Linux, compton, have only one specific window transparent

Question

Is it possible to configure compton so that only gnome-terminal becomes transparent? it should be transparent both when the window is active and inactive. I use i3wm as my wm.

Here is my compton.conf:

# Shadow
#shadow = true;
#no-dnd-shadow = true;
#no-dock-shadow = true;
#clear-shadow = true;
#shadow-radius = 7;
#shadow-offset-x = -7;
#shadow-offset-y = -7;
# shadow-opacity = 0.7;
# shadow-red = 0.0;
# shadow-green = 0.0;
# shadow-blue = 0.0;
shadow-exclude = [
    "name = 'Notification'",
    "class_g = 'Conky'",
    "class_g ?= 'Notify-osd'",
    "class_g = 'Cairo-clock'",
    "_GTK_FRAME_EXTENTS@:c"
];
# shadow-exclude = "n:e:Notification";
# shadow-exclude-reg = "x10+0+0";
# xinerama-shadow-crop = true;

# Opacity
#menu-opacity = 0.8;
opacity-rule = ["80:class_g = 'gnome-terminal'"];
#inactive-opacity = 0.8;
#active-opacity = 1.0;
#frame-opacity = 0.7;
inactive-opacity-override = false;
alpha-step = 0.06;
# inactive-dim = 0.2;
# inactive-dim-fixed = true;
# blur-background = true;
# blur-background-frame = true;
blur-kern = "3x3box";
# blur-kern = "5,5,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1";
# blur-background-fixed = true;
blur-background-exclude = [
    "window_type = 'dock'",
    "window_type = 'desktop'",
    "_GTK_FRAME_EXTENTS@:c"
];
#opacity-rule = [ "20:class_g = 'gnome-terminal'" ];

# Fading
fading = false;
# fade-delta = 30;
#fade-in-step = 0.03;
#fade-out-step = 0.03;
 no-fading-openclose = true;
 no-fading-destroyed-argb = true;
fade-exclude = [ ];

# Other
backend = "xrender";
mark-wmwin-focused = true;
mark-ovredir-focused = true;
# use-ewmh-active-win = true;
detect-rounded-corners = true;
detect-client-opacity = true;
refresh-rate = 0;
vsync = "none";
dbe = false;
paint-on-overlay = true;
# sw-opti = true;
# unredir-if-possible = true;
# unredir-if-possible-delay = 5000;
# unredir-if-possible-exclude = [ ];
focus-exclude = [ "class_g = 'Cairo-clock'" ];
detect-transient = true;
detect-client-leader = true;
invert-color-include = [ ];
# resize-damage = 1;

# GLX backend
# glx-no-stencil = true;
glx-copy-from-front = false;
# glx-use-copysubbuffermesa = true;
# glx-no-rebind-pixmap = true;
glx-swap-method = "undefined";
# glx-use-gpushader4 = true;
# xrender-sync = true;
# xrender-sync-fence = true;

# Window type settings
wintypes:
{
  tooltip = { fade = true; shadow = true; opacity = 0.75; focus = true; };
};

Solution

You have it almost right, you misspelled a G as g. You can use xprop to find the WM_CLASS, if you want to create specific rules for an application:

$ xprop WM_CLASS <Click on gnome-terminal>
WM_CLASS(STRING) = "gnome-terminal-server", "Gnome-terminal"

That means your rule has to look like this:

opacity-rule = [ "40:class_g = 'Gnome-terminal'" ];

or use a case insensitive rule:

opacity-rule = [ "40:class_g *?= 'gnome-terminal'" ];