbd "C-c C-1") 'my-subed-mpv-jump-to-25%)
(define-key subed-mode-map (kbd "C-c C-2") 'my-subed-mpv-jump-to-50%)
(define-key subed-mode-map (kbd "C-c C-3") 'my-subed-mpv-jump-to-75%)
(define-key subed-mode-map (kbd "C-c C-4") 'my-subed-mpv-jump-to-90%)

I release the code included in this comment under Creative Commons Zero License

[rodrigo-morales-1]

I just found out that subed defines two functions for jumping to specific parts of the audio:

subed-mpv-jump-to-current-subtitle

subed/subed/subed-mpv.el

Lines 326 to 332 in 07567d5

	(defun subed-mpv-jump-to-current-subtitle ()
	"Move playback position to start of currently focused subtitle if possible."
	(interactive)
	(let ((cur-sub-start (subed-subtitle-msecs-start)))
	(when cur-sub-start
	(subed-debug "Seeking player to focused subtitle: %S" cur-sub-start)
	(subed-mpv-jump cur-sub-start))))

subed-mpv-jump-to-current-subtitle-near-end

subed/subed/subed-mpv.el

Lines 334 to 341 in 07567d5

	(defun subed-mpv-jump-to-current-subtitle-near-end ()
	"Move playback position to near the end of the current subtitle."
	(interactive)
	(let ((cur-sub-end (subed-subtitle-msecs-stop)))
	(when cur-sub-end
	(setq cur-sub-end (- cur-sub-end subed-sample-msecs))
	(subed-debug "Seeking player to end of focused subtitle: %S" cur-sub-end)
	(subed-mpv-jump cur-sub-end))))

Both functions are mapped by default.

subed/subed/subed.el

Lines 73 to 74 in 07567d5

	(define-key subed-mode-map (kbd "M-j") #'subed-mpv-jump-to-current-subtitle)
	(define-key subed-mode-map (kbd "M-J") #'subed-mpv-jump-to-current-subtitle-near-end)

I think these already defined functions cover most use cases. In order to cover more use cases, we could provide a macro that helps users define functions, which would be similar to the functions I defined in #69 (comment), for jumping to specific parts (25%, 30%, 50%, 75%).

[rodrigo-morales-1]

I had another idea that has to do with jumping to specific parts in mpv: We could define functions for seeking backward or seeking forward (5s, 10s) from the current playback position.

My use case: I sometimes use programs for automatically generating subtitles for audio files, those subtitles contain some errors, so I need to manually fix them (e.g. split long subtitles, correct time span). While I'm splitting subtitles, I need to listen the part that comes after the point and then go back to the point I was in to make sure that I'm splitting a subtitle in the right position, in this case I wish I could seek backward.

[sachac]

That could be fun. =) I think I've mostly been using middle-click in subed-waveform to sample sound without affecting the playback position, but something that's more keyboard-focused might work for your workflow. Have you figured out keyboard shortcuts and functions that make sense to you? (No rush, just curious!)

[sachac]

Or maybe if there was the idea of some kind of register, like the way you can save Emacs positions to a register and then jump back to them...

[rodrigo-morales-1]

Have you figured out keyboard shortcuts and functions that make sense to you? (No rush, just curious!)

@sachac In GNU Emacs 29.4, M-; is mapped to comment-dwim and M-' is mapped to abbrev-previx-mark, both in global-map. I have never used those functions while editing subtitles so I ended up using M-; for seeking backward and M-' for seeking forward. Below I share a minimal reproducible example for the functions that I defined.

In the video below, I show how seeking helps me listen specific parts of a subtitle. Note that the benefit of seeking is that I don't have to listen the entire subtitle again in order to listen a specific part. The following list explains what happens in the video:

1. The point is on the first subtitle
2. I move the point to the second subtitle
3. I play the audio
4. I pause mpv
5. I search the string a to move the point where I want to split the subtitle
6. I press M-; (bound to my-subed-mpv-seek-backward) repeatedly to seek backward so that I can listen one more time the part where I think I should split the subtitle. When seeking backward, the vertical line which is shown in the waveform is moved.
7. I pause mpv
8. I press M-; (bound to my-subed-mpv-seek-backward) to seek backward again to move the vertical line to the specific part where I want to split the subtitle.
9. I press M-. to split the subtitle (by default, M-. is bound to subed-split-subtitle in subed-srt-mode-map)

output.webm

I used the command in the first code block below to download the sample video and the sample subtitle file shown in the video. I created the file /tmp/subtitles/init.el with the contents of the second code block. I started Emacs using the command in the third code block below.

rm -rf /tmp/subtitles \
  && mkdir /tmp/subtitles \
  && curl \
       --output /tmp/subtitles/video.ogv \
       'https://upload.wikimedia.org/wikipedia/commons/4/41/Creative_Commons_and_Commerce.ogv' \
  && curl \
       --data-urlencode 'format=json' \
       --data-urlencode 'action=query' \
       --data-urlencode 'prop=revisions' \
       --data-urlencode 'rvprop=content' \
       --data-urlencode 'rvslots=*' \
       --data-urlencode 'formatversion=2' \
       --data-urlencode 'titles=TimedText:Creative_Commons_and_Commerce.ogv.en.srt' \
       'https://commons.wikimedia.org/w/api.php' | jq --raw-output '.query.pages.[].revisions.[].slots.main.content' > /tmp/subtitles/video.en.srt

(add-to-list 'load-path "/tmp/subed/subed")

(require 'subed-srt)
(require 'subed-waveform)

(add-hook 'subed-mode-hook 'subed-enable-sync-player-to-point)
(add-hook 'subed-mode-hook 'subed-enable-loop-over-current-subtitle)
(add-hook 'subed-mode-hook 'subed-enable-replay-adjusted-subtitle)
(add-hook 'subed-mode-hook 'subed-waveform-minor-mode)
(add-hook 'subed-mode-hook 'my-subed-custom-keys-minor-mode)

(defun my-subed-mpv-seek-backward ()
  (interactive)
  (subed-mpv-seek -100))

(defun my-subed-mpv-seek-forward ()
  (interactive)
  (subed-mpv-seek 100))

(define-minor-mode
  my-subed-custom-keys-minor-mode
  "Custom keys for `subed-mode'."
  :keymap (let ((map (make-sparse-keymap)))
            (define-key map (kbd "M-;") 'my-subed-mpv-seek-backward)
            (define-key map (kbd "M-'") 'my-subed-mpv-seek-forward)
            map))

emacs \
  --quick \
  --load /tmp/subtitles/init.el \
  /tmp/subtitles/video.en.srt

Sidenote: I initially created the issue for functions that would allow to jump to the 25%, 50% and 75% of the audio, but now I believe that seeking is more needed, so we should focus our attention on seeking and then we can focus on those functions for jumping to 25%, 50% and 75%.