The simplest outline shader (via material)

Very simple outline shader, but I think it’s useful.

It create outline by drawing scaled up object with inverted backface culling, so:

• requires correct setting of normal vectors
• breaks transparency obtained by backface culling (alpha-based transparency works ok)
• works OK with background objects
• outline is drawed around object (oustide, not on the object)

How to use:

1. add next pass (with shader material) to the object that should have outline
2. set this shader in this material
3. adjust parameters (outline color and size, note: size is depending on object size – it’s scale factor)

Tested in Godot 4.3 with Forward+ (Vulkan) rendering. Inspirited by this video.

Shader code

shader_type spatial;
render_mode blend_mix, cull_front, unshaded;

uniform vec4 color : source_color = vec4(1,0,0,1);
uniform float size : hint_range(1.0, 1.5, 0.01) = 1.05;

void vertex() {
	VERTEX *= size;
}

void fragment() {
	ALBEDO = color.rgb;
	ALPHA = color.a;
}

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