gimp script-fu: drawing a simple line - scheme

Question

I'm trying to draw a line using gimp scripting. In the following script: I create a new image 512x512 and a I try to draw a diagonal (0,0)->(512,512).
But the image (tmp.xcf) remains transparent.
What am i doing wrong ?
(define (drawdiagonal W H)
(let* (
(img 0)
(bg 0)
(points (cons-array 4 'double) )
)
(set! img (car (gimp-image-new W H 0)))
(gimp-image-undo-group-start img)
(gimp-context-push)
(set! bg (car (gimp-layer-new img W H RGBA-IMAGE "background" 100 NORMAL-MODE)))
(gimp-image-add-layer img bg 0)
(gimp-drawable-set-visible bg TRUE)
(gimp-image-set-active-layer img bg)
(gimp-context-set-brush-size 10.0)
(gimp-context-set-opacity 100)
(gimp-context-set-paint-mode NORMAL-MODE)
(gimp-context-set-foreground '(255 127 0))
(gimp-selection-all img)
(aset points 0 0)
(aset points 1 0)
(aset points 2 W)
(aset points 3 H)
(gimp-paintbrush-default bg 4 points)
(gimp-context-pop)
(gimp-image-undo-group-end img)
(gimp-xcf-save 1 img img "tmp.xcf" "tmp.xcf")
(display "DONE")
)
)
(drawdiagonal 512 512) (gimp-quit 0)
usage:
cat test.scm | gimp -i -b -

Answer 1

It seems you are mostly missing:
(gimp-context-set-brush "Some brush")
Additionally, the 3rd argument there (2nd img) should be a drawable:
(gimp-xcf-save 1 img img "tmp.xcf" "tmp.xcf")
PS: do yourself a favor and write your script in Python. See here for an example.
Edit post-Gimp 2.10: there is now an API to stroke paths in "Line" mode which is the best way to obtain clean results. The relevant calls are:
pdb.gimp_context_set_stroke_method(STROKE_LINE)
pdb.gimp_context_set_line_cap_style(...)
pdb.gimp_context_set_line_join_style(...)
pdb.gimp_context_set_line_miter_limit(...)
pdb.gimp_context_set_line_width(width)
pdb.gimp_drawable_edit_stroke_item(drawable, path)

Answer 2

For those who may come along later, based on xenoid's hints, I made this using gimp-pencil.
[I found no documentation for the 'item/path' of gimp_drawable_edit_stroke_item]
Could be tweaked to use gimp-brush...
(define (util-draw-path drawable args . path)
;; Draw a zig-zag line:
;; (util-draw-path layer `((width 10) (color ,RED)) 10 50 40 80 70 30 110 90 150 80)
(let ((save (util-assq 'save args) #t)
(color (util-assq 'color args)) ; else use context value
(width (util-assq 'width args)) ; else use context value
(miter (util-assq 'miter args)) ; else use context value
(join (util-assq 'join args)) ; else use context value
(cap (util-assq 'cap args)) ; else use context value
(stroke (util-assq 'stroke args)) ; else use context value
)
(and save (gimp-context-push))
(and miter (gimp-context-set-line-miter-limit miter)) ; default: 10, default mitre up to 60 pixels
(and stroke (gimp-context-set-stroke-method stroke)) ; default STROKE-PAINT-METHOD
(and cap (gimp-context-set-line-cap-style cap)) ; CAP-ROUND, CAP-SQUARE
(and join (gimp-context-set-line-join-style join)) ; JOIN-MITER, JOIN-ROUND, JOIN-BEVEL
(and color (gimp-context-set-foreground color)) ;
(and width (gimp-context-set-line-width width)) ; default: 6
(let ((vec (apply vector path)))
(gimp-pencil drawable (vector-length vec) vec))
(and save (gimp-context-pop))
))
;;; (util-assq key list [default]) -> value or [default or #f]
(macro (util-assq form)
(let ((key (cadr form)) (lis (caddr form))
(els (if (pair? (cdddr form)) (cadddr form) #f)))
`(let ((v (assq ,key ,lis)))
(if v (cadr v) ,els))))