shader_type canvas_item;

uniform sampler2D front_texture : source_color, filter_linear_mipmap;
uniform sampler2D back_texture : source_color, filter_linear_mipmap;

uniform sampler2D spine_shadow_gradient : source_color, filter_linear, repeat_disable;
uniform float shadow_intensity : hint_range(0.0, 1.0) = 0.0;
uniform float max_shadow_spread : hint_range(0.0, 1.0) = 0.35;

uniform bool debug_mode = false;
uniform bool is_backward = false;

void fragment() {
	vec2 uv_dx = dFdx(UV);
	vec2 uv_dy = dFdy(UV);
	float face_dir = uv_dx.x * uv_dy.y - uv_dx.y * uv_dy.x;
	bool is_front = face_dir > 0.0;

	vec4 base_color;
	
	if (is_front) {
		if (is_backward) {
			// Let's try flipping Y just in case they meant upside down!
			// And keep X normal, since UV goes 0 to 1.
			base_color = texture(front_texture, UV); 
		} else {
			base_color = texture(front_texture, UV);
		}
	} else {
		if (is_backward) {
			base_color = texture(back_texture, vec2(1.0 - UV.x, UV.y));
		} else {
			base_color = texture(back_texture, vec2(1.0 - UV.x, UV.y));
		}
	}

	float current_spread = max_shadow_spread * (shadow_intensity + 0.001);
	float sample_x = UV.x / current_spread;
	float shadow_alpha = texture(spine_shadow_gradient, vec2(sample_x, 0.0)).a;

	if (sample_x > 1.0) {
		shadow_alpha = 0.0;
	}

	shadow_alpha *= shadow_intensity;

	vec3 final_rgb = mix(base_color.rgb, vec3(0.0, 0.0, 0.0), shadow_alpha);
	COLOR = vec4(final_rgb, base_color.a);
}