True Emacs Transparency

February 1, 2024

Before Emacs 29, we can set the alpha frame parameter to adjust the opacity of the frame. The issue with alpha is that is set the opacity of the entire window, which means both text foreground and background become transparent together. But what we want is the text remains opaque while the background becomes transparent.

The solution came from a patch make by Håkon Flatval. The patch adds the alpha-background frame parameter which make only the background of a frame transparent.

From Emacs 29, it supports true transparency. The alpha-background frame parameter set the background transparency of the frame. Unlike the alpha frame parameter, this only controls the transparency of the background while keeping foreground elements such as text fully opaque. It should be an integer between 0 and 100, where 0 means completely transparent and 100 means completely opaque (default).

Here I make a helper function to adjust transparency.

(defun dotemacs-adjust-transparency (frame incr)
  "Adjust the background transparency of FRAME by increment INCR."
  (let* ((oldalpha (or (frame-parameter frame 'alpha-background) 100))
         (newalpha (+ incr oldalpha)))
    (when (and (<= 0 newalpha) (>= 100 newalpha))
      (set-frame-parameter frame 'alpha-background newalpha))))
(keymap-global-set "C-M-8" (lambda () (interactive) (dotemacs-adjust-transparency nil -2)))
(keymap-global-set "C-M-9" (lambda () (interactive) (dotemacs-adjust-transparency nil 2)))
(keymap-global-set "C-M-0" (lambda () (interactive) (set-frame-parameter nil 'alpha-background 100)))