默认红色描边，确保物体具有MeshInstance3D，shader放在Material Overlay栏即可。

shader_type spatial;
render_mode cull_front, depth_draw_always, unshaded;

uniform bool outline_enabled = false;
uniform vec4 outline_color : source_color = vec4(1.0, 0.0, 0.0, 1.0);
uniform float outline_size = 0.01;

void vertex() {
    if (outline_enabled) {
        VERTEX += NORMAL * outline_size;
    }
}

void fragment() {
    if (outline_enabled) {
        ALBEDO = outline_color.rgb;
        ALPHA = outline_color.a;
    } else {
        ALPHA = 0.0; // 透明，显示原始材质
    }
}