.. _transforms:

==========
Transforms
==========

Transforms are used in order to turn a :doc:`displayable <displayables>` into
another displayable. There are several kinds of transforms, and various ways to
create them. The built-in transforms are used to control where an image is
placed on the screen, while user-defined transforms can cause more complex
effects, like motion, zoom, rotation, up to complex color effects.

Transforms can be applied to images by passing them to the ``at`` clause of the
:ref:`show <show-statement>` or scene statements. The following applies the
``right`` transform to the ``eileen happy`` image::

    show eileen happy at right

Multiple transforms can be applied by separating them with commas. These
transforms are applied from left-to-right. ::

    show eileen happy at halfsize, right


Applying transforms to displayables in Python
=============================================

There are several ways to apply transform ``t`` to displayable ``d`` in Python:

#. The most universal and most recommended way is ``At(d, t)`` (see below). It
   works with all transforms.

#. ``d(child=t)`` works with all :ref:`ATL transforms <atl>`.

#. ``t(d)`` works with all :ref:`Python transforms <transforms-python>`, as well
   as with ATL transforms that don't have any positional parameters.

.. include:: inc/disp_at

.. note::
    The resulting value may not be able to be displayed, if there remains
    parameters of the transform that have not been given a value, as can be the
    case with transforms defined using the :ref:`transform-statement`.

.. note::
    The resulting value may still be a transform that can be applied to yet
    another displayable (overriding its previous child) ; that's the case with
    ATL transforms which are still usable as transforms even when having their
    child set.


Built-in Transforms
===================

Ren'Py ships with a number of transforms defined by default. These transforms
position things on the screen. Here's a depiction of where each built-in
transform will position an image.

.. code-block:: none

                 +-----------------------------------------------------------+
                 |topleft, reset               top                   topright|
                 |                                                           |
                 |                                                           |
                 |                                                           |
                 |                                                           |
                 |                          truecenter                       |
                 |                                                           |
                 |                                                           |
                 |                                                           |
                 |                                                           |
    offscreenleft|left                   center, default                right|offscreenright
                 +-----------------------------------------------------------+

The :var:`offscreenleft` and :var:`offscreenright` transforms position images
off the edges of the screen. These transforms can be used to move things off
the screen (remember to hide them afterwards, to ensure that they do not consume
resources).

The transforms are:

.. var:: center

    Centers horizontally, and aligns to the bottom of the screen.

.. var:: default

    Centers horizontally, and aligns to the bottom of the screen. This can be
    redefined via :var:`config.default_transform` to change the default
    placement of images shown with the show or scene statements.

.. var:: left

    Aligns to the bottom-left corner of the screen.

.. var:: offscreenleft

    Places the displayable off the left side of the screen, aligned to the
    bottom of the screen.

.. var:: offscreenright

    Places the displayable off the right side of the screen, aligned to the
    bottom of the screen.

.. var:: reset

    Resets the transform to the default values of each property, removing any
    properites set before it.

.. var:: right

    Aligns to the bottom-right corner of the screen.

.. var:: top

    Centers horizontally, and aligns to the top of the screen.

.. var:: topleft

    Aligns to the top-left corner of the screen.

.. var:: topright

    Aligns to the top-right corner of the screen.

.. var:: truecenter

    Centers both horizontally and vertically.


.. _atl:

ATL - Animation and Transformation Language
===========================================

The Animation and Transformation Language (ATL) is a high-level language which
can create animations, move displayables across the screen, set their position,
apply transformations, and more. These can be changed over time, and in response
to events.

ATL transform objects, which are created using the :ref:`transform-statement`
down below, are displayables and can be used as such (even though they will be
transparent when their child displayable is not set) : they can be passed to a
screen's :ref:`sl-add` element, or to a :ref:`show-expression-statement`
statement, or to the :func:`renpy.show` function.

Ren'Py script statements
------------------------

There are three Ren'Py script statements which can include ATL code.

