(define-shader-entity background (renderable)
  ((name :initform 'background)
   (texture :initform (error "BACKGROUND REQUIRED") :accessor texture)
   (vertex-array :initform (// 'trial:trial 'trial::fullscreen-square) :accessor vertex-array)))

(defmethod render ((background background) (program shader-program))
  (let ((vao (vertex-array background)))
    (setf (uniform program "view_size") (vec2 (max 1 (width *context*)) (max 1 (height *context*))))
    (setf (uniform program "view_matrix") (minv *view-matrix*))
    (setf (uniform program "tex_image") 0)
    (gl:active-texture :texture0)
    (gl:bind-texture :texture-2d (gl-name (texture background)))
    (unwind-protect
         (%gl:draw-elements :triangles (size vao) :unsigned-int 0)
      (gl:bind-vertex-array 0))))

(defmethod stage :after ((entity background) (area staging-area))
  (stage (vertex-array entity) area)
  (stage (texture entity) area))

(define-class-shader (background :vertex-shader)
  "layout (location = 0) in vec3 vertex;
layout (location = 1) in vec2 vertex_uv;
uniform sampler2D tex_image;
uniform mat4 view_matrix;
uniform vec2 view_size;
uniform vec2 parallax = vec2(0.5,0.5);
uniform vec2 offset = vec2(0, 0);
uinform vec2 scaling = vec2(0.5, 0.5);
out vec2 map_coord;
out vec2 world_xy;
void main(){
  gl_Position = vec4(vertex, 1);
  world_xy = (view_matrix*vec4(vertex_uv*view_size,0,1)).xy;
  vec2 size_a = textureSize(tex_image, 0).xy;
  map_coord = (view_matrix * vec4(vertex_uv*view_size*parallax, 0, 1)).xy;
  map_coord += size_a/2 + offset;
  map_coord /= parallax * scaling * size_a;
}")

(define-class-shader (background :fragment-shader)
  "uniform sampler2D tex_image;
in vec2 map_coord;
in vec2 world_xy;
out vec4 color;
void main(){
  color = texture(tex_image, map_coord);
}")