MSAA Changes colors and adds borders around sprites

[TheNeikos]

When setting a Msaa level above 1 sample the color changes:

Msaa 1:

Msaa 2:

Msaa 4:

Code:

use bevy::prelude::*;
use bevy::render::pass::ClearColor;

fn main() {
    env_logger::init();
    App::build()
        .add_resource(Msaa { samples: 4 })
        .add_resource(ClearColor(Color::rgb(0.5, 0.5, 0.0)))
        .add_default_plugins()
        .add_startup_system(setup.system())
        .run();
}

struct Square(u64);

fn setup(
    mut commands: Commands,
    mut materials: ResMut<Assets<ColorMaterial>>,
    asset_server: Res<AssetServer>,
) {
    commands
        .spawn(Camera2dComponents::default())
        .spawn(UiCameraComponents::default())
        .spawn((Square(1), Translation::new(0., 20., 0.)))
        .with_children(|parent| {
            parent.spawn(SpriteComponents {
                material: materials.add(Color::rgb(1.0, 0.2, 0.2).into()),
                sprite: Sprite {
                    size: Vec2::new(25., 25.),
                },
                translation: Translation::new(0., 0., 0.),
                ..Default::default()
            });
        });
}

My platform is Linux with an NVIDIA Corporation GP107GLM [Quadro P620] GPU

[julhe]

Hmm, I can't reproduce this under Windows 10 x64 with a AMD R9 390.

Could you test this https://github.com/gfx-rs/wgpu-rs/tree/master/examples/msaa-line? Change the MSAA level with the left and right arrow key and see if the problem is also present there.

[TheNeikos]

@julhe No, it doesn't happen there

[TheNeikos]

Another test on Win10 with an GTX 1060:

Msaa 1:

Msaa 4:

Msaa 8:

It getting blurrier doesn't make too much sense imo as the sprite itself should be directly on a pixel edge (and isn't Msaa meant to make things crisp?)

Another weird thing is the background. It is set to rgb(0.5, 0.5, 0.0) which translated to non-float values would be rgb(128, 128, 0) (or 127 depending on rounding mode). But at Msaa 1 it displays it with rgb(187, 187, 0)! Which is also what I'm getting on Windows. So it seems that I got it the wrong way around! Msaa { 1 } actually clears everything in the wrong color on Linux, and on Windows it seems to be always wrong.

Am I understanding this correctly?

[cart]

As @StarArawn mentioned on discord, the weird multisampling behavior might be related to: gfx-rs/wgpu#725. I think we should treat this issue as scoped to that problem.

A little bit of blurriness is to be expected with MSAA. It makes jagged edges crisper by taking multiple samples and blending them together (which is the literal definition of a blur operation). This makes jagged edges crisper, but it can make already straight lines a little bit blurry.

I do think we have another issue though. I also get the same "incorrect color" results on my computer (linux, proprietary nvidia drivers). I'm pretty sure the incorrect "base" colors are related to bevy rendering with the srgb color space. I have a feeling we aren't doing a proper conversion here. The math certainly lines up with improper color handling.

Desired linear color value: 0.5
SRGB to Linear Conversion: ((0.5 + 0.055) / 1.055) ^ 2.4 = 0.214
Rendering with Color::rgb(0.214, 0.214, 0.0) and color picking = rgb(128, 128, 0)

So by using an SRGB->Linear conversion before rendering, we get the expected output.

I'm pretty sure we're locked into an SRGB swap chain, so we might need to do these conversions internally.

[cart]

As a followup, loaded images do use the correct colors (and also use the SRGB image type), so I think the fix to the "base color" problem is it convert the Color type before uploading to the shader.

[TheNeikos]

@cart I understand that some things might get a bit more blurry, but it also adds a discolored edge to sprites which it imo should not do

For example, in my game this is what it looks with Msaa 8:

and without:

[cart]

@TheNeikos hmm yeah thats not desirable. I'm honestly uncertain if thats a bevy issue, a wgpu issue, or a general MSAA issue.

[cwfitzgerald]

Fully agree that this is just gfx-rs/wgpu#725. It manifests exactly as the photos show; borked srgb conversion of the background plus bad MSAA. It's been a long-standing issue with intel (at least on linux), and just needs to be worked around at some layer.

[rparrett]

gfx-rs/wgpu#725 is apparently fixed since wgpu 0.11.

Anyone affected still able to reproduce with bevy main?

[cart]

I think its probably safe to close this for now and then re-open if this resurfaces.