Hello there!
I have found a few other issues related to sRGB, but they simply do not help me and I am still incredibly confused. My problem is that ContextBuilder::with_srgb does not seem to have any effect. Check out this minimal example (using glium):
#[macro_use]
extern crate glium;
fn main() {
#[allow(unused_imports)]
use glium::{glutin, Surface};
let events_loop = glutin::EventsLoop::new();
let wb = glutin::WindowBuilder::new();
let cb = glutin::ContextBuilder::new().with_srgb(false); // MARK A -------
let display = glium::Display::new(wb, cb, &events_loop).unwrap();
#[derive(Copy, Clone)]
struct Vertex {
position: [f32; 2],
}
implement_vertex!(Vertex, position);
let shape = vec![
Vertex { position: [-1.0, -1.0] },
Vertex { position: [-1.0, 1.0] },
Vertex { position: [ 1.0, -1.0] },
Vertex { position: [ 1.0, 1.0] },
];
let vertex_buffer = glium::VertexBuffer::new(&display, &shape).unwrap();
let indices = glium::index::NoIndices(glium::index::PrimitiveType::TriangleStrip);
let vertex_shader_src = r#"
#version 140
in vec2 position;
void main() {
gl_Position = vec4(position, 0.0, 1.0);
}
"#;
let fragment_shader_src = r#"
#version 140
out vec4 color;
void main() {
color = vec4(vec3(0.5), 1.0);
}
"#;
let program = glium::Program::new(
&display,
glium::program::ProgramCreationInput::SourceCode {
vertex_shader: vertex_shader_src,
tessellation_control_shader: None,
tessellation_evaluation_shader: None,
geometry_shader: None,
fragment_shader: fragment_shader_src,
transform_feedback_varyings: None,
outputs_srgb: false, // MARK B ----------------------------------
uses_point_size: false,
}
).unwrap();
loop {
let mut target = display.draw();
target.draw(&vertex_buffer, &indices, &program, &uniform!{}, &Default::default()).unwrap();
target.finish().unwrap();
}
}
There are two places of interest which I will explain later.
.with_srgb(_) // MARK Aoutputs_srgb: _, // MARK B
My test setup is the following: I run this program and take a screenshot and then inspect what color value the pixels have. For the four possible configurations, I get the following values:
| | .with_srgb(false) | .with_srgb(true) |
| - | - | - |
| outputs_srgb: false | #bbbbbb | #bbbbbb |
| outputs_srgb: true | #808080 | #808080 |
So as far as I understand: there is the GL_FRAMEBUFFER_SRGB flag. If that flag is false, OpenGL does not perform any conversion from fragment shader to frame buffer. If it is enabled, however, OpenGL assumes that the shader output is linear RGB and will thus -- if the frame buffer has an sRGB format -- convert the shader output to sRGB. In glium, GL_FRAMEBUFFER_SRGB is controlled by the outputs_srgb parameter of the program. If the latter is false, GL_FRAMEBUFFER_SRGB is set to true and the other way around.
Additionally, I would expect glutin to create a framebuffer with the format specified by .with_srgb(_). As such, I have the following expectations:
- ✗
with_srgb(false) and outputs_srgb: false: the framebuffer should be linear RGB, meaning that no conversion should happen, regardless of GL_FRAMEBUFFER_SRGB. As such, I would expect #808080, but I got #bbbbbb.
- ✔
with_srgb(true) and outputs_srgb: false: the framebuffer should be sRGB and since the shader does not output sRGB, I expect OpenGL to convert. As such, I expect #bbbbbb, which I actually got.
- ✔
with_srgb(false) and outputs_srgb: true: as above, the framebuffer should be linear RGB, meaning that no conversion should happen, regardless of GL_FRAMEBUFFER_SRGB. As such, I would expect #808080 which I luckily also got.
- ✔
with_srgb(true) and outputs_srgb: true: the framebuffer is sRGB, but the shader also outputs in that color space, so no conversion should happen. As such I expect #808080 which I got.
This issue is about the first situation. As far as I can tell, this is just wrong.
I tested the above on several platforms: Ubuntu 18.04, MacOS and Windows. I always got the same results (well, on MacOS and Windows the #bbbbbb was slightly off, but still way more than #808080).
(I also did a test with GLFW in case that's interesting. I used the simple example and changed line 63 to " gl_FragColor = vec4(vec3(0.5), 1.0);\n". Running that, I get a #808080 triangle.)
Am I doing something wrong? Am I completely misunderstanding color spaces? Is this a bug in glutin/glium/...? Would be super great if someone could help me!
![[Windows] Unable to create non-SRGB framebuffer](https://avatars.githubusercontent.com/u/7419664?v=4)
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