Consistent screen-space coordinates

[irate-devil]

Objective

Make the coordinate systems of screen-space items (cursor position, UI, viewports, etc.) consistent.

Solution

Remove the weird double inversion of the cursor position's Y origin. Once in bevy_winit to the bottom and then again in bevy_ui back to the top.
This leaves the origin at the top left like it is in every other popular app framework.

Update the world_to_viewport, viewport_to_world, and viewport_to_world_2d methods to flip the Y origin (as they should since the viewport coordinates were always relative to the top left).

Migration Guide

Window::cursor_position now returns the position of the cursor relative to the top left instead of the bottom left.
This now matches other screen-space coordinates like RelativeCursorPosition, UI, and viewports.

The world_to_viewport, viewport_to_world, and viewport_to_world_2d methods on Camera now return/take the viewport position relative to the top left instead of the bottom left.

If you were using world_to_viewport to position a UI node the returned y value should now be passed into the top field on Style instead of the bottom field.
Note that this might shift the position of the UI node as it is now anchored at the top.

If you were passing Window::cursor_position to viewport_to_world or viewport_to_world_2d no change is necessary.

[irate-devil]

I had to rework the labels in the blend_modes example a bit to keep their position the same as before.
Not a usability regression, it's just that for the labels to display correctly they need to be 'anchored' to the bottom left which happened to align with what world_to_viewport returned before.

Solved by adding a parent node and using bottom: Val::Px(0.), for the text itself.

[alice-i-cecile]

Yeah I'm happy with the changes here!

[superdump]

I think this looks good but need to recheck when I’m less tired.

[nablabla]

please provide a formula to convert, guessing is so frustrating.
That is what i did, it should be a minor change. Or is there a easier way?

let Some(screen_pos) = window.cursor_position()
let window_size = Vec2::new(window.width() as f32, window.height() as f32);
// convert screen position [0..resolution] to ndc [-1..1] (gpu coordinates)
let ndc = - (screen_pos / window_size); // * 2.0 - Vec2::ONE;
// matrix for undoing the projection and camera transform
let ndc_to_world = camera_transform.compute_matrix() * camera.projection_matrix().inverse();
// use it to convert ndc to world-space coordinates
let world_pos_3d = ndc_to_world.project_point3(ndc.extend(-1.0));
println!("world_pos_3d: {}/{}", world_pos_3d.x, world_pos_3d.y);
for (clickable, trans) in clickable.iter_mut() {
  // find the top-most instance
  let model_pos = trans.compute_matrix().inverse().project_point3(world_pos_3d).truncate();			
  if   clickable.check_clicked(model_pos) && trans.translation.z > clicked_z {
    clicked_z = trans.translation.z;
    clicked = Some(clickable);
  }
}

[nablabla]

please provide a formula to convert, guessing is so frustrating. That is what i did, it should be a minor change. Or is there a easier way?

let Some(screen_pos) = window.cursor_position()
let window_size = Vec2::new(window.width() as f32, window.height() as f32);
// convert screen position [0..resolution] to ndc [-1..1] (gpu coordinates)
let ndc = - (screen_pos / window_size); // * 2.0 - Vec2::ONE;
// matrix for undoing the projection and camera transform
let ndc_to_world = camera_transform.compute_matrix() * camera.projection_matrix().inverse();
// use it to convert ndc to world-space coordinates
let world_pos_3d = ndc_to_world.project_point3(ndc.extend(-1.0));
println!("world_pos_3d: {}/{}", world_pos_3d.x, world_pos_3d.y);
for (clickable, trans) in clickable.iter_mut() {
  // find the top-most instance
  let model_pos = trans.compute_matrix().inverse().project_point3(world_pos_3d).truncate();			
  if   clickable.check_clicked(model_pos) && trans.translation.z > clicked_z {
    clicked_z = trans.translation.z;
    clicked = Some(clickable);
  }
}

Okay, I found it out, to restore the previous behaviour, simply do this:
let screen_pos = Vec2::new(screen_pos.x, window.height() as f32 - screen_pos.y);
I think something like this should be in the migration guide