;;; Sly
;;; Copyright (C) 2014, 2015 David Thompson <davet@gnu.org>
;;;
;;; Sly is free software: you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation, either version 3 of the License, or
;;; (at your option) any later version.
;;;
;;; Sly is distributed in the hope that it will be useful, but WITHOUT
;;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
;;; or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
;;; License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with this program.  If not, see
;;; <http://www.gnu.org/licenses/>.

;;; Commentary:
;;
;; Viewports.
;;
;;; Code:

(define-module (sly render viewport)
  #:use-module (ice-9 match)
  #:use-module (srfi srfi-9)
  #:use-module (gl)
  #:use-module (gl low-level)
  #:use-module (sly wrappers gl)
  #:use-module (sly math rect)
  #:use-module (sly utils)
  #:use-module (sly render color)
  #:export (make-viewport
            viewport?
            viewport-area
            viewport-clear-color
            viewport-clear-flags
            null-viewport
            %standard-clear-flags
            apply-viewport
            clear-viewport))
;;;
;;; Viewport
;;;

(define-record-type <viewport>
  (%make-viewport area clear-color clear-flags)
  viewport?
  (area viewport-area)
  (clear-color viewport-clear-color)
  (clear-flags viewport-clear-flags))

(define %standard-clear-flags '(color-buffer depth-buffer))

(define* (make-viewport area #:optional #:key (clear-color black)
                        (clear-flags %standard-clear-flags))
  "Create a viewport that covers the rectangle AREA of the window.
Fill the viewport with CLEAR-COLOR when clearing the screen.  Clear
the buffers denoted by the list of symbols in CLEAR-FLAGS.  Possible
values for CLEAR-FLAGS are 'color-buffer', 'depth-buffer',
'accum-buffer', and 'stencil-buffer'."
  (%make-viewport area clear-color clear-flags))

(define null-viewport (make-viewport (make-rect 0 0 0 0)))

(define clear-buffer-mask
  (memoize
   (lambda (flags)
     (apply logior
            ;; Map symbols to OpenGL constants.
            (map (match-lambda
                  ('depth-buffer 256)
                  ('accum-buffer 512)
                  ('stencil-buffer 1024)
                  ('color-buffer 16384))
                 flags)))))

(define (apply-viewport viewport)
  "Set the OpenGL state for VIEWPORT.  Clip rendering to the viewport
area, set the clear color, and clear necessary buffers."
  (gl-enable (enable-cap scissor-test))
  (let* ((rect (viewport-area viewport))
         (x (inexact->exact (rect-x rect)))
         (y (inexact->exact (rect-y rect)))
         (width (inexact->exact (rect-width rect)))
         (height (inexact->exact (rect-height rect))))
    (gl-viewport x y width height)
    (gl-scissor x y width height))
  (match (viewport-clear-color viewport)
    (($ <color> r g b a)
     (gl-clear-color r g b a))))

(define (clear-viewport viewport)
  "Clear the relevant OpenGL buffers VIEWPORT."
  (gl-clear (clear-buffer-mask (viewport-clear-flags viewport))))