2D outline/inline

Adds an outer or inner stroke to a texture. Just attach the shader to a material and the material to a CanvasItem, like a Sprite.

This shader is robust and configurable (color, width, pattern, placement, and the ability to add margins to the texture to make room for an outline). However, for textures with anti-aliased edges this shader might provide a better result.

Shader code
shader_type canvas_item;

uniform vec4 color : hint_color = vec4(1.0);
uniform float width : hint_range(0, 10) = 1.0;
uniform int pattern : hint_range(0, 2) = 0; // diamond, circle, square
uniform bool inside = false;
uniform bool add_margins = true; // only useful when inside is false

void vertex() {
	if (add_margins) {
		VERTEX += (UV * 2.0 - 1.0) * width;
	}
}

bool hasContraryNeighbour(vec2 uv, vec2 texture_pixel_size, sampler2D texture) {
	for (float i = -ceil(width); i <= ceil(width); i++) {
		float x = abs(i) > width ? width * sign(i) : i;
		float offset;
		
		if (pattern == 0) {
			offset = width - abs(x);
		} else if (pattern == 1) {
			offset = floor(sqrt(pow(width + 0.5, 2) - x * x));
		} else if (pattern == 2) {
			offset = width;
		}
		
		for (float j = -ceil(offset); j <= ceil(offset); j++) {
			float y = abs(j) > offset ? offset * sign(j) : j;
			vec2 xy = uv + texture_pixel_size * vec2(x, y);
			
			if ((xy != clamp(xy, vec2(0.0), vec2(1.0)) || texture(texture, xy).a == 0.0) == inside) {
				return true;
			}
		}
	}
	
	return false;
}

void fragment() {
	vec2 uv = UV;
	
	if (add_margins) {
		vec2 texture_pixel_size = vec2(1.0) / (vec2(1.0) / TEXTURE_PIXEL_SIZE + vec2(width * 2.0));
		
		uv = (uv - texture_pixel_size * width) * TEXTURE_PIXEL_SIZE / texture_pixel_size;
		
		if (uv != clamp(uv, vec2(0.0), vec2(1.0))) {
			COLOR.a = 0.0;
		} else {
			COLOR = texture(TEXTURE, uv);
		}
	} else {
		COLOR = texture(TEXTURE, uv);
	}
	
	if ((COLOR.a > 0.0) == inside && hasContraryNeighbour(uv, TEXTURE_PIXEL_SIZE, TEXTURE)) {
		COLOR.rgb = inside ? mix(COLOR.rgb, color.rgb, color.a) : color.rgb;
		COLOR.a += (1.0 - COLOR.a) * color.a;
	}
}
Tags
2d, border, configurable, inline, outline, stroke
The shader code and all code snippets in this post are under CC0 license and can be used freely without the author's permission. Images and videos, and assets depicted in those, do not fall under this license. For more info, see our License terms.

Related shaders

2D Outline and Rainbow outline 2 in 1

Colorful Outline

Post Effect Outline Shader for GLES2

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5 Comments
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ubX
ubX
1 year ago

Noice

Apprentice
Apprentice
1 year ago

Nice code.

Muhannad
Muhannad
1 year ago

great! thanks

morningking
morningking
5 months ago

thanks.

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