Some sparse questions

Daniel Kochmański daniel at
Sun Jan 19 22:01:30 UTC 2020

Hey Andrea,

> Follow some comments, in particular I think that what you wrote about
> transformations is wrong.

I'm pretty certain about my interpretation of how mediums behave (it is
reinforced by hours of tinkering with the console backend). As of sheets
and their transformations -- I think it that the topic is
underspecified; it might be that my understanding is lacking and/or
incorrect. IMO it is important to keep a separation between mediums and
sheets in mind. That is: medium is an entity on its own rights and the
sheet interface is built on top of it (that is when interpeting mediums
we should not into account sheet specification).

>> - with-output-to-pdf-stream
>> - with-output-to-ps-stream
>> - with-output-to-raster-image-stream
> Ok so your suggestion is to modify the actual WITH-OUTPUT-TO-PDF-STREAM
> macro without introduce new WITH-OUTPUT-TO-PDF-FILE macro, I will try to
> do it.

Preciesly. Thank you.

>>> 4) transformation and region machinery
>>> Is it the transformation and region machinery correct? Or it is works
>>> only in some standard situaions? For example I think that we could
>>> obtain a Zoom effect simply by: (setf (sheet-transformation
>>> some-sheet) (make-scaling-transformation 1.5 1.5)) but this doesn't
>>> work on McCLIM (I try it in the listener demo of clim-tos and there it
>>> works). Follow some more specific topics:
>> I did not investigate how the zooming effect could be conformingly
>> achieved, but I think that modifying the SHEET-TRANSFORMATION is not the
>> way to do that. If I were speculating I'd be more inclined to tinker
>> with the medium transformation. No guarantees it will work (even if it
>> is allowed by the spec which I'm not sure it is).
> Why not? If you change the medium-transformation instead of
> sheet-transformation you can not obtain zoom effect because in the
> replay of output-record the medium-transformation is not take in
> account. The Spec. (16.2) says for replay-output-record:
> "Displays the output captured by the output record record on the output
> recording stream stream, exactly as it was originally captured (subject
> to subsequent modifications). The current user transformation, line
> style, text style, ink, and clipping region of stream are all ignored
> during the replay operation.  Instead, these are gotten from the output
> record."
> The "user transformation" *is* the medium-transformation as stated in
> the Spec (10.1):
> "
> medium-transformation   medium                                   [Generic Function]
> The current user transformation for the medium medium. This
> transformation is used to transform the coordinates supplied as
> arguments to drawing functions to the coordinate system of the drawing
> plane. See Chapter 5 for a complete description of transformations. The
> :transformation drawing option temporarily changes the value of
> medium-transformation.
> "

10.1 talks about drawing on the sheet. medium-transformation transforms
coordinates supplied to the drawing function to coordinates of the
mirror on which we draw (putting aside the question that are device
coordinates or native coordinates). When drawing operation is performed
on the output recording stream, then medium-transformation is recorded
and then it is reinstationed when we replay the output record (so the
current transformation is indeed ignored).

    (stream :transformation *drawing-transformation*)
  (draw-rectangle* stream 0 0 10 10))

    (medium :transformation *ignored-transformation*)
  (replay stream))

That does not mean that replaying output records is done with the
medium-transformation set to +identity-transformation+. It means, that
the transformation from the *time of drawing* is used. So when we replay
the output record, medium-transformation is
*drawing-transformation*. Same goes for other recorded medium
components. What exactly is recorded depends on the output record class.

It is implemented as an around method on replay-output-record
specialized on gs-transformation-mixin. There is a catch though:
basic-medium eagerly transforms coordinates, so backends are not
expected to apply the medium-transformation to coordinates (unless their
medium does not inherit from the basic-medium[1]). That's why zooming is
not easy to achieve. That's also why hardware transformations would
require custom medium. I'm positive though that both could be
implemented (and zooming would be easier).

[1] "An :around on basic-medium for each of the drawing functions will
have already transformed the user coordinates to medium coordinates
before the most specific, implementation-dependent method is called." --
12.7.2 Medium-specific Drawing Functions.

>>> 4a) Setting sheet-native-transformation:
>>> McCLIM set directly the SHEET-NATIVE-TRANSFORMATION with
>>> %%SET-SHEET-NATIVE-TRANSFORMATION. I think this is not correct the
>>> sheet-native-transformation must be computated and only cached never
>>> set directly, otherwise you can lost the parent trasformation.
>> It is an internal interface which makes possible to update a native
>> transformation when we update the mirror geometry. You are right that it
>> should never be used to change the native transformation, it is only
>> used to update it when it changes (when we know what we are
>> doing). %% prefix means "dangerous".
> the sheet-native-transformation method already have the caching
> mechanism. I think that the only things to do when we update the mirror
> geometry is to invalidate-cached-transformations and
> invalidate-cached-regions. The next call of sheet-native-transformation
> will recompute it.

You are right that invalidating cache would be correct.
%%set-sheet-native-transformation is used as an optimization. i.e
docstring of update-mirror-geometry which uses it:

  "This function reflects the current sheet region and sheet
transformation to the mirror. It also sets up the native
transformation. This function is supposed to be called whenever one of
the following happens:

  - the sheet's transformation changed
  - the sheet's region changed
  - the parent's native transformation changed
  - the parent's transformation changed
  - the parent's mirror region changed

Also if the sheet's native transformation changes, the mirror's contents
need to be redrawn, which is achieved by calling

Since changing the sheet's native transformation might thus be
expensive, this function tries to minimize changes to it. (although it
does not try very hard)."

This writer method is also used in a macro which allows swapping mirrors
(which is an ugly hack w which should go!).

> I think this is not right respect to Spec and certainly is not what
> McCLIM does. Try yourself open a clim-listener move the window in some
> place of the screen and try: (sheet-native-tranformation
> (frame-top-level-sheet *application-frame*)) I always have the identity
> transformation as answer.

Thank you for elaborating on parts. You've certainly convinced me that
my rationale about transformations has holes, I'm not sure what is the
right behavior anymore. If you create an issue on github we may move he
discussion there. Before we move forward with code changes we need to
get a common understanding of which coordinate system does what. I need
to do some reading on my part (most notably 8, 10 and 12 chapters of the
spec), but currently we have some open PRs and I don't think I have
enough energy to jump at that now.

Best regards,

Daniel Kochmański ;; aka jackdaniel | Przemyśl, Poland
TurtleWare - Daniel Kochmański      |

"Be the change that you wish to see in the world." - Mahatma Gandhi

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