# How To?

This is a list of frequently ask questions answering basic "How to do X, Y, Z?" questions.

using Javis

function ground(args...)
background("black")
sethue("white")
end

video = Video(500, 500)
Background(1:100, ground)
# The provided snippets!

render(video; pathname="how_to.gif")

## How can I move a circle from A to B?

First of all you need to define an Object which draws a circle.

circ = Object(1:100, (args...)->circle(O, 50, :fill))

and then you need the a translation with anim_translate to move the circle.

act!(circ, Action(anim_translate(O, Point(100, 100))))

The circle then moves from the origin (center of frame) 100 px down and to the right.

## How can I define frames?

There are currently three different ways to define frames inside Javis. The simplest one is to define the UnitRange like 1:100 as above such that the action is called for every frame from 1 to 100.

Examples:

Object(1:100, (args...)->circle(O, 50, :fill))
Object(1:50, (args...)->circle(O, 70, :stroke))

It is relatively often the case that the following action should work with the same frames as the previous action this can be done with.

Examples:

Object(1:100, (args...)->circle(O, 50, :fill))
Object(:same, (args...)->circle(O, 20, :stroke), Point(100, 100))
Object((args...)->circle(O, 20, :stroke), Point(-100, 100))

so either use the symbol :same or just don't mention frames.

The last option is to define frames relative to the previous frame. More precisely the end of the last frame.

Examples:

Object(1:50, (args...)->circle(O, 50, :fill))
Object(RFrames(1:50), (args...)->circle(O, 20, :stroke), Point(100, 100))

This is the same as:

Object(1:50, (args...)->circle(O, 50, :fill))
Object(51:100, (args...)->circle(O, 20, :stroke), Point(100, 100))

## How can I make an object fade in from the background?

Let's make the standard circle we used before appear from the background.

circ = Object(1:100, (args...)->circle(O, 50, :fill))
act!(circ, Action(1:50, appear(:fade)))

this is using a change in opacity to show the circle.

There are two other options :scale and :fade_line_width. :scale also works for every kind of Object whereas :fade_line_width only works if you only draw the stroke instead of using fill.

Example:

circ = Object(1:100, (args...)->circle(O, 50, :stroke))
act!(circ, Action(1:50, appear(:fade_line_width)))

Additionally you can use all of these three options for the disappear functionality.

NOTE: An Action gets also called for frames after the last specified action frame such that disappeared objects stay disappeared. This can be turned off by using ; keep = false as an argument to the Action.

## How can I move one object based on another object?

In this case we need to define our own circ function which draws the circle and returns the center point of the circle.

function circ(point, radius, action)
return point
end

Now we define two actions:

1. Drawing a circle and saving the position my_circle
2. Drawing a rectangle above the circle
my_circle = Object(1:100, (args...)->circ(O, 50, :stroke))
act!(my_circle, Action(anim_translate(100, 100)))
Object(1:100, (args...)->rect(pos(my_circle)+Point(-10, -100), 20, 20, :fill))

In this animation the position of the circle is saved inside my_circle and can be used with pos(my_circle) inside the rect function.

## How can I show a text being drawn?

A text or latex rendering can appear as any other object with appear(:fade) and appear(:scale), However, it also has a special appear functionality called :draw_text.

You can use

my_text = Object(1:100, (args...) -> text("Hello World!"; halign = :center))
act!(my_text, Action(1:15, sineio(), appear(:draw_text)))
act!(my_text, Action(76:100, sineio(), disappear(:draw_text)))

to let the text "Hello World!" appear from left to right in an animated way.

## How can I have an object follow a path?

We need to create a path by providing a list of points that the object can follow. All objects that return a list of points can be used directly like star and poly for others a list of points must be provided as the input.

An action can look like this:

my_star = Object(1:100, (args...) -> star(O, 20, 5, 0.5, 0, :fill))
act!(my_star, Action(1:100, follow_path(star(O, 200))))

in this case a star is following the path of a bigger star.

NOTE: the star inside follow_path should have the action=:none which is the default for most Luxor functions.

NOTE: Unfortunately the above currently only works for some Luxor functions like ngon and star but not for circle and rect as they return true instead of the points.

In that case you need to define a function like:

function ground(args...)
background("white")
sethue("black")
end

function luxor2poly(func::Function)
newpath()
func()
closepath()
return pathtopoly()[1]
end

video = Video(600, 400)
Background(1:100, ground)
my_star = Object(1:100, (args...) -> star(O, 20, 5, 0.5, 0, :fill))
act!(my_star, Action(1:100, follow_path(luxor2poly(()->rect(O, 100, 100, :path)))))
render(video; pathname="follow_path.gif")

Another possibility is to specify a vector of points like this:

act!(
my_star,
Action(
1:100,
sineio(),
follow_path([Point(100, 100), Point(-20, -150), Point(-80, -10)]; closed = false),
),
)

In this case I want the star to follow a path consisting of two edges and I use ; closed=false to specify that it's just two edges and not a closed triangle.

An interesting possibility is to define paths using Bézier curves which can be defined with Luxor (see: Polygons to Bézier paths and back again)

One example of this can be seen in our example section

## How can I see a live view of the animation?

A live view of the animation can be useful for creating an animation where one doesn't need the overhead of building a gif or mp4 all the time. It also has the advantage that it's very easy to jump to a specific frame.

The live viewer can be called with adding ; liveview=true to the render call.

NOTE: If liveview=true the tempdirectory and pathname arguments are ignored and no file is created.