Apply scale factor to ImageMeasure sizes

[ickshonpe]

Objective

In Bevy main, the unconstrained size of an ImageBundle or AtlasImageBundle UI node is based solely on the size of its texture and doesn't change with window scale factor or UiScale.

Solution

• The size field of each ImageMeasure should be multiplied by the current combined scale factor.
• Each ImageMeasure should be updated when the combined scale factor is changed.

Example:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .insert_resource(UiScale { scale: 1.5 })
        .add_systems(Startup, setup)
        .run();
}

fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
    commands.spawn(Camera2dBundle::default());
    commands.spawn(NodeBundle {
        style: Style {
            // The size of the "bevy_logo_dark.png" texture is 520x130 pixels
            width: Val::Px(520.),
            height: Val::Px(130.),
            ..Default::default()
        },
        background_color: Color::RED.into(),
        ..Default::default()
    });
    commands
        .spawn(ImageBundle {
            style: Style {
                position_type: PositionType::Absolute,
                ..Default::default()
            },
            image: UiImage::new(asset_server.load("bevy_logo_dark.png")),
            ..Default::default()
        });
}

The red node is given a size with the same dimensions as the texture. So we would expect the texture to fill the node exactly.

• Result with Bevy main branch bb59509:
  

• Result with this PR (and Bevy 0.10.1):
  

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Changelog

bevy_ui::widget::image

• Update all ImageMeasures on changes to the window scale factor or UiScale.
• Multiply ImageMeasure::size by the window scale factor and UiScale.

Migration Guide

[mockersf]

Could you add a comment on ImageMeasure that it should be in physical pixels? (like UiImageSize)

[ickshonpe]

Could you add a comment on ImageMeasure that it should be in physical pixels? (like UiImageSize)

Done,
Scale factor is so confusing, just realised the UiImageSize comment is wrong too It's the size of the texture not the size in physical or logical pixels, changed it as well. And it's a component, not a field.

[ickshonpe]

This fix needs to be included with 0.11 I think, otherwise updating any UI projects using images from 0.10 is going to be really painful.

[TimJentzsch]

Do we want this to be editable by the user? Or should this be pub(crate)?

[ickshonpe]

Yes it's intended to be pub. That way someone can use ImageMeasure for their own custom widget if they want.

Otherwise, it doesn't matter that it's pub as measure funcs can't be accessed by users once they've been added to the layout tree.