.. _transform-statement:

Transform Statement
~~~~~~~~~~~~~~~~~~~

The ``transform`` statement creates a new transform. The syntax is:

.. productionlist:: script
    atl_transform : "transform" `qualname` ( "(" `parameters` ")" )? ":"
                  :    `atl_block`

The transform statement is run at :ref:`init time <init-phase>`. The transform
may take a list of parameters, which works much the same way as a Python
function definition, except that several kinds of parameters are currently
forbidden, though they may be allowed in the future:

#. Positional-only parameters
#. Keyword-only parameters without a default value
#. Variadic positional parameters (``*args``)
#. Variadic keyword parameters (``**kwargs``)

..
    update the special child section with this section

The created object cannot be used as a transform until and unless all its
parameters have been given a value.

*See also :* :ref:`atl-partial`

`qualname`, the name of the transform, must be a set of dot-separated Python
identifiers. The transform created by the ATL block will be bound to that name,
within the provided :ref:`store <named-stores>` if one was provided. ::

    transform left_to_right:
        xalign 0.
        linear 2 xalign 1.
        repeat

    transform ariana.left:
        xalign .3

    transform animated_ariana_disp:
        "ariana"
        pause 1.
        "ariana_reverse"
        pause 1.
        repeat

The created object is both a transform and a displayable, but as opposed to the
``image`` statement, it is created as a variable (or a constant), rather than in
the namespace of :ref:`images <defining-images>`.

.. _atl-image-statement:

Image Statement with ATL Block
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The second way to include ATL code in a script is as part of an :ref:`image
statement <image-statement>`. As its inline counterpart, it binds an image name
(which may contain spaces) to the given transform. As there is no way to supply
with parameters, it's only useful if the transform defines an animation. The
syntax is:

.. productionlist:: script
    atl_image : "image" `image_name` ":"
              :    `atl_block`

::

    image animated_ariana_img:
        "ariana"
        pause 1.
        "ariana_reverse"
        pause 1.
        repeat

Scene and Show Statements with ATL Block
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The final way to use ATL is as part of a :ref:`show <show-statement>` or scene
statement. This wraps the image that's being shown inside an ATL transformation
which is created on the fly and applied to the image. The syntax is:

.. productionlist:: script
    atl_show  : `stmt_show` ":"
              :    `atl_block`
    atl_scene : `stmt_scene` ":"
              :    `atl_block`

::

    show eileen happy:
        xalign 1.

    scene bg washington:
        zoom 2.

..
    include here, after #4405, how to change the child then get back to it

ATL Syntax and Statements
-------------------------

ATL statements may be inline, or make up a block within the ATL block in which
it is written. With some exceptions described in the relevant statements, the
statements in an ATL block are executed in order, from top to bottom.

If an ATL statement requires an expression to be evaluated, such evaluation
occurs when the transform is first executed (that is when using a ``show``
statement, or displaying the transform as part of a screen), and not when the
particular ATL statement is executed.

The following are the ATL statements.

.. _inline-contains-atl-statement:

Inline Contains Statement
~~~~~~~~~~~~~~~~~~~~~~~~~

The inline contains statement takes a single expression evaluating to a
:doc:`displayable <displayables>`.

.. productionlist:: atl
    atl_contains : "contains" `simple_expression`

This statement sets (or replaces) the child of the current ATL transform to the
value of the expression, making it useful for animation. ::

    transform an_animation:
        "1.png"
        pause 2
        "2.png"
        pause 2
        repeat

    image move_an_animation:
        contains an_animation

        # If we didn't use contains, we'd still be looping
        # and would never reach here.
        xalign 0.0
        linear 1.0 yalign 1.0

The :ref:`displayable-atl-statement` is less explicit and bears ambiguity with
the transform expression statement, but it allows for a
:doc:`transition <transitions>` to be used for replacing the child. This
statement can be particularly useful when an ATL transform wishes to contain,
rather than include, a second ATL transform.

Number Statement
~~~~~~~~~~~~~~~~

The number statement consists of a simple expression evaluating to an integer or
floating-point number. It can optionally be preceded by the keyword "pause".

.. productionlist:: atl
    atl_number : "pause"? `simple_expression`

It is used as a number of seconds to pause execution for. ::

    image atl example:
        # Displays logo_base.png
        contains "logo_base.png"

        # Pause for 1.0 seconds.
        pause 1.0

        # Show logo_bw.png, with a dissolve.
        "logo_bw.png" with Dissolve(0.5, alpha=True)

        # Pause for 3 seconds
        3

        repeat

Properties Statement
~~~~~~~~~~~~~~~~~~~~

This statement sets one or more transform properties to a new value.

