man: include a default shader

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

man/picom.1.asciidoc

@@ -365,17 +365,45 @@ The shader must defined a function, 'vec4 window_shader()', which would be the e
 
 The following uniform/input variables are made available to the shader:
 
-	in vec2 texcoord;             // texture coordinate of the fragment
-	
-	uniform float opacity;        // opacity of the window (0.0 - 1.0)
-	uniform float dim;            // dimming factor of the window (0.0 - 1.0, higher means more dim)
-	uniform float corner_radius;  // corner radius of the window (pixels)
-	uniform float border_width;   // estimated border width of the window (pixels)
-	uniform bool invert_color;    // whether to invert the color of the window
-	uniform sampler2D tex;        // texture of the window
-	uniform sampler2D brightness; // estimated brightness of the window, 1x1 texture
-	uniform float max_brightness; // configured maximum brightness of the window (0.0 - 1.0)
-	uniform float time;           // time in milliseconds, counting from an unspecified starting point
+[source,glsl]
+----
+in vec2 texcoord;             // texture coordinate of the fragment
+
+uniform float opacity;        // opacity of the window (0.0 - 1.0)
+uniform float dim;            // dimming factor of the window (0.0 - 1.0, higher means more dim)
+uniform float corner_radius;  // corner radius of the window (pixels)
+uniform float border_width;   // estimated border width of the window (pixels)
+uniform bool invert_color;    // whether to invert the color of the window
+uniform sampler2D tex;        // texture of the window
+uniform sampler2D brightness; // estimated brightness of the window, 1x1 texture
+uniform float max_brightness; // configured maximum brightness of the window (0.0 - 1.0)
+uniform float time;           // time in milliseconds, counting from an unspecified starting point
+----
+
+The default behavior of picom window rendering can be replicated by the following shader:
+
+[source,glsl]
+----
+#version 330
+in vec2 texcoord;             // texture coordinate of the fragment
+
+uniform sampler2D tex;        // texture of the window
+
+// Default window post-processing:
+// 1) invert color
+// 2) opacity / transparency
+// 3) max-brightness clamping
+// 4) rounded corners
+vec4 default_post_processing(vec4 c);
+
+// Default window shader:
+// 1) fetch the specified pixel
+// 2) apply default post-processing
+vec4 window_shader() {
+    vec4 c = texelFetch(tex, ivec2(texcoord), 0);
+    return default_post_processing(c);
+}
+----
 
 The interface is expected to be mostly stable.