From 71bd34ac5bc1df8bae3974d53180d3918fa987d5 Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonygarnockjones@gmail.com>
Date: Fri, 23 Oct 2015 21:56:47 -0400
Subject: [PATCH] Add interframe time delta to frame-events

---
 prospect-gl/2d.rkt             | 20 ++++++++++++--------
 prospect-gl/examples/basic.rkt | 22 +++++++++++-----------
 2 files changed, 23 insertions(+), 19 deletions(-)

diff --git a/prospect-gl/2d.rkt b/prospect-gl/2d.rkt
index 8079e83..c7844f4 100644
--- a/prospect-gl/2d.rkt
+++ b/prospect-gl/2d.rkt
@@ -34,7 +34,7 @@
 (struct window (width height) #:transparent)
 
 ;; Message sent by world. Describes frame about to be rendered.
-(struct frame-event (counter timestamp target-frame-rate) #:transparent)
+(struct frame-event (counter timestamp elapsed-ms target-frame-rate) #:transparent)
 
 ;; Message sent by world. Describes a key event. Key is a sealed
 ;; key-event%. `press?` is #t when the key is pressed (or
@@ -223,6 +223,7 @@
 
     (define counter 0)
     (define start-time (current-inexact-milliseconds))
+    (define prev-frame-time start-time)
     (define/public (sim-time)
       (- (current-inexact-milliseconds) start-time))
 
@@ -239,18 +240,16 @@
     (define world (make-world boot-actions))
     (define event-queue (make-queue))
 
-    (define target-frame-rate 30)
-    (define frame-count 0)
+    (define target-frame-rate 60)
 
     ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
     (define (sleep-and-refresh)
-      (define target-sim-time (* frame-count (/ target-frame-rate)))
+      (define target-sim-time (* counter (/ target-frame-rate)))
       (define sleep-time (- target-sim-time (/ (sim-time) 1000.0)))
       (when (positive? sleep-time)
         (sleep/yield sleep-time))
-      (refresh)
-      (set! frame-count (+ frame-count 1)))
+      (refresh))
 
     (define/public (set-target-frame-rate! r)
       (set! target-frame-rate r))
@@ -320,8 +319,13 @@
     (define/override (on-paint)
       (with-gl-context
         (lambda ()
-          (inject-event! (message (frame-event counter (sim-time) target-frame-rate)))
-          (set! counter (+ counter 1))
+          (let ((this-frame-time (sim-time)))
+            (inject-event! (message (frame-event counter
+                                                 this-frame-time
+                                                 (- this-frame-time prev-frame-time)
+                                                 target-frame-rate)))
+            (set! counter (+ counter 1))
+            (set! prev-frame-time this-frame-time))
           (quiesce!)
           (unless initialised?
             (glBlendFunc GL_ONE GL_ONE_MINUS_SRC_ALPHA) ;; premultiplied
diff --git a/prospect-gl/examples/basic.rkt b/prospect-gl/examples/basic.rkt
index 4cf126c..d7edf7d 100644
--- a/prospect-gl/examples/basic.rkt
+++ b/prospect-gl/examples/basic.rkt
@@ -45,35 +45,35 @@
                 (patch-project/set/single p key-pressed-projection))
               (define new-keys-down (set-subtract (set-union keys-down added) removed))
               (transition (list x y new-keys-down) '())]
-             [(message (at-meta (frame-event _ _ target-frame-rate)))
+             [(message (at-meta (frame-event _ _ elapsed-ms _)))
               (define-values (old-x old-y) (values x y))
-              (define speed (/ 360.0 target-frame-rate))
-              (let* ((x (if (set-member? keys-down 'left) (- x speed) x))
-                     (x (if (set-member? keys-down 'right) (+ x speed) x))
-                     (y (if (set-member? keys-down 'up) (- y speed) y))
-                     (y (if (set-member? keys-down 'down) (+ y speed) y)))
+              (define distance (* 0.360 elapsed-ms))
+              (let* ((x (if (set-member? keys-down 'left) (- x distance) x))
+                     (x (if (set-member? keys-down 'right) (+ x distance) x))
+                     (y (if (set-member? keys-down 'up) (- y distance) y))
+                     (y (if (set-member? keys-down 'down) (+ y distance) y)))
                 (and (not (and (= x old-x) (= y old-y)))
                      (move-to x y keys-down)))]
              [_ #f]))
          (list 100 100 (set))
          (update-sprites
           (simple-sprite -0.5 100 100 (image-width CC) (image-height CC) CC))
-         (sub (frame-event ? ? ?) #:meta-level 1)
+         (sub (frame-event ? ? ? ?) #:meta-level 1)
          (sub (key-pressed ?))))
 
 (define (spawn-frame-counter)
   (spawn (lambda (e s)
            (match e
-             [(message (at-meta (frame-event counter elapsed-ms _)))
-              (and (> elapsed-ms 0)
-                   (let ((i (text (format "~a fps" (/ counter (/ elapsed-ms 1000.0))) 22 "black")))
+             [(message (at-meta (frame-event counter sim-time-ms _ _)))
+              (and (> sim-time-ms 0)
+                   (let ((i (text (format "~a fps" (/ counter (/ sim-time-ms 1000.0))) 22 "black")))
                      (transition s (update-sprites (simple-sprite -10 300 10
                                                                   (image-width i)
                                                                   (image-height i)
                                                                   i)))))]
              [_ #f]))
          (void)
-         (sub (frame-event ? ? ?) #:meta-level 1)))
+         (sub (frame-event ? ? ? ?) #:meta-level 1)))
 
 (2d-world (spawn-keyboard-integrator)
           (spawn-background)