Transparency shader not working in v12

[CyberShadow]

Platform

NixOS

GPU, drivers, and screen setup

Intel Graphics on Linux v6.11.2, single monitor (laptop - Thinkpad X1 Extreme Gen3)

name of display: :0
display: :0  screen: 0
direct rendering: Yes
Extended renderer info (GLX_MESA_query_renderer):
    Vendor: Intel (0x8086)
    Device: Mesa Intel(R) UHD Graphics (CML GT2) (0x9bc4)
    Version: 24.2.2
    Accelerated: yes
    Video memory: 15942MB
    Unified memory: yes
    Preferred profile: core (0x1)
    Max core profile version: 4.6
    Max compat profile version: 4.6
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 3.2
OpenGL vendor string: Intel
OpenGL renderer string: Mesa Intel(R) UHD Graphics (CML GT2)
OpenGL core profile version string: 4.6 (Core Profile) Mesa 24.2.2
OpenGL core profile shading language version string: 4.60
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile

OpenGL version string: 4.6 (Compatibility Profile) Mesa 24.2.2
OpenGL shading language version string: 4.60
OpenGL context flags: (none)
OpenGL profile mask: compatibility profile

OpenGL ES profile version string: OpenGL ES 3.2 Mesa 24.2.2
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.20

Environment

i3 version 4.23

picom version

v12.2

Diagnostics

[ 2024-10-11 14:21:33.370 open_config_file_at WARN ] This compositor has been renamed to "picom", the old config file paths is deprecated. Please replace the "compton"s in the path with "picom"
**Version:** v12.2

### Extensions:

* Shape: Yes
* RandR: Yes
* Present: Present

### Misc:

* Use Overlay: No
  (Another compositor is already running)
* Config file specified: None
* Config file used: /home/vladimir/.config/compton.conf

### Drivers (inaccurate):

modesetting
[ 2024-10-11 14:21:33.416 egl_init WARN ] The egl backend is still experimental, use with care.

### Backend: egl

* Driver vendors:
 * EGL: Mesa Project
 * EGL driver: iris
 * GL: Intel
* GL renderer: Mesa Intel(R) UHD Graphics (CML GT2)

### Backend: glx

* Driver vendors:
 * GLX: Mesa Project and SGI
 * GL: Intel
* GL renderer: Mesa Intel(R) UHD Graphics (CML GT2)
* Accelerated: 1

Configuration:

Configuration file

backend = "glx";

rules: ({
  match = "class_g = 'Emacs'";
  shader = "compton/color-key-transparency-171925.glsl";
})

Steps of reproduction

1. Create and set up a color-key transparency shader, e.g.:

   #version 330
   
   in vec2 texcoord;
   uniform sampler2D tex;
   
   vec4 window_shader() {
           vec4 c = texelFetch(tex, ivec2(texcoord), 0);
           {
                   if (c == vec4(23.0/255.0, 25.0/255.0, 37.0/255.0, 1.0)) // #171925
                           c = vec4(0, 0, 0, 0.7);
           }
           return c;
   }

2. Set Emacs to use the same background color that the shader looks for (#171925), e.g. by activating the deeper-blue color theme
3. Move the cursor around a bit

Expected behavior

Window background should be transparent, e.g.:

(screenshot taken with picom 11.2 with the equivalent config window-shader-fg-rule = [ "compton/color-key-transparency-171925.glsl:class_g = 'Emacs'" ];)

Current Behavior

Looks like the window contents is drawn on top of the previous copy repeatedly, instead of on the background, causing the window to be opaque and a ghosting effect, e.g.:

Stack trace

N/A

OpenGL trace

Let me know if you need this.

Other details

Hopefully the screenshots should be OK.

[yshui]

i think you need --force-win-blend for shader that adds transparency (to windows that are opaque) to work

[CyberShadow]

Thanks. I see the exact same problem with

backend = "glx";

force-win-blend = true;

rules: ({
  match = "class_g = 'Emacs'";
  shader = "compton/color-key-transparency-171925.glsl";
})

[yshui]

thanks for testing! there is probably a bug in how force_win_blend is handled in v12.

[yshui]

can you test the commit above?

[CyberShadow]

Works great, thank you!

Would it make sense to have force-win-blend as a rules setting, instead of just a global?

[yshui]

Would it make sense to have force-win-blend as a rules setting, instead of just a global?

Yes.

[yshui]

but it's going to be in v13 though, too late for v12.