[racket] Fwd: Transformations and hc-append

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