diff --git a/README.org b/README.org
index ee0a1c0..b773813 100644
--- a/README.org
+++ b/README.org
@@ -11,38 +11,36 @@ on Linux and macOS.
 
 ** API
 
+High-level API:
+- ~F _display width‿height~ creates a window of the given dimensions
+  and displays its left operand. ~F~ should be a function returning a
+  color as an RGB triplet in the (0,1) range, and taking a list of two
+  elements ~x‿y~, where ~x~ and ~y~ are the pixel coordinates.
+
+Low-level API:
 - ~OpenWindow w‿h‿t~ opens a window of width ~w~ and height ~h~, with
   title ~t~. Returns a window handle.
 - ~CloseWindow w~ closes a window given its handle ~w~.
 - ~F _render w~ runs the function ~F~ at each coordinate of the window
-  ~w~ and set the corresponding pixel color. ~F~ should be a function
-  returning a color as an RGB triplet in the (0,1) range, and taking a
-  list of three elements ~x‿y‿t~, where ~x~ and ~y~ are the pixel
-  coordinates and ~t~ is the time.
+  ~w~ and set the corresponding pixel color. ~F~ is a function with
+  the same arguments as for ~_display~.
 
 ** Example
 
 See [[./example.bqn][example.bqn]].
 
 #+begin_src bqn
-f←•Import "fenster.bqn"
+f←•Import"fenster.bqn"
 
 size←500
 
-w←f.OpenWindow size‿size‿"bqn-fenster example"
-
-Rainbow←{𝕊x‿y‿t:
+Rainbow←{𝕊x‿y:
   x‿y↩x‿y÷size
-  t↩(100|t)÷100
   h←(x+y)÷2
   f.HSVtoRGB⟨360×h,1,1⟩
 }
 
-Rainbow f._render w
-
-•Delay 3
-
-f.CloseWindow w
+Rainbow f._display size‿size
 #+end_src
 
 [[./example.png]]
diff --git a/example.bqn b/example.bqn
index 75d75e2..dfeedd1 100644
--- a/example.bqn
+++ b/example.bqn
@@ -2,17 +2,10 @@ f←•Import"fenster.bqn"
 
 size←500
 
-w←f.OpenWindow size‿size‿"bqn-fenster example"
-
-Rainbow←{𝕊x‿y‿t:
+Rainbow←{𝕊x‿y:
   x‿y↩x‿y÷size
-  t↩(100|t)÷100
   h←(x+y)÷2
   f.HSVtoRGB⟨360×h,1,1⟩
 }
 
-Rainbow f._render w
-
-•Delay 3
-
-f.CloseWindow w
+Rainbow f._display size‿size
diff --git a/example.png b/example.png
index b788c74..7361a87 100644
Binary files a/example.png and b/example.png differ
diff --git a/fenster.bqn b/fenster.bqn
index 168578c..7580bf6 100644
--- a/fenster.bqn
+++ b/fenster.bqn
@@ -1,7 +1,10 @@
 ⟨
+  FensterLoop,
   OpenWindow,
   CloseWindow,
   _render,
+  RenderArray
+  _display,
   Black,
   White,
   Gray,
@@ -11,6 +14,8 @@
   HSVtoRGB
 ⟩⇐
 
+## FFI functions
+
 FensterFFI←"lib.so"⊸•FFI
 
 fensterOpen←FensterFFI"i8"‿"fenster_open"‿">*:i8"
@@ -24,7 +29,12 @@ fensterGetWidth←FensterFFI"u32"‿"fenster_get_width"‿">*:i8"
 fensterGetHeight←FensterFFI"u32"‿"fenster_get_height"‿">*:i8"
 fensterGetPixel←FensterFFI"u32"‿"fenster_get_pixel"‿"*:i8"‿"u32"‿"u32"
 fensterSetPixel←FensterFFI""‿"fenster_set_pixel"‿"*:i8"‿"u32"‿"u32"‿"u32"
+fensterSetArray←FensterFFI""‿"fenster_set_array"‿"*:i8"‿"u32"‿"u32"‿"u32"‿"u32"‿"*u32"
 
+## Low-level functions
+
+# Open a window with given width, height, and title
+# Returns the window's handle.
 OpenWindow←{𝕊w‿h‿t:
   f←FensterInit⟨w,h,1+≠t,t∾@⟩
   FensterOpen f
@@ -32,17 +42,19 @@ OpenWindow←{𝕊w‿h‿t:
   f
 }
 
+# Close a window.
 CloseWindow←{𝕊f:
   FensterClose f
   FensterLoop f
 }
 
+# Render a function in the window.
+# The function should take a list x‿y of coordinates and return an r‿𝕘‿b color.
 _render←{Func _𝕣 f:
   w←FensterGetWidth f
   h←FensterGetHeight f
-  t←FensterTime⟨⟩
   {𝕊x‿y:
-    r‿g‿b←Func x‿y‿t
+    r‿g‿b←Func x‿y
     c←+´(⌽256⋆↕3)×⌊r‿g‿b×255
     FensterSetPixel f‿x‿y‿c
   }¨⥊(↕w)⋈⌜↕h
@@ -50,6 +62,24 @@ _render←{Func _𝕣 f:
   f
 }
 
+# Render an array directly.
+RenderArray←{buf𝕊f:
+  bufw‿bufh←≢buf
+  FensterSetArray⟨f,0,0,bufw,bufh,buf⟩
+}
+
+## High-level function
+
+# Open a window, render a function in it, and wait for the user to close it.
+_display←{Func _𝕣 w‿h:
+  win←OpenWindow w‿h‿"bqn-fenster"
+  Func _render win
+  ⊢•_while_(¬FensterLoop)win
+  CloseWindow win
+}
+
+## Utilities
+
 Black←0‿0‿0˙
 White←1‿1‿1˙
 Gray←{𝕩‿𝕩‿𝕩}
diff --git a/lib.c b/lib.c
index dc668dd..6c6cea6 100644
--- a/lib.c
+++ b/lib.c
@@ -29,3 +29,16 @@ uint32_t fenster_get_pixel(struct fenster *f, int i, int j) {
 void fenster_set_pixel(struct fenster *f, int i, int j, uint32_t color) {
   fenster_pixel(f, i, j) = color;
 }
+
+void fenster_set_array(struct fenster *f, int start_i, int start_j,
+                       int bufwidth, int bufheight,
+                       uint32_t buf[bufwidth * bufheight]) {
+  int max_i = start_i + bufwidth < f->width ? bufwidth : f->width - start_i;
+  int max_j = start_j + bufheight < f->height ? bufheight : f->height - start_j;
+  for (int i = 0; i < max_i; i++) {
+    for (int j = 0; j < max_j; j++) {
+      fenster_pixel(f, start_i + i, start_j + j) = buf[j * bufwidth + i];
+    }
+  }
+  fenster_loop(f);
+}