Flipping

[lehitoskin]

I'm trying to take a pict I have and flip it horizontally/vertically, but in the docs the only thing I can find that's even kind of related is the rotate function. It appears there's currently no way to flip a pict without converting it to another format first. What I've ended up doing is converting it to a bitmap, then to a flomap, flipping it, converting it back to a bitmap, and then back to a pict. This process is quite slow and messy.

Could flipping a pict be added to the library? It would be very much appreciated.

[mflatt]

You can scale by a negative number to implement flipping, but you'll probably have to also use inset to fix the bounding box (which I think will go negative).

[lehitoskin]

If I use (scale pct -1), it spits out the same result as if I had used (rotate pct pi). I can't get it to flip properly, no matter what negative value I use.

[AlexKnauth]

You can use a negative width-factor with a positive height-factor to do this: (scale pct -1 1)

[rfindler]

Should we add a `flip-horizontal` and `flip-vertical` function to the library, since the implementation isn't obvious? Robby ​

[lehitoskin]

IMHO it would be immensely helpful to have those functions available. It's especially confusing when you're searching through the docs for flip-vertical and flip-horizontal, but they are provided from 2htdp/image instead of pict.

[LeifAndersen]

Agreed. I'm running into this right now. Especially because doing a (scale -1 1) ends up with a negative bounding box, so it ends up needing to be placed in a larger context.

[spdegabrielle]

is this still an issue?

If not please consider adding the label good first issue so it is findable:

Issues labeled good first issue in in Racket GitHub repositories

[rfindler]

I think the main clues are are here in the discussion, so I added the "good first issue" label. I had to create it, tho, so I may have done that wrong -- a sanity check would be welcome.

[soegaard]

Here is a starting point for horizontal flipping: #lang racket (require racket/match) (require pict) (define (compose-trans t1 t2) (match* (t1 t2) [((vector a d b e c f) (vector g j h k i l)) (vector (+ (* a g) (* b j)) (+ (* d g) (* e j)) (+ (* a h) (* b k)) (+ (* d h) (* e k)) (+ (* a i) (* b l) c) (+ (* d i) (* e l) f))])) (define (compose-trans* t . ts) (let loop ([t t] [ts ts]) (match ts ['() t] [(cons t0 ts) (loop (compose-trans t t0) ts)]))) (define (make-translate h k) (vector 1 0 0 1 h k)) (define (make-flip-x) ; around y-axis (vector -1 0 0 1 0 0)) (define (flip-x p) (define w (pict-width p)) (define h (pict-height p)) (dc (λ (dc x y) (let () (define w (pict-width p)) (define old-t (send dc get-initial-matrix)) (define new-t (compose-trans* (make-translate (+ (/ w 2)) 0) (make-flip-x) (make-translate (- (/ w 2)) 0))) (send dc set-initial-matrix new-t) (draw-pict p dc x y) (send dc set-initial-matrix old-t))) w h)) (standard-fish 100 50) (flip-x (standard-fish 100 50)) Den tor. 18. jun. 2020 kl. 22.57 skrev Robby Findler < notifications@github.com>:

…

I think the main clues are are here in the discussion, so I added the "good first issue" label. I had to create it, tho, so I may have done that wrong -- a sanity check would be welcome. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#29 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AADQXRJPEGIRNTNVV4I6Q6TRXJ5URANCNFSM4C6DXN4Q> .

-- -- Jens Axel Søgaard

[jbromley]

I'd like to take a shot at fixing this. I've only started using Racket in the last few months (mostly algorithms problems), but I do have a decent background in Common Lisp and Clojure. Is there anything I need to do other than fork the repo, clone it, and start digesting the code? Are there are Racket developer/contributor docs I should look at?

[rfindler]

There really should be a document somewhere about this (I think @bennn made one a while back?) but in the meantime, here is what I recommend:

git clone https://github.com/racket/racket.git
cd racket
make
# get coffee (but really pass no args;
# this makes a build that's contained in the 
# directory you just checked out)
mkdir extra-pkgs
cd extra-pkgs
# make sure `bin` from the racket checkout
# above is in your path or use absolute paths
raco pkg update --clone pict
# accept the defaults when prompted

At this point, you should be able to mess around in the pict directory and see the results when you run programs like the one above. You can also add another remote and push to it, in order to create an eventual pull request.

hth!

[jbromley]

Cool, thanks. That is a reasonable start. I'm assuming that once I've got something, I can make a merge request from a feature branch. I don't have to do the fork, modify, and make pull request workflow that other software uses? Thanks again! Jay

…

On Wed, May 19, 2021 at 3:43 PM Robby Findler ***@***.***> wrote: There really should be a document somewhere about this (I think @bennn <https://github.com/bennn> made one a while back?) but in the meantime, here is what I recommend: git clone https://github.com/racket/racket.git cd racket make # get coffee (but really pass no args; # this makes a build that's contained in the # directory you just checked out) mkdir extra-pkgs cd extra-pkgs # make sure `bin` from the racket checkout # above is in your path or use absolute paths raco pkg update --clone pict # accept the defaults when prompted At this point, you should be able to mess around in the pict directory and see the results when you run programs like the one above. You can also add another remote and push to it, in order to create an eventual pull request. hth! — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#29 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AABTBIWUGUBRYGCEL2KQZKTTOQ5IHANCNFSM4C6DXN4Q> .

[rfindler]

You're probably ahead of me here, but are you asking for write access to this repo?