Stabilize snapping 2D transforms to pixel

[adamscott]

tl;dr

This PR simplifies greatly the snapping of 2D transforms to pixel. Instead of relying on flooring sprites, CanvasItems (so twice flooring for sprites) and the camera transform, this PR only rounds each CanvasItem position.

Co-authorship

This PR is heavily inspired by #84380 by @KeyboardDanni, so I added them to the commit as co-author.

Why?

There's some issues with pixel perfect both due to:

• the numerous places where flooring occurs;
• the flooring itself;

Superseding

• Supersedes Replace floor() with round() in pixel snap to transform #84380
• Supersedes Remove 2D transform snapping to rely instead on the GPU #87058

Tests

• Passes Replace floor() with round() in pixel snap to transform #84380 PixelPerfectTest.zip

#84380 test

master

master.webm

This PR

87297.webm

Merlin: Scale of the Magic

(author note: the project was edited to add debug info)

master

master.webm

This PR

87297.webm

Production edit: closes godotengine/godot-roadmap#79

Bugsquad edit:

• Fixes Vulkan: 2D Pixel Snap not working #56793
• Fixes Sprite "crunch" when placed in certain positions on screen #84632

[markdibarry]

Just tested with both pixel perfect test projects, and both look as expected.

[akien-mga]

Great research everyone!

Let's give this a spin and see how pixel art users like it in the next dev snapshot (4.3-dev4).

[akien-mga]

Thanks!

[alvinhochun]

A bit late to the party, but I have the feeling that instead of rounding, it should have been floor(x + 0.5) instead. This is because rounding to nearest has different behaviour for positive and negative values when the fractional part is 0.5.

Consider this: We position a sprite at x=0.5, which is snapped to x=1 on screen. We move it to the left by 1px. Where should the sprite be at on screen?

The expectation should be x=0. However, since the subpixel position is now x=-0.5, the sprite is actually snapped to x=-1 on screen thanks to rounding. Which means the supposedly 1px move becomes a 2px move on screen.