.. productionlist:: atl
    atl_properties : `atl_property`+

.. productionlist:: atl
    atl_property : `transform_property` `simple_expression`

The statement first gives a series (at least one) of property names, each
followed by the new value to set it to. See :ref:`transform-properties` for a
list of transform properties, their meaning and the values they take. ::

    transform rightoid:
        xalign .9

    transform ariana.left:
        xanchor .3 xpos 100

Interpolation Statement
~~~~~~~~~~~~~~~~~~~~~~~

The interpolation statement is the main way of getting smoothly animated
transformations.

.. productionlist:: atl
    atl_interp : ((`warper` `simple_expression`) | ("warp" `simple_expression` `simple_expression`)) (`atl_interp_target`+ | (":"
               :    `atl_interp_target`+ ))

.. productionlist:: atl
    atl_interp_target : (`atl_property`+ ("knot" `simple_expression`)* )
                      : | `atl_transform_expression`
                      : | "clockwise"
                      : | "counterclockwise"
                      : | ("circles" `simple_expression`)

Some sample interpolations::

    show logo base:
        # Show the logo at the upper right side of the screen.
        xalign 1.0 yalign 0.0

        # Take 1.0 seconds to move things back to the left.
        linear 1.0 xalign 0.0

        # Take 1.0 seconds to move things to the location specified in the
        # truecenter transform. Use the ease warper to do this.
        ease 1.0 truecenter

        # Set the location to circle around.
        anchor (0.5, 0.5)

        # Use circular motion to bring us to spiral out to the top of
        # the screen. Take 2 seconds to do so.
        linear 2.0 yalign 0.0 clockwise circles 3

        # Use a spline motion to move us around the screen.
        linear 2.0 align (0.5, 1.0) knot (0.0, .33) knot (1.0, .66)

        # Changes xalign and yalign at the same time.
        linear 2.0 xalign 1.0 yalign 1.0

        # The same thing, using a block.
        linear 2.0:
            xalign 1.0
            yalign 1.0

The first part of the interpolation is used to select a function that time-warps
the interpolation. That means, a function that maps linear time to non-linear
time, see :ref:`warpers` for more information about that. Selecting a warper can
either be done by giving the name of a registered warper, or by giving the
keyword "warp" followed by an expression giving a warping function.

In either case, it's followed by a number giving the number of seconds the interpolation should take. ::

    transform builtin_warper:
        xpos 0
        ease 5 xpos 520

    init python:
        def my_warper(t):
            return t**4.4

    define my_warpers = [my_warper]

    transform accessed_as_function:
        xpos 0
        warp my_warpers[0] 5 xpos 520
        warp my_warper 3 xpos 100

The interpolation will persist for the given amount of time, and at least one
frame.

When :doc:`transform_properties` are given, the value each is given is the value
it will be set to at the end of the interpolation statement. This can be tweaked
in several ways:

* If the value is followed by one or more knots, then spline motion is used. The
  starting point is the value of the property at the start of the interpolation,
  the end point is the given value, and the knots are used to control the
  spline. A quadratic curve is used for a single knot, Bezier is used when there
  are two and Catmull-Rom is used for three or more knots. In the former two
  cases, the knot or knots are simply control nodes. For Catmull-Rom, the first
  and last knot are control nodes (often outside the displayed path) and the
  other knots are points the path passes through.

* If the interpolation statement contains a "clockwise" or "counterclockwise"
  clause, circular motion is used. In that case, Ren'Py will compare the start
  and end locations (which are set by :tpref:`pos`, :tpref:`align`,
  :tpref:`angle` and :tpref:`radius`, ...) and figure out the polar coordinate
  center (which is :tpref:`around`). Ren'Py will then compute the number of
  degrees it will take to go from the start angle to the end angle, in the
  specified direction of rotation. If the circles clause is given, Ren'Py will
  ensure that the appropriate number of circles will be made.

* Otherwise, the value is linearly interpolated between the start and end
  locations.

It is also possible to interpolate a :ref:`transform-expression-atl-statement`,
which should in this case be an ATL transform containing only a single
properties statement. The properties from the transform will be processed as if
they were written directly in this interpolation.

A warper may be followed by a colon (:). In that case, it may be followed by one
or more lines, in an indented block, containing the clauses described above.
This lets you break an interpolation of many different things up into several
lines.

Pass Statement
~~~~~~~~~~~~~~

.. productionlist:: atl
    atl_pass : "pass"

The ``pass`` statement is a simple statement that causes nothing to happen : a
no-op. This can be used when there's a desire to separate statements, like when
two sets of choice statements (see below) would otherwise be back-to-back. It
can also be useful when the syntax requires a block to be created but you need
it to be empty, for example to make one of the choice blocks not do anything.

