feat: add cover color-changing-cover

[enklht]

This pull request introduces a new (pseudo) semitransparent cover function that provides a method for hiding content in slides. The function changes the text color of elements to a specified color if they contain text. If an element does not contain text, it can use hide as a fallback.

This implementation has been successfully tested with the default examples and the specific example provided in issue #112.

[OrangeX4]

Can we achieve translucency by modifying the alpha value instead of fixing a color? We can think about the hacky code that I have as proof of concept below

#set page(width: 20em, height: auto)

#let update-alpha(constructor: rgb, color, alpha) = constructor(..color.components(alpha: true).slice(0, -1), alpha)

#show par: it => {
  if repr(text.fill).split("(").at(0) == "rgb" {
    set text(fill: update-alpha(text.fill, 50%))
    it
  } else {
    it
  }
}

// ----------------
// content
// ----------------


#set text(rgb("#000000"))

Test

#set text(rgb("#ff0000"))

Test

[enklht]

What do you think about this approach?

The transparentize method should handle most of the heavy lifting, which means we may not need to rely on many "hacks."

[OrangeX4]

The table and grid types also have children, I'm not sure if this should be included?

[enklht]

I understand. It might be good to have an option to toggle the behaviour, or exclude those elements (using util.is-sequence?)

[OrangeX4]

Since the table has borders, we should use util.is-sequence as default, but it can indeed be made into a configuration.

[enklht]

Quick Update: I discovered that the new alpha-changing-cover doesn't function perfectly when the text color is modified within the paused items.

see https://typst.app/project/wL4tuMqQ3FQArqLDmRrDj0

[enklht]

Update: The enhanced function now provides improved handling of color-changing texts. This is achieved by utilizing the context feature and the update-alpha function. This works well with the example I made the other day.

Note

The decision to use update-alpha instead of transparentize was deliberate. Unlike transparentize, which applies relative changes that can accumulate to cause nested texts to vanish, update-alpha allows to specify absolute text opacities. This approach ensures consistent visibility for nested text elements.

[enklht]

While this is slightly tangential to the current PR, I'd like to propose an improvement to the update-alpha function:

#let update-alpha(color, alpha) = color.opacify(100%).transparentize(alpha)

This revised implementation can handle all color constructors uniformly. It first ensures full opacity with opacify(100%), then applies the desired transparency using transparentize(alpha).

[OrangeX4]

If update-alpha is used, we need to consider non-rgb cases, like #set text(luma(80%)).

[enklht]

My latest proposal addresses that issue!

[OrangeX4]

Thanks!