Color space issues with SRGB framebuffer in OpenGL

[parasyte]

Short info header:

• GFX version: 0.17.1
• OS: macOS 10.13.4
• GPU: AMD Radeon R9 M370X 2048 MB graphics / Intel Iris Pro 1536 MB graphics

Today I started updating the gfx dependency in imgui-rs. One of the things I noticed is that the imgui-gfx-renderer looks washed out compared to imgui-glium-renderer. It took me all day to find the cause, which turned out to be a color space mismatch between the (linear space) colors provided by imgui in the vertex attribute buffer, and the gfx_device_gl backend forcefully enabling SRGB on the framebuffer.

I was able to reproduce it by patching the gamma example to use the purple title bar color from imgui instead of the grayscale. Here's the patch:

diff --git a/examples/gamma/main.rs b/examples/gamma/main.rs
index af29c5b4..b95a8c82 100644
--- a/examples/gamma/main.rs
+++ b/examples/gamma/main.rs
@@ -31,7 +31,7 @@ pub type DepthFormat = gfx::format::DepthStencil;
 gfx_defines!{
     vertex Vertex {
         pos: [f32; 2] = "a_Pos",
-        color: [f32; 3] = "a_Color",
+        color: [f32; 4] = "a_Color",
     }

     pipeline pipe {
@@ -41,13 +41,14 @@ gfx_defines!{
 }

 const QUAD: [Vertex; 4] = [
-    Vertex { pos: [ -0.5,  0.5 ], color: [0.0, 0.0, 0.0] },
-    Vertex { pos: [ -0.5, -0.5 ], color: [0.0, 0.0, 0.0] },
-    Vertex { pos: [  0.5, -0.5 ], color: [1.0, 1.0, 1.0] },
-    Vertex { pos: [  0.5,  0.5 ], color: [1.0, 1.0, 1.0] }
+    // Use imgui TitleBg color: https://github.com/ocornut/imgui/blob/v1.52/imgui.cpp#L744
+    Vertex { pos: [ -0.5,  0.5 ], color: [0.27, 0.27, 0.54, 0.83] },
+    Vertex { pos: [ -0.5, -0.5 ], color: [0.27, 0.27, 0.54, 0.83] },
+    Vertex { pos: [  0.5, -0.5 ], color: [0.27, 0.27, 0.54, 0.83] },
+    Vertex { pos: [  0.5,  0.5 ], color: [0.27, 0.27, 0.54, 0.83] }
 ];

-const CLEAR_COLOR: [f32; 4] = [0.5, 0.5, 0.5, 1.0];
+const CLEAR_COLOR: [f32; 4] = [1.0, 1.0, 1.0, 1.0];

 type SurfaceData = <<ColorFormat as Formatted>::Surface as SurfaceTyped>::DataType;

diff --git a/examples/gamma/shader/quad_120.glslv b/examples/gamma/shader/quad_120.glslv
index a64fe208..44a270ed 100644
--- a/examples/gamma/shader/quad_120.glslv
+++ b/examples/gamma/shader/quad_120.glslv
@@ -1,10 +1,10 @@
 #version 120

 attribute vec2 a_Pos;
-attribute vec3 a_Color;
+attribute vec4 a_Color;
 varying vec4 v_Color;

 void main() {
-    v_Color = vec4(a_Color, 1.0);
+    v_Color = a_Color;
     gl_Position = vec4(a_Pos, 0.0, 1.0);
 }
diff --git a/examples/gamma/shader/quad_150_core.glslv b/examples/gamma/shader/quad_150_core.glslv
index 2ce6461b..c438e2ba 100644
--- a/examples/gamma/shader/quad_150_core.glslv
+++ b/examples/gamma/shader/quad_150_core.glslv
@@ -1,10 +1,10 @@
 #version 150 core

 in vec2 a_Pos;
-in vec3 a_Color;
+in vec4 a_Color;
 out vec4 v_Color;

 void main() {
-    v_Color = vec4(a_Color, 1.0);
+    v_Color = a_Color;
     gl_Position = vec4(a_Pos, 0.0, 1.0);
 }
diff --git a/examples/gamma/shader/quad_300_es.glslv b/examples/gamma/shader/quad_300_es.glslv
index f0f59952..3df5b91e 100644
--- a/examples/gamma/shader/quad_300_es.glslv
+++ b/examples/gamma/shader/quad_300_es.glslv
@@ -1,10 +1,10 @@
 #version 300 es

 in vec2 a_Pos;
-in vec3 a_Color;
+in vec4 a_Color;
 out vec4 v_Color;

 void main() {
-    v_Color = vec4(a_Color, 1.0);
+    v_Color = a_Color;
     gl_Position = vec4(a_Pos, 0.0, 1.0);
 }

And here's a screenshot, showing the color rendered incorrectly:

When I disable the SRGB framebuffer (using unsafe code), the rendering works as expected:

// XXX: Fix incorrect render results
unsafe {
    extern crate gfx_gl;
    device.with_gl(|gl| {
        gl.Disable(gfx_gl::FRAMEBUFFER_SRGB);
    });
}

---

As a final data point, here are some screenshots of the imgui-rs hello_gfx example; the latter screenshot has the SRGB-disable workaround to fix the #colors:

May be related to: #992 #997

[parasyte]

Related: ocornut/imgui#578

[amitksinha]

https://www.mapd.com/docs/latest/getting-started/hwRefCfgGuide/
Currently, the optimal GPUs on which to run the MapD Analytics Platform are:

NVIDIA Tesla P40
NVIDIA Tesla P100

Will it handle any GPU card such as radeon?

[amitksinha]

may be i posted the question incorrectly.
Should I take that mapd will support any GPU, or is it vendor specific?
thanks

[parasyte]

This actually looks like a problem with imgui colors in its default style. The gfx sRGB framebuffer is rendering colors exactly as they should appear. (It's just that color spaces always confuse the hell out of me.)

The TitleBg color above, for example is the sRGB components divided by 255 to incorrectly place them into linear space without a gamma transform. (128,128,128) in sRGB is not mid-gray, but (0.5,0.5,0.5) in linear-space is mid-gray. That's the mistake they made.

So I'm going to close this in favor of #997