Add sobel filter (edge detection)

shaderlib/sobel.gdshaderinc

@@ -0,0 +1,50 @@
+
+/*
+	Edge Detection (Sobel Filter and Gaussian Blur) by FencerDevLog, adapted
+	original code: https://godotshaders.com/shader/edge-detection-sobel-filter-and-gaussian-blur/
+	license of the original code: CC0
+*/
+
+vec3 _convolution(sampler2D tex, vec2 uv, vec2 pixel_size) {
+	vec3 conv = vec3(0.0);
+	// Gaussian blur kernel
+	float gauss[25] = {
+		0.00390625, 0.015625, 0.0234375, 0.015625, 0.00390625,
+		0.015625, 0.0625, 0.09375, 0.0625, 0.015625,
+		0.0234375, 0.09375, 0.140625, 0.09375, 0.0234375,
+		0.015625, 0.0625, 0.09375, 0.0625, 0.015625,
+		0.00390625, 0.015625, 0.0234375, 0.015625, 0.00390625
+	};
+	for (int row = 0; row < 5; row++) {
+		for (int col = 0; col < 5; col++) {
+			conv += texture(tex, uv + vec2(float(col - 2), float(row - 2)) * pixel_size).rgb * gauss[row * 5 + col];
+		}
+	}
+	return conv;
+}
+
+vec4 sobel(sampler2D tex, vec2 uv) {
+	vec2 pixel_size = 1.0/vec2(textureSize(tex, 0));
+	vec3 pixels[9];  // Sobel kernel
+	// [0, 1, 2]
+	// [3, 4, 5]
+	// [6, 7, 8]
+	for (int row = 0; row < 3; row++) {
+		for (int col = 0; col < 3; col++) {
+			vec2 uv_ = uv + vec2(float(col - 1), float(row - 1)) * pixel_size;
+			pixels[row * 3 + col] = _convolution(tex, uv_, pixel_size);
+		}
+	}
+
+	// Sobel operator
+	vec3 gx = (
+		pixels[0] * -1.0 + pixels[3] * -2.0 + pixels[6] * -1.0
+		+ pixels[2] * 1.0 + pixels[5] * 2.0 + pixels[8] * 1.0
+	);
+	vec3 gy = (
+		pixels[0] * -1.0 + pixels[1] * -2.0 + pixels[2] * -1.0
+		+ pixels[6] * 1.0 + pixels[7] * 2.0 + pixels[8] * 1.0
+	);
+	vec3 sobel = sqrt(gx * gx + gy * gy);
+	return vec4(sobel, 1.0);
+}