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Re: SVG hack for display engine

From: Anand Tamariya
Subject: Re: SVG hack for display engine
Date: Thu, 18 Nov 2021 05:17:47 +0530

Please describe the intended use of these APIs and the hook to provide
the display feature(s) you have in mind, and how will that work in
concert with the display engine.  I don't think I have a clear picture
of that yet.

Also, please describe what happens with use of these APIs when just a
small part of the Emacs display needs to be redrawn.

Maybe a demo ( https://youtu.be/1343m3LY4mo ) will help you visualize my suggestion. In the video, an event loop is started in the scratch buffer. All the edit operations update text and text property in a temporary buffer, say a.txt. Based on this information, in-memory SVG DOM is updated. Ultimately, the display engine updates the view with this SVG image. In cases like this, the mode can have full control over the display if so desired.

Since you have a problem accessing my repo, I've added the code of a function I call in my event loop at the end of this mail.

With a tighter integration, Emacs would define a new data structure {svg, x, y, w, h} and two new variables with this type - new_svg and prev_svg. Then the sequence of operations would be:
- Display engine refreshes the view bitmap with current logic - full or partial refresh as the case may be.
- If it's a graphical display
- prev_svg = new_svg
- Run display-svg-hook. The registered user function will call (svg-render) function passing required information. This API sets the value in new_svg.
- If the bounding box of new_svg < the bounding box of prev_svg, display engine refreshes the area of prev_svg in the view bitmap.
- Generate a bitmap for new_svg and superimpose it on the view bitmap.
- Display the view bitmap to the user.

Hope this helps.

(defun formula-canvas-mode (key svg)
  (let* ((func (lookup-key global-map (vector key)))
         (i 0)
         (size 20)
         mkey win
         text x y id point pos start end opts)
    ;; (message "Before: %s %s" key func)
    (unless func
      (setq mkey (lookup-key local-function-key-map (vector key))
            func (lookup-key global-map mkey)))
    ;; (message "After: %s %s" key func)

    (with-current-buffer (get-buffer-create "a.txt")
      (when func
        (condition-case var
            (if (eq func 'self-insert-command)
                (funcall func 1 key)
              (funcall func 1))
          (error (message (format "Error: %s" (car var)))))

          (setq win (get-buffer-window))
          (goto-char (window-start win))
          ;; Need to keep popup at the end
          ;; (setq svg (subseq svg 0 2))
          (dom-remove-node svg (car (dom-by-id svg "_popup")))
          (while (< (point) (window-end win))
            (setq start (point)
                  end (progn (end-of-line) (point))
                  text (buffer-substring start end)
                  id (number-to-string i)
                  x 0
                  y (* (1+ i) size))
            (setq i (1+ i))
            ;; Text
            (svg-text svg text :id id :x x :y y :xml:space "preserve")))

        ;; Cursor
        (setq pos (posn-col-row (posn-at-point (point) win))
              point (* 4 (car pos))
              y (* size (cdr pos)))
        (svg-rectangle svg point y 5 size
                       :id "_cursor" :fill "black" :opacity "0.4")
        ;; Popup
        (setq opts '("a" "b" "c" ))
        (if opts
            (formula-canvas-popup svg point (+ y size) opts))
        (svg-possibly-update-image svg)

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