game: add shader to indicate low hp

assets/shaders/screen_low_hp.gdshader

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+shader_type canvas_item;
+
+uniform sampler2D screen_texture : hint_screen_texture;
+uniform float damage_amount : hint_range(0.0, 1.0) = 0.0;
+uniform float noise_strength : hint_range(0.0, 1.0) = 0.3;
+uniform float pulse_strength : hint_range(0.0, 1.0) = 0.2;
+uniform float time;
+
+// TODO: uniform vec2 player_uv; // use this to calculate player offset to camera
+
+float random(vec2 uv) {
+    return fract(sin(dot(uv.xy ,vec2(12.9898,78.233))) * 43758.5453);
+}
+
+void fragment() {
+    // Distance from screen center
+    vec2 center = vec2(0.5, 0.5);
+    float dist = distance(SCREEN_UV, center);
+    float edge_factor = smoothstep(0.4, 0.7, dist); // stronger at edges
+	float edge_factor_gray = smoothstep(0.0, 1.0-damage_amount, dist);
+
+    // Noise flicker
+    float n = random(SCREEN_UV * 200.0 + time * 2.0); 
+    float noise = (n - 0.5) * noise_strength;
+
+    // Pulsing intensity
+    float pulse = sin(time * 5.0) * pulse_strength;
+
+    // Final intensity
+    float final_amount = damage_amount * edge_factor + noise + pulse;
+    final_amount = clamp(final_amount, 0.0, 1.0);
+	
+	vec4 base = texture(screen_texture, SCREEN_UV);
+	float gray = dot(base.rgb, vec3(0.299, 0.587, 0.114));
+	base.rgb = mix(base.rgb, vec3(gray), edge_factor_gray * damage_amount);
+	
+    vec4 red_tint = vec4(1.0, 0.0, 0.0, 1.0);
+    COLOR = mix(base, red_tint, final_amount);
+}

assets/shaders/screen_low_hp.gdshader.uid

@@ -0,0 +1 @@
+uid://dnwiwj1ak1bu5