Repeat Statement
~~~~~~~~~~~~~~~~

The ``repeat`` statement is a simple statement that causes the block containing
it to resume execution from the beginning.

.. productionlist:: atl
    atl_repeat : "repeat" (`simple_expression`)?

If the expression is present, then it is evaluated to give an integer number of
times the block will execute. (So a block ending with ``repeat 2`` will execute
at most twice in total, and ``repeat 1`` does not repeat.)

The repeat statement must be the last statement in a block::

    show logo base:
        xalign 0.0
        linear 1.0 xalign 1.0
        linear 1.0 xalign 0.0
        repeat

Block Statement
~~~~~~~~~~~~~~~

The ``block`` statement simply contains a block of ATL statements.

.. productionlist:: atl
    atl_block_stmt : "block" ":"
                   :      `atl_block`

This can be used to group statements that will repeat::

    show logo base:
        alpha 0.0 xalign 0.0 yalign 0.0
        linear 1.0 alpha 1.0

        block:
            linear 1.0 xalign 1.0
            linear 1.0 xalign 0.0
            repeat

Parallel Statement
~~~~~~~~~~~~~~~~~~

The ``parallel`` statement defines a set of ATL blocks to execute in parallel.

.. productionlist:: atl
    atl_parallel : ("parallel" ":"
                 :    `atl_block`)+

Parallel statements are greedily grouped into a parallel set when more than
one parallel block appears consecutively in a block. The set of all parallel
blocks are then executed simultaneously. The parallel statement terminates when
the last block terminates.

The blocks within a set should be independent of each other, and manipulate
different :doc:`transform_properties`. When two blocks change the same property,
the result is undefined. ::

    show logo base:
        parallel:
            xalign 0.0
            linear 1.3 xalign 1.0
            linear 1.3 xalign 0.0
            repeat
        parallel:
            yalign 0.0
            linear 1.6 yalign 1.0
            linear 1.6 yalign 0.0
            repeat

Choice Statement
~~~~~~~~~~~~~~~~

The ``choice`` statement defines one of a set of
potential choices. Ren'Py will pick one of the choices in the set, and
execute the ATL block associated with it, and then continue execution after
the last choice in the choice set.

.. productionlist:: atl
   atl_choice : ("choice" (`simple_expression`)? ":"
              :     `atl_block`)+

Choice statements are greedily grouped into a choice set when more than one
choice statement appears consecutively in a block. If the `simple_expression`
is supplied, it is a floating-point weight given to that block, otherwise 1.0
is assumed. ::

    image eileen random:
        choice:
            "eileen happy"
        choice:
            "eileen vhappy"
        choice:
            "eileen concerned"

        pause 1.0
        repeat

The ``pass`` statement can be useful in order to break several sets of choice
blocks into several choice statements, or to make an empty choice block.

Animation Statement
~~~~~~~~~~~~~~~~~~~

The ``animation`` statement must be the first statement in an ATL block, and
tells Ren'Py that the block uses the animation timebase.

.. productionlist:: atl
    atl_animation : "animation"

As compared to the normal showing timebase, the animation timebase starts when
an image or screen with the same tag is shown. This is generally used to have
one image replaced by a second one at the same apparent time. For example::

    image eileen happy moving:
        animation
        "eileen happy"
        xalign 0.0
        linear 5.0 xalign 1.0
        repeat

    image eileen vhappy moving:
        animation
        "eileen vhappy"
        xalign 0.0
        linear 5.0 xalign 1.0
        repeat

    label start:
        show eileen happy moving
        pause
        show eileen vhappy moving
        pause

This example will cause Eileen to change expression when the first pause
finishes, but will not cause her position to change, as both animations share
the same animation time, and hence will place her sprite in the same place.
Without the animation statement, the position would reset when the player
clicks.

On Statement
~~~~~~~~~~~~

The ``on`` statement defines an event handler.

