Implement visible characters enter/exit FX in RichTextLabel

[jordigcs]

Resolves godotengine/godot-proposals#9137

Adds visibility_transition property to RichTextLabel, which takes a RichTextTransition resource.
This resource functions similarly to RichTextEffect, but instead of _process_custom_fx, there are 2 separate funcs for enter or exit fx.

Example

@tool
extends RichTextTransition

@export var speed:float = 2.5;

func _process_enter_fx(char_fx: CharFXTransform) -> bool:
	char_fx.color.a = lerp(0.0, 1.0, char_fx.elapsed_time * speed);
	char_fx.offset.y = lerp(-15.0, 0.0, char_fx.elapsed_time * speed);
	return char_fx.color.a >= 1;

func _process_exit_fx(char_fx: CharFXTransform) -> bool:
	char_fx.color.a = lerp(1.0, 0.0, char_fx.elapsed_time * speed);
	char_fx.offset.y = lerp(0.0, 15.0, char_fx.elapsed_time * speed);
	return char_fx.color.a <= 0;

Implementation Details

When a visibility_transition is set, making changes to visible_characters or visible_ratio will call _update_characters_to_transition, which will calculate the characters removed or added, and map a CharacterToTransition struct to the character. The CharacterToTransition struct contains data on if the character is entering or exiting, along with the transition's time elapsed. Before FX is processed, the RichTextLabel will check if the character currently being drawn is in the characters_to_transition map, and use the CharacterToTransition struct to call either _process_enter_fx or _process_exit_fx on the assigned RichTextTransition. If either of these methods return true, the character is done transitioning, and removed from the "to transition" map. Due to the nature of VisibleCharactersBehaviour::CHARS_BEFORE_SHAPING, having a RichTextTransition assigned restricts the visible character clipping behaviour to after shaping, to allow for exit fx.

[jordigcs]

Here's a showcase of the feature with large amounts of text. For performance, the RichTextTransition will stop applying transitions when there is a large jump (>20 chars) in the amount of visible text.

[bruvzg]

This will only work with "Characters After Shaping" mode, since it's assuming that visible range is character indices, but in other modes these are glyph indices (and should track processed_glyphs_step instead of character index).

Screen.Recording.2024-05-09.at.11.34.51.mov

[IntangibleMatter]

Here's a showcase of the feature with large amounts of text. For performance, the RichTextTransition will stop applying transitions when there is a large jump (>20 chars) in the amount of visible text.

While this is a good plan (when I attempted this I didn't cap it, and it got... very bad), I do think that setting a hard cap doesn't make a ton of sense. A better option might be to add this as a variable to the class (such as max_transition_characters).

(Though if it has to be a hardcoded cap, make it a power of 2. This is a programming thing after all :p)

[IntangibleMatter]

I think that it makes more sense to return false when finished, to better match the way it's done in RichTextEffect. Instead of treating it as done = true treat it as processing effect = false. This means less problems when converting from richtexteffect. Same goes for _process_exit_effects.

[RedMser]

This PR might help towards fixing godotengine/godot-proposals#6286 (by allowing detecting on when exactly a specific character got revealed)

A better option might be to add this as a variable to the class (such as max_transition_characters).

Agreed on this point, for a use-case like this it would be important to run the transition for every character.

[RedMser]

Similar to what I've done in #66600, it would be nice to create a GDScript template for RichTextTransition as well.

[jordigcs]

Force pushing the branch closed the PR for some reason? Unsure why that happened.
I've fixed typos. Working on adding support for the Glyph clipping modes. I will mark this as draft for now.