feat(patches/alpha-background): add experimental alpha-background patch

[jimeh]

Resolves #111

[jimeh]

@jonrubens I hope you don't mind if I borrow the changes in your ns-alpha-background branch as a optional patch for use in my build script. If you have any objections, I'm happy to drop it.

The patch feature includes a static *.patch file for Emacs 29.x, and dynamically downloads a patch for Emacs 30.x and 31.x from GitHub.

If you don't mind others making use of your alpha-background changes, I think it'd be nice to get a patch submitted to emacs-plus as well. They already maintain a number of very nice patches.

[jonrubens]

You can absolutely include my change as a patch.

FYI, I have submitted the code to the Emacs master branch, so hopefully it will get pulled in for Emacs 31.

[jimeh]

@jonrubens Not sure if you've noticed, but when stepping through alpha-background with +1 or -1, I notice some odd flickering and color changes with my config. Essentially, every second or third value yields a slightly more yellowish background color than others, making it look like a slight flicker when quickly stepping up/down the alpha-background one value at a time.

I'm not sure if it's simply how the math works out when applying transparency to the colors of my theme, or if it's some odd bug with alpha-background. A similar thing seems to happen when setting alpha as well, but it's harder to tell cause the intensity curve is much stronger in the 90-100 range than it is for alpha-background.

If you're curious to dig deeper, I've got screenshots ranging with values 70 to 100 for both alpha-background and alpha, with Emacs placed in front of a solid white background:

• https://share.jimeh.me/emacs-alpha-background-levels-YM5icz490jr0.zip
• https://share.jimeh.me/emacs-alpha-levels-0DxdWdZLH5aR.zip

I used my transparency helpers for quickly changing the value, and here's a variant that uses alpha-background instead of alpha:

siren-transparency.el:

;;; siren-transparency.el --- jimeh's Emacs Siren: Emacs Frame transparency.

;;; Commentary:

;; Basic helpers and configuration for frame transparency.

;;; Code:

(require 'siren-ui)

(defgroup siren-transparency nil
  "Options for the Siren transparency."
  :group 'siren-ui)

(defcustom siren-transparency-default-level 90
  "The default frame transparency level for Emacs frames."
  :type 'number
  :set (lambda (symbol value)
         (set-default symbol value)
         (setf (alist-get 'alpha-background default-frame-alist) value))
  :group 'siren)

(defun siren-transparency-decrease ()
  "Decrease level of transparency for the current frame."
  (interactive)
  (let ((current (or (frame-parameter nil 'alpha-background) 100)))
    (if (> current 0)
        (let ((new-level (+ current -1)))
          (siren-transparency new-level)
          (message "Frame transparency set to %s" new-level))
      (message "This is a minimum value of transparency!"))))

(defun siren-transparency-increase ()
  "Increase level of transparency for the current frame."
  (interactive)
  (let ((current (or (frame-parameter nil 'alpha-background) 100)))
    (if (< current 100)
        (let ((new-level (+ current +1)))
          (siren-transparency new-level)
          (message "Frame transparency set to %s" new-level))
      (message "This is a minimum value of transparency!"))))

(defun siren-transparency (num)
  "Set level of transparency for the current frame by providing NUM."
  (interactive "nEnter transparency level in range 0-100: ")
  (set-frame-parameter nil 'alpha-background (cond ((> num 100) 100)
                                                   ((< num 0) 0)
                                                   (t num))))

;; Keybindings
(siren-general-define-key
 "C-M-|" 'siren-transparency
 "C-M-<" 'siren-transparency-decrease
 "C-M->" 'siren-transparency-increase)

(provide 'siren-transparency)
;;; siren-transparency.el ends here

[jonrubens]

I haven't ever noticed the color flicker. It's been a while since I wrote the code but my first pass guess is that it is something in how MacOS does transparency and not on the emacs side of things. I might dig into that further but I don't think there is anything in what I wrote that breaks down the color values to RGB besides simply applying the alpha component. It could be a consequence of how MacOS rounds when taking a 0-100 multiplier and converts to 0-255.

As for the intensity curve, that is something that bothered me and it never felt right. I wanted the transparency at 90% alpha to match 90% alpha-background but I couldn't figure out why it doesn't. I had an idea to fix it by translating the alpha-background curve to match, but it felt to hacky when there must be some layer being drawn that should be able to be fixed.