.. productionlist:: atl
   atl_on : "on" `name` ("," `name`)* ":"
          :      `atl_block`

``on`` blocks are greedily grouped into a single statement. On statement can
handle a single event name, or a comma-separated list of event names.

This statement is used to handle events. When an event is handled, handling of
any other event ends and handing of the new event immediately starts. When an
event handler ends without another event occurring, the ``default`` event is
produced (unless the ``default`` event is already being handled).

Execution of the on statement will never naturally end. (But it can be ended
by the time statement, or an enclosing event handler.)

See the event statement for a way to produce arbitrary events, and see
:ref:`external-atl-events` for a list of naturally-produced events. ::

    show logo base:
        on show:
            alpha 0.0
            linear .5 alpha 1.0
        on hide:
            linear .5 alpha 0.0

    transform pulse_button:
        on hover, idle:
            linear .25 zoom 1.25
            linear .25 zoom 1.0

.. _transform-expression-atl-statement:

Transform Expression Statement
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This statement includes another ATL transform as part of the current ATL block.

.. productionlist:: atl
    atl_transform_expression : `simple_expression`

This only applies if the ATL transform has **not** been supplied a child (see
the top of the page for how to do that), otherwise it will be interpreted as a
:ref:`displayable-atl-statement`. The contents of the provided ATL transform
are included at the location of this statement. ::

    transform move_right:
        linear 1.0 xalign 1.0

    image atl example:
        # Display logo_base.png
        "logo_base.png"

        # Run the move_right transform.
        move_right

.. _displayable-atl-statement:

Displayable Statement
~~~~~~~~~~~~~~~~~~~~~

The displayable statement consists of a simple Python expression evaluating to
a :doc:`displayable <displayables>`, optionally followed by a with clause
containing a second simple expression.

.. productionlist:: atl
    atl_displayable : `simple_expression` ("with" `simple_expression`)?

It is used to set or replace the child of the transform when the statement
executes.

If a ``with`` clause is present, the second expression is evaluated as a
:doc:`transition <transitions>`, and the transition is applied between the old
child and the new child. Be careful in that not all transitions will work in
this situation, notably :ref:`dict-transitions` and :var:`move- <move>` and
:var:`ease- <ease>` transitions. ::

    image atl example:
        # Displays logo_base.png
        "logo_base.png"

        # Pause for 1.0 seconds.
        1.0

        # Show logo_bw.png, with a dissolve.
        "logo_bw.png" with Dissolve(0.5, alpha=True)

.. warning::

    If passing any child-less transform is pointless as it will make the
    transform transparent and ineffective, passing child-less ATL transforms may
    be interpreted as a :ref:`transform-expression-atl-statement`, which will
    yield different results.

If the expression evaluates to an ATL transform **with** a child, the execution
of this ATL block will only continue after the includee's ATL code runs.

Contains Block Statement
~~~~~~~~~~~~~~~~~~~~~~~~

The contains block, like its
:ref:`inline counterpart <inline-contains-atl-statement>`, sets the child of the
transform but in a different way.

.. productionlist:: atl
    atl_counts : "contains" ":"
               :    `atl_block`

One or more contains blocks will be greedily grouped together inside a single
contains statement, wrapped inside a :func:`Fixed`, and set as the child of the
transform.

Each block should define a displayable to use, otherwise an error will occur.
The contains statement executes instantaneously, without waiting for the
children to complete. ::

    image test double:
        contains:
            "logo.png"
            xalign 0.0
            linear 1.0 xalign 1.0
            repeat

        contains:
            "logo.png"
            xalign 1.0
            linear 1.0 xalign 0.0
            repeat

