Tracking issue: #2282

Changes

This is the second part of gradients, it includes the following changes:

• Adds the gradient type
• Adds the gradient.linear constructor for creating a gradient with the following args:
  • stops (variadic): either a color or a color and a ratio in an array
  • dir (defaults to ltr): the direction in which the ratio goes
  • space: the color space in which to interpolate the stops
  • relative: what is the gradient relative to (see later)
• Layout engine changes
  • Frame can now be FrameKind::Soft or FrameKind::Hard, this is information being kept of simply to determine what is the "container" of an element
  • block, box, columns, page, polygon, and frame produce hard frames, anything else produces a soft frame
  • hard frames are never inlined (we need to keep track of the transform)
• Adds the following method to the gradient type:
  • stops to get the color stops of the gradient
  • space to get the interpolation color space of the gradient
  • relative returns the "relative-ness" of a gradient (see later)
  • dir returns the angle of the gradient
  • kind returns the constructor used to create this gradient (currently only gradient.linear)
  • sample samples a color at a given ratio t
  • samples samples many colors (variadic) at a series of given ratios t
  • sharp turns the gradient into its sharp variant (see later)
  • repeat repeats the stops of the gradient n times, optionally mirror every other repetition
• Adds the following presets color maps (see later for an image with all of them): turbo, cividis, rainbow, spectral, viridis, inferno, magma, plasma, rocket, mako, vlag, icefire, flare, and crest. They can all be used (except rainbow) for data visualization.
• PDF/SVG/PNG export of gradients
• The following test:
  • aspect ratio correction (see later)
  • directions of gradients
  • all presets
  • relative argument
  • repeat method
  • sharp method
  • gradients are strokes
  • modified methods.typ to add tests to all methods in gradient
  • test that gradient on text (see later) gives an error
  • test that all transforms work well (tested because of a bug I had introduced)
• Adds a size + gradient operation for creating a stroke
• Gradient on text causes an error with a hint saying it will be available soon:tm: (see later)

Also adds the following niceties:

• Adds better code completion for colors (was missing the new color spaces)
• Adds a quadrant method on angle that allows getting the quadrant in which the angle is, it is used when computing gradients, I figured it was a nice method to expose to the user.
• Small SVG export file size improvements

Presets

Gradients on text

Gradients on text are currently disabled, they will be included in Part 4 of the gradient series of PRs. Currently, they give an error when someone tries to use them:

Relative-ness

Gradients can be relative to either the shape they are painted on, or to the nearest parent containers. This is controlled by the relative argument of the constructors. By default, gradients are relative to the shape they are painted on, unless they are painted on text, in which case they are relative to the parent.

The way the parent is determined is as follows:

• For shapes that are placed at the root/top level of the document, the parent is the page itself.
• For other shapes, the parent is the first block or box that contains the shape. This includes the boxes and blocks that are implicitly created by show rules. For example, a rotate will not affect the parent of a gradient, but a grid will.

The following code exemplifies this, if they were not in the same relative coordinate space for their fills, the result would not look uniform and continuous.

#set page(width: 100pt, height: 100pt, margin: 0pt)
#set block(spacing: 0pt)

#let fill = gradient.linear(..gradient.icefire, relative: "parent", dir: ttb)

#place(top + left, square(fill: fill, size: 50pt))
#place(center + horizon, rotate(45deg, square(fill: fill, size: 50pt)))
#place(bottom + right, square(fill: fill, size: 50pt))

Sharp gradient

Additionally, the user can sample the gradient to make a "sharp" variant (i.e with sharp color bars), this is especially useful for creating a list of colors from a color map. Below a result of such a sharp gradient can be seen along with it's non-sharp version:

Aspect ratio correction

Due to the way gradients are handled, they would normally ignore the aspect ratio of the shape and the dir of the gradient would become incorrect for gradients with a width to height ratio (aspect ratio) of anything but 1. This is handled in angle.rs with a small method. The reason why I mention it is because making this (imo necessary) correction leads to a larger file size as it decrease how well the exported gradient data can be re-used in PDF and SVG export.