[racket] Fwd: Transformations and hc-append

Thanks very much for looking into this. I'm sure the docs will benefit from
what you figure out!

Robby


On Tue, Feb 11, 2014 at 3:54 PM, Jens Axel Søgaard <jensaxel at soegaard.net>wrote:

> 2014-02-11 20:07 GMT+01:00 Robby Findler <robby at eecs.northwestern.edu>:
> > I'm not sure I can give you more than just hints (and certainly not a
> > specification)
>
> I suddenly realized I should look in the Cairo docs, to see their
> intended usage.
>
> > but probably the difference is that pict->bitmap is drawing
> > the pict at (0,0) in the DC and scribble is drawing it at some random
> place.
> > But yeah, I'm sorry, that's still just a guess.
>
> After some experiments I think the reason is that Cairo has both user
> space coordinates (aka logical coordinates) and pattern space. When a
> pattern is
> created it has the same coordinate system has the user space, but if
> a transformation is applied (to user space) the spaces become different.
>
> I haven't yet fully grokked what the implications are.
>
> It is still odd that DrRacket and Scribble render the same pict
> differently though.
> If I call (convert the-pict 'png-bytes) in DrRacket I get the correct
> image, and
> Scribble doesn't do anything more.
>
> > And the examples at that link are really fantastic!
>
> Thanks!
>
> > On Tue, Feb 11, 2014 at 1:03 PM, Jens Axel Søgaard <
> jensaxel at soegaard.net>
> > wrote:
> >>
> >> 2014-02-11 19:39 GMT+01:00 Robby Findler <robby at eecs.northwestern.edu>:
> >> > Is the problem that you need to use (more) absolute coordinates in the
> >> > coordinate arguments to linear-gradient% (either that or set the
> origin
> >> > of
> >> > the dc, in the case that you wanted to create the brush only once)?
> >>
> >> The problem is that it is unspecified (as far as I know) whether brushes
> >> used in picts are specified using absolute or relative coordinates. For
> >> solid
> >> brushes this makes no difference. For non-solid brushes (gradients and
> >> textures) there is a choice to be made. I got surprised by the
> >> behavior and thought
> >> of it as a bug (I was looking for one - I have an example where a pict
> >> shows up perfectly in DrRacket but renders differently in Scribble).
> >>
> >> If I in shady make a new gradient each time the pict is to be drawn, I
> get
> >> the
> >> behavior I want:
> >>
> >>                            (new linear-gradient%
> >>                                 [x0 x] [y0 y] [x1 (+ x (* 2 r))] [y1
> >> y] ; horizontal gradient
> >>                                 [stops (list (list 0   red)   ; (0,0)
> >> to ( r,0) red->green
> >>                                              (list 1/2 green) ; (r,0)
> >> to (2r,0) green->blue
> >>                                              (list 1   blue))])
> >>
> >> Since the pict constructors such as filled-rectangle use the pen, there
> >> were no way to figure out what was intended.
> >>
> >> The last example here: shows that Scribble converts p and (pict->bitmap
> p)
> >> differently. I am still puzzled by this.
> >>
> >>
> >>
> http://soegaard.github.io/docs/metapict/metapict.html#%28part._example-rgb-triangle%29
> >>
> >> /Jens Axel
> >>
> >>
> >> > On Tue, Feb 11, 2014 at 10:49 AM, Jens Axel Søgaard
> >> > <jensaxel at soegaard.net>
> >> > wrote:
> >> >>
> >> >> The intent of hc-append is that (hc-append p1 p2) draws p1 and p2
> >> >> beside each other.
> >> >>
> >> >> The question is whether (hc-append p p) should draw to identical
> picts?
> >> >>
> >> >> When I use a non-solid brush I get the two ps are drawn differently.
> >> >> In (hc-append p1 p2) I was expecting a transformation (translation)
> on
> >> >> the brush.
> >> >>
> >> >> As a demonstration I offer the following program (a bit long, but
> >> >> hopefully clear.
> >> >>
> >> >> See a syntax-highligthed version here:
> http://pasterack.org/pastes/7953
> >> >> The DrRacket output is here: http://imgur.com/5BRiY0Z
> >> >>
> >> >> /Jens Axel
> >> >>
> >> >>
> >> >> #lang racket
> >> >> (require pict)
> >> >>
> >> >> ; debug : value pict -> pict
> >> >> ;  return a pict, that when drawn prints the
> >> >> ;  brush and drawing context transformation
> >> >> (define (debug who pict)
> >> >>   (dc (lambda (dc x y)
> >> >>         (define b   (send dc get-brush))