Function Statement
~~~~~~~~~~~~~~~~~~

The ``function`` statement allows ATL to use Python code.

.. productionlist:: atl
    atl_function : "function" `simple_expression`

The functions have the same signature as those used with :func:`Transform`:

* The first argument is a transform object. :doc:`transform_properties` can be
  set as attributes on this object.

* The second argument is the shown timebase, the number of seconds since the
  function began executing.

* The third argument is the animation timebase, which is the number of
  seconds something with the same tag has been on the screen.

* If the function returns a number, it will be called again after that number of
  seconds has elapsed. (0 seconds means to call the function as soon as
  possible.) If the function returns None, control will pass to the next ATL
  statement.

This function should not have side effects other than changing the transform
object in the first argument, and may be called at any time with any value as
part of prediction.

Note that ``function`` is not a transform property, and that it doesn't have the
exact same behavior as :func:`Transform`\ 's `function` parameter. ::

    init python:
        def slide_vibrate(trans, st, at, /):
            if st > 1.0:
                trans.xalign = 1.0
                trans.yoffset = 0
                return None
            else:
                trans.xalign = st
                trans.yoffset = random.randrange(-10, 11)
                return 0

    label start:
        show logo base:
            function slide_vibrate
            pause 1.0
            repeat

Time Statement
~~~~~~~~~~~~~~

The ``time`` statement is a control statement.

.. productionlist:: atl
    atl_time : "time" `simple_expression`

It contains a single expression, which is evaluated to give a time expressed as
seconds from the start of execution of the containing block. When the time given
in the statement is reached, the following statement begins to execute. This
transfer of control occurs even if a previous statement is still executing, and
causes any such prior statement to immediately terminate.

Time statements are implicitly preceded by a pause statement with an infinite
time. This means that if control would otherwise reach the time statement, it
waits until the time statement would take control.

When there are multiple time statements in a block, they must strictly
increase in order. ::

    image backgrounds:
        "bg band"
        xoffset 0
        block:
            linear 1 xoffset 10
            linear 1 xoffset 0
            repeat # control would never exit this block

        time 2.0
        xoffset 0
        "bg whitehouse"

        time 4.0
        "bg washington"

