GLSL outline shaders made for Flutter with Umbra

outline_in.glsl

uniform vec2 imageResolution;
uniform float lineThickness;
uniform vec4 lineColor;
uniform sampler2D image;

bool check_bounds(vec4 a, vec4 b, vec4 c, vec4 d, vec4 e, vec4 f, vec4 g, vec4 h) {
	if (a.a < 0.5) return true;
	if (b.a < 0.5) return true;
	if (c.a < 0.5) return true;
	if (d.a < 0.5) return true;
	if (e.a < 0.5) return true;
	if (f.a < 0.5) return true;
	if (g.a < 0.5) return true;
	if (h.a < 0.5) return true;
	return false;
}

vec4 fragment(vec2 uv, vec2 fragCoord) {
	vec2 TEXTURE_PIXEL_SIZE = vec2(1.0) / imageResolution;

	vec2 size = TEXTURE_PIXEL_SIZE * lineThickness;
	vec4 color = texture(image, uv);
	
	if (color.a > 0.5) {
		if (check_bounds(
			texture(image, uv + vec2(-size.x, -size.y)), // top left
			texture(image, uv + vec2(0, -size.y)), // top center
			texture(image, uv + vec2(size.x, -size.y)), // top right
			texture(image, uv + vec2(-size.x, 0)), // center left
			texture(image, uv + vec2(size.x, 0)), // center right
			texture(image, uv + vec2(-size.x, size.y)), // bottom left
			texture(image, uv + vec2(0, size.y)), // bottom center
			texture(image, uv + vec2(size.x, size.y)) // bottom right
		)) {
			color = lineColor;
		}
		// Ensure that the bounding box is outlined if required.
		if (uv.x < size.x) color = lineColor;
		if (uv.y < size.y) color = lineColor;
		if (uv.y > 1.0 - size.y) color = lineColor;
		if (uv.x > 1.0 - size.x) color = lineColor;
	}

	return color;
}

outline_out.glsl

uniform vec2 imageResolution;
uniform float lineThickness;
uniform vec4 lineColor;
uniform sampler2D image;

bool check_bounds(vec4 a, vec4 b, vec4 c, vec4 d, vec4 e, vec4 f, vec4 g, vec4 h) {
	if (a.a > 0.5) return true;
	if (b.a > 0.5) return true;
	if (c.a > 0.5) return true;
	if (d.a > 0.5) return true;
	if (e.a > 0.5) return true;
	if (f.a > 0.5) return true;
	if (g.a > 0.5) return true;
	if (h.a > 0.5) return true;
	return false;
}

vec4 fragment(vec2 uv, vec2 fragCoord) {
	vec2 TEXTURE_PIXEL_SIZE = vec2(1.0) / imageResolution;

	vec2 size = TEXTURE_PIXEL_SIZE * lineThickness;
	vec4 color = texture(image, uv);
	
	if (color.a < 0.5) {
		if (check_bounds(
			texture(image, uv + vec2(-size.x, -size.y)), // top left
			texture(image, uv + vec2(0, -size.y)), // top center
			texture(image, uv + vec2(size.x, -size.y)), // top right
			texture(image, uv + vec2(-size.x, 0)), // center left
			texture(image, uv + vec2(size.x, 0)), // center right
			texture(image, uv + vec2(-size.x, size.y)), // bottom left
			texture(image, uv + vec2(0, size.y)), // bottom center
			texture(image, uv + vec2(size.x, size.y)) // bottom right
		)) {
			color = lineColor;
		}
	}

	if (color.a > 0.5) {
		// Ensure that the bounding box is outlined if required.
		if (uv.x < size.x) color = lineColor;
		if (uv.y < size.y) color = lineColor;
		if (uv.y > 1.0 - size.y) color = lineColor;
		if (uv.x > 1.0 - size.x) color = lineColor;
	}

	return color;
}