> >> >>         (define bt  (send b  get-transformation))
> >> >>         (define dct (send dc get-transformation))
> >> >>         (displayln (list who 'x x 'y y 'brush: bt 'dc: dct))
> >> >>         (draw-pict pict dc x y))
> >> >>       (pict-width pict) (pict-height pict)))
> >> >>
> >> >> (define r 20) ; use same box side for the entire example
> >> >>
> >> >> ; a black filled rectangle
> >> >> (define (rect) (filled-rectangle r r))
> >> >>
> >> >> ;;; Examine whether hc-append does any transformation.
> >> >> "Expected Image:          Two squares a black and a red"
> >> >> "Expected Transformation: Same for A and B. Some difference for C."
> >> >> (debug 'A
> >> >>        (hc-append (debug 'B (rect))
> >> >>                   (debug 'C (colorize (rect) "red" ))))
> >> >>
> >> >> ;;; --------------------------------------------------------------
> >> >> (require racket/draw)
> >> >>
> >> >> ; colors
> >> >> (define (color: name) (send the-color-database find-color name))
> >> >> (define red   (color: "red"))
> >> >> (define green (color: "green"))
> >> >> (define blue  (color: "blue"))
> >> >>
> >> >> ; square-path : real real real real -> path%
> >> >> ;     make square with side r and upper left corner (x,y)
> >> >> (define (square-path x y w h)
> >> >>   (define p (new dc-path%))
> >> >>   (send p move-to    x      y)
> >> >>   (send p line-to    x   (+ y h))
> >> >>   (send p line-to (+ x w)(+ y h))
> >> >>   (send p line-to (+ x w)(+ y 0))
> >> >>   (send p line-to (+ x 0)(+ y 0))
> >> >>   p)
> >> >>
> >> >> ; fill : pict -> pict
> >> >> ;   draw a path around pict using current pen and brush
> >> >> (define (fill pict)
> >> >>   (define w (pict-width pict))
> >> >>   (define h (pict-height pict))
> >> >>   (dc (lambda (dc x y)
> >> >>         (draw-pict pict dc x y)
> >> >>         (send dc draw-path (square-path x y w h)))
> >> >>       w h))
> >> >>
> >> >> ; shady : pict -> pict
> >> >> ;   Draws pict with a brush given by a linear, horizontal
> >> >> ;   gradient from (0,0) to (0,2r). The colors are red->green->blue.
> >> >> (define (shady pict)
> >> >>   (dc (lambda (dc x y)
> >> >>         ; get old brush
> >> >>         (define b (send dc get-brush))
> >> >>         ; make new brush, only change gradient
> >> >>         (define new-brush
> >> >>           (new brush%
> >> >>                [color          (send b get-color)]
> >> >>                [style          (send b get-style)]
> >> >>                [stipple        (send b get-stipple)]
> >> >>                [gradient       (new linear-gradient%
> >> >>                                 [x0 0] [y0 0] [x1 (* 2 r)] [y1 0] ;
> >> >> horizontal gradient
> >> >>                                 [stops (list (list 0   red)   ; (0,0)
> >> >> to ( r,0) red->green
> >> >>                                              (list 1/2 green) ; (r,0)
> >> >> to (2r,0) green->blue
> >> >>                                              (list 1   blue))])]
> >> >>                [transformation (send b get-transformation)]))
> >> >>         ; use new brush to draw the pict
> >> >>         (send dc set-brush new-brush)
> >> >>         (draw-pict pict dc x y)
> >> >>         ; restore old brush
> >> >>         (send dc set-brush b))
> >> >>       (pict-width pict) (pict-height pict)))
> >> >>
> >> >> (define-syntax (echo stx) (syntax-case stx () [(_ expr) #'(values
> 'expr
> >> >> expr)]))
> >> >>
> >> >> (newline) (newline)
> >> >> "Expected: A (black) rectangle"
> >> >> (echo (rect))
> >> >> "Expected: A rectangle filled with nothing (default brush is empty)"
> >> >> (echo (fill (rect)))
> >> >> "Expected: A rectangle filled with linear gradient (red to green)"
> >> >> (echo (shady (fill (rect))))
> >> >> "Expected: Two red-to-green rectangles"
> >> >> (echo (hc-append (shady (fill (rect))) (shady (fill (rect)))))
> >> >> "Expected: Two red-to-green rectangles"
> >> >> (echo (let () (define p (shady (fill (rect)))) (hc-append p p)))
> >> >>
> >> >>
> >> >>
> >> >> --
> >> >> Jens Axel Søgaard
> >> >>
> >> >> ____________________
> >> >>   Racket Users list:
> >> >>   http://lists.racket-lang.org/users
> >> >
> >> >
> >>
> >>
> >>
> >> --
> >> --
> >> Jens Axel Søgaard
> >
> >
>
>
>
> --
> --
> Jens Axel Søgaard
>
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