Event Statement
~~~~~~~~~~~~~~~

The ``event`` statement is a simple statement that causes an event with the
given name to be produced.

.. productionlist:: atl
    atl_event : "event" `name`

When an event is produced inside a block, the block is checked to see if an
event handler for the given name exists. If it does, control is transferred
to the event handler. Otherwise, the event propagates to any containing event
handler.


.. _external-atl-events:

External events
---------------

The following events are triggered automatically within an ATL transform:

``start``
    A pseudo-event, triggered on entering an ``on`` statement, if no event of
    higher priority has happened.

``show``
    Triggered when the transform is shown using the show or scene statement, and
    no image with the given tag exists.

``replace``
    Triggered when transform is shown using the ``show`` statement, replacing an
    image with the given tag.

``hide``
    Triggered when the transform is hidden using the ``hide`` statement or its
    Python equivalent.

    Note that this isn't triggered when the transform is eliminated via the
    :ref:`scene-statement` or exiting the :ref:`context` it exists in, such as
    when exiting the game menu.

``replaced``
    Triggered when the transform is replaced by another. The image will not
    actually hide until the ATL block finishes.

``hover``, ``idle``, ``selected_hover``, ``selected_idle``, ``insensitive``, ``selected_insensitive``
    Triggered when a button containing this transform, or a button contained by
    this transform, enters the named state.

.. _atl-partial:

ATL curry and partial parameter passing
---------------------------------------

An ATL transform defined using the :ref:`transform-statement` can have its
parameters set at different times. When calling an ATL transform like a
function, the resulting value is still a transform, and the parameters that were
passed a value are treated as though the value is the new default value of the
parameter.

For example::

    transform screamer(child, screamee, wait_time=2, flash_time=.1):
        child
        pause wait_time
        screamee
        pause flash_time
        child

    # doing this doesn't raise an error (it would if it were a Python function)
    define shorter_screamer = screamer(wait_time=1)

    define eileen_excited_screamer = screamer(screamee="eileen excited", flash_time=.2)

    label start:
        show hhannahh happy at screamer(screamee="hhannahh surprised", wait_time=1.5)
        "Here is one way"

        show eileen vhappy at eileen_excited_screamer
        "Here is another"

        show patricia sad at eileen_excited_screamer(screamee="patricia wow")
        "And you can also do this"

Note that the ``shorter_screamer`` transform, just as the ``screamer``
transform, cannot be used directly like ``show eileen at screamer``, since their
``screamee`` parameters do not have a value.

Note also that, like labels and screens, the default values of the parameters of
a transform directly created by the :ref:`transform-statement` will be evaluated
at the time when the transform is *called*, not at the time when it is
*defined*.

However, the transform resulting from calling another transform (such as
``shorter_screamer`` in the example above, or also the transform applied to
patricia) has all the default values of its parameters already evaluated,
whether they come from the evaluation of the default values in the original
transform (such as ``shorter_screamer``\ 's `flash_time` parameter, or
patricia's transform's `wait_time` parameter), or from values passed to it in a
call earlier in the line (such as ``shorter_screamer``\ 's `wait_time`
parameter, or patricia's transform's `screamee` and `flash_time` parameters).

.. _atl-child-param:

Special Child Parameter
-----------------------

If an ATL transform has a parameter named "child" and that parameter receives a
value, **regardless of the kind of parameter or the way it receives a value**
(by a positional argument or by keyword, and even if the parameter is
positional-only or keyword-only, and defaulted or required), then in parallel
the child of the transform is set to the value of the parameter.

..
    address the special case of **kwargs if and when allowed

Note that the default value of the parameter doesn't count, the parameter has to
receive a value from the outside.

Conversely, when that ATL transform is used as a transform, the ``child=``
keyword argument will be passed, and so in addition to setting the child, if a
parameter is there to receive it (excluding positional-only parameters, since it
is passed by keyword), it will have the child's value when the transform
executes.

For example, this enables to swap between the supplied child and another
displayable::

    transform lucy_jump_scare(child):
        # the child is implicitly set as the child of the transform
        pause 5

        # Jump scare
        "lucy mad"
        pause .2

        # Go back to the original child
        child

It can also be used to place the original child inside a ``contains`` block::

    transform marquee(width, height=1.0, duration=2.0, child=None):
        xcenter 0.5
        ycenter 0.5

        crop (0, 0, 0.5, 500)

        contains:
            child
            xanchor 0.0 xpos 1.0
            linear duration xanchor 1.0 xpos 0.0

The `old_widget` and `new_widget` keyword-able parameters (meaning that they
should not be positional-only) have a special use as part of
:ref:`atl-transitions`.


.. _warpers:

Warpers
=======

A warper is a function that can change the amount of time an interpolation
statement considers to have elapsed. They are defined as functions from t to t',
where t and t' are floating point numbers, with t ranging from 0.0 to 1.0 over
the given amount of time. (If the statement has 0 duration, then t is 1.0 when
it runs.) t' should start at 0.0 and end at 1.0, but can be greater or less. The
following warpers are defined by default.

``pause``
    Pause, then jump to the new value. If ``t == 1.0``, ``t' = 1.0``. Otherwise,
    ``t' = 0.0``.

``linear``
    Linear interpolation. ``t' = t``

``ease``
    Start slow, speed up, then slow down. ``t' = .5 - math.cos(math.pi * t) / 2.0``

``easein``
    Start fast, then slow down. ``t' = math.cos((1.0 - t) * math.pi / 2.0)``

``easeout``
    Start slow, then speed up. ``t' = 1.0 - math.cos(t * math.pi / 2.0)``

In addition, most of Robert Penner's easing functions are supported. To
make the names match those above, the functions have been renamed
