Added and improved screen shaders, switched to viewport stretch mode, improved overall visual style

core/shaders/screen_effects.gdshader

@@ -1,41 +1,54 @@
 shader_type canvas_item;
 render_mode unshaded;
 
+// uniforms
+
 uniform sampler2D screen_texture : hint_screen_texture, repeat_disable, filter_linear_mipmap;
 
-// simple edge detection
+// constants
+
+const vec3 BLACK = vec3(0);
+const float glow_strength = 0.5;
+const float glow_size_lod = 3.0;
+const int outline_radius = 1;
+
+// simple edge detection based on black
+
 bool is_edge(int radius, vec2 screen_uv, vec2 screen_pixel_size) {
-	bool ed_found_black = false;
+	bool ed_found_bg = false;
 	bool ed_found_color = false;
 	// check neighbor pixels
 	for (int x = -radius; x < radius+1; x++) {
 		for (int y = -radius; y < radius+1; y++) {
 			vec4 p = texture(screen_texture, screen_uv + (vec2(float(x), float(y)) * screen_pixel_size));
-			if (p.rgb == vec3(0.0)) {
-				ed_found_black = true;
+			if (p.rgb == BLACK) {
+				ed_found_bg = true;
 			} else {
 				ed_found_color = true;
 			}
 		}
 	}
-	return ed_found_black && ed_found_color;
+	return ed_found_bg && ed_found_color;
 }
 
+//
+
 void fragment() {
 	// get screen texture
 	vec4 screen = texture(screen_texture, SCREEN_UV);
 	// apply edge detection
-	int radius = max(int(0.003 * (1.0/SCREEN_PIXEL_SIZE.y)), 1); // with a relative radius
-	if (is_edge(radius, SCREEN_UV, SCREEN_PIXEL_SIZE)) {
-		COLOR = screen;
-	} else {
-		COLOR = vec4(0, 0, 0, 1);
+	if (is_edge(outline_radius, SCREEN_UV, SCREEN_PIXEL_SIZE)) {
+		if (screen.rgb != BLACK) { // would be incompatible with target backgrounds != black
+			COLOR = screen;
+		}
 	}
 	// apply glow
 	// blur a relative radius -> x * log(1.0/SCREEN_PIXEL_SIZE.y)
 	// using log, because the LOD value seems to affect the blur radius exponentially
-	vec4 glow = textureLod(screen_texture, SCREEN_UV, 0.5 * log(1.0/SCREEN_PIXEL_SIZE.y));
-	if (screen.rgb == vec3(0.0)) {
-		COLOR = mix(COLOR, glow, 0.4) + (glow * 0.3);
+	vec4 glow = textureLod(screen_texture, SCREEN_UV, glow_size_lod);
+	if (screen.rgb == BLACK) {
+		COLOR += glow * glow_strength;
 	}
+	// set alpha to 1
+	COLOR.a = 1.0;
 }

core/shaders/screen_effects_2.gdshader

@@ -0,0 +1,29 @@
+shader_type canvas_item;
+render_mode unshaded;
+
+// uniforms
+
+uniform sampler2D screen_texture : hint_screen_texture, repeat_disable, filter_linear_mipmap;
+uniform sampler2D vignette_mask;
+
+// constants
+
+const float rgb_distortion_v_mult = 0.9;
+const float rgb_distortion_v_exp = 2.0;
+const float rgb_distortion_v_min = 0.04;
+const float rgb_distortion_v_max = 0.12;
+
+//
+
+void fragment() {
+	float v = texture(vignette_mask, SCREEN_UV).r;
+	// rgb & blur distortion
+	float v_ = clamp(
+		pow(v * rgb_distortion_v_mult, rgb_distortion_v_exp),
+		rgb_distortion_v_min,
+		rgb_distortion_v_max
+	);
+	COLOR.r = textureLod(screen_texture, SCREEN_UV - (v_ * 0.01), v).r;
+	COLOR.g = textureLod(screen_texture, SCREEN_UV + (v_ * 0.01), v).g;
+	COLOR.b = textureLod(screen_texture, SCREEN_UV, v).b;
+}

core/shaders/vignette_mask.tres

@@ -0,0 +1,10 @@
+[gd_resource type="GradientTexture2D" load_steps=2 format=3 uid="uid://bwlgypt4xjocj"]
+
+[sub_resource type="Gradient" id="Gradient_h26vj"]
+
+[resource]
+gradient = SubResource("Gradient_h26vj")
+width = 640
+height = 640
+fill = 1
+fill_from = Vector2(0.5, 0.5)