somewhat. Graphs of these standard functions can be found at
https://easings.net/.

.. include:: inc/easings

These warpers can be accessed in the ``_warper`` read-only module, which contains
the functions listed above. It is useful for things in Ren'Py which take a
time-warping function, such as :func:`Dissolve`, which you can use like::

    with Dissolve(1, time_warp=_warper.easein_quad)

New warpers can be defined using the ``renpy.atl_warper`` decorator, in a ``python
early`` block. It should be placed in a file that is parsed before any file
that uses the warper. This looks like::

    python early hide:

        @renpy.atl_warper
        def linear(t):
            return t


.. _replacing-transforms:

Replacing Transforms
====================

When an ATL transform, a built-in transform or a transform defined using the
:class:`Transform` class is replaced by another transform of these categories,
the properties of the outgoing transform are inherited by the incoming
transform. That inheritance doesn't apply for other kinds of transforms.

When the :ref:`show statement <show-statement>` has multiple transforms in the
``at`` list, the transforms are matched from last to first, until one list runs
out. For example::

    show eileen happy at a, b, c
    "Dialogue !"
    show eileen happy at d, e

The ``c`` transform will be replaced by ``e``, the ``b`` transform will be
replaced by ``d``, and nothing replaces the ``a`` transform.

At the moment of replacement, if both transforms are of suitable kinds, the
values of the properties of the old transform are copied to the new transform.
If the old transform was animated, the current intermediate value is inherited.
For example::

    transform bounce:
        linear 3.0 xalign 1.0
        linear 3.0 xalign 0.0
        repeat

    transform headright:
        linear 15 xalign 1.0

    label example:
        show eileen happy at bounce
        pause
        show eileen happy at headright
        pause

In this example, the image will bounce from left to right and back until the
player clicks. When that happens, the ``xalign`` property of the ``bounce``
transform will be used to initialize the ``xalign`` property of the
``headright`` transform, and so the image will move from where it was when the
player first clicked.

The position properties (:tpref:`xpos`, :tpref:`ypos`, :tpref:`xanchor`,
:tpref:`yanchor`, and properties setting them such as :tpref:`xalign` or
:tpref:`radius`\ / :tpref:`angle`) have a special rule for inheritance : a value
set in the child will override a value set in the parent. That is because a
displayable may have only one position, and a position that is actively set
takes precedence.

Finally, when a ``show`` statement does not include an ``at`` clause, the same
transforms are used, so no inheritance is necessary. To reset all transform
properties, hide and then show the displayable again. To break the animations
applied to a displayable (but keep the position), you can use::

    show eileen happy at some_animation
    "Wow, so this is what antigravity feels like !"

    show eileen:
        pass
    "But I'm happy when it settles down."


The Transform Class
===================

One equivalent to to the simplest ATL transforms is the Transform class.

.. class:: Transform(child=None, *, function=None, reset=False, **properties)

    Creates a transform which applies operations such as cropping, rotation,
    scaling or alpha-blending to its child. A transform object has fields
    corresponding to the :doc:`transform properties <transform_properties>`,
    which it applies to its child.

    `child`
        The child the transform applies to.

    `reset`
        If True, the transform will reset properties to their default values
        when it is shown, rather than inheriting those properties from the
        transforms it replaces.

    .. function:: function(trans: Transform, st: float, at: float, /) -> int|None

        If not None, this function will be called when the transform is
        rendered, with three positional arguments:

        * The transform object.
        * The shown timebase, in seconds.
        * The animation timebase, in seconds.

        The function should return a delay, in seconds, after which it will be
        called again, or None to be called again at the start of the next
        interaction.

        This function should not have side effects other than changing the
        Transform object in the first argument, and may be called at any time
        with any value as a part of prediction.

    Additional keyword arguments are values that transform properties are set
    to. These particular transform properties will be set each time the
    transform is drawn, and so may not be changed after the Transform object is
    created. Fields corresponding to other transform properties, however, can be
    set and changed afterwards, either within the function passed as the
    `function` parameter, or immediately before calling the :meth:`update`
    method.

    .. attribute:: hide_request

        This attribute is set to true when the function is called, to indicate
        that the transform is being hidden.

    .. attribute:: hide_response

        If ``hide_request`` is true, this can be set to false to prevent the
        transform from being hidden.

    .. attribute:: original_child

        This is the child Displayable that was set when the transform was created,
        before any calls to set_child. This may be None if no child was during creation.

    .. method:: set_child(child)

        Call this method with a new `child` to change the child of this
        transform.

    .. method:: update()

        This should be called when a transform property field is updated outside
        of the function passed as the `function` argument, to ensure that the
        change takes effect.

.. _transforms-python:

Callables as transforms
=======================

Finally, simple Python callables can be used as transforms. These callables
should take a single :doc:`displayable <displayables>` as an argument, and
return a new Displayable. For example::

    init python:

        # this transform uses the right and left transforms
        def right_or_left(d):
            if switch:
                return At(d, right)
            else:
                return At(d, left)

That means that certain builtins which take a displayable and return a displayable,
such as :func:`Flatten`, are also transforms and can be used as such.
