render: Merge <group> into <model>.

* sly/render/model.scm (<model>): Add 'transform' and 'children' fields.
  (model-transform, model-children): New accessors.
  (make-model): Add 'transform' and 'children' keyword arguments.
  (set-transform-identity!): New procedure.
  (draw-model): Recursively draw model and all children.
  (model-paint, model-blend): Renamed.
  (paint, blend, group, group*, move, place): New procedures.
* sly/render/group.scm: Delete.
* Makefile.am (SOURCES): Remove it.
* examples/2048/2048.scm: Update example.
* examples/animation.scm: Likewise.
* examples/font.scm: Likewise.
* examples/joystick.scm: Likewise.
* examples/mines/mines.scm: Likewise.
* examples/simple.scm: Likewise.
* examples/tilemap.scm: Likewise.
* README.org (Example): Update sample code.

11 files changed, 171 insertions, 252 deletions

diff --git a/Makefile.am b/Makefile.am
index 59b59a9..f9bc879 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -49,7 +49,6 @@ SOURCES = 					\
   sly/render/framebuffer.scm			\
   sly/render/mesh.scm				\
   sly/render/model.scm				\
-  sly/render/group.scm				\
   sly/render/texture.scm			\
   sly/render/shader.scm				\
   sly/render/shape.scm				\
diff --git a/README.org b/README.org
index 7316c06..c534276 100644
--- a/README.org
+++ b/README.org
@@ -33,16 +33,15 @@
                   (sly window)
                   (sly math vector)
                   (sly render camera)
-                  (sly render group)
+                  (sly render model)
                   (sly render sprite))
 
      (define scene
-       (group-move (vector2 320 240)
-                   (group (load-sprite "gnu.png"))))
+       (move (vector2 320 240) (load-sprite "gnu.png")))
 
      (define camera (orthographic-camera 640 480))
 
-     (add-hook! draw-hook (lambda _ (draw-group scene camera)))
+     (add-hook! draw-hook (lambda _ (draw-model scene camera)))
 
      (with-window (make-window #:title "Hello, world!")
        (start-game-loop))
diff --git a/examples/2048/2048.scm b/examples/2048/2048.scm
index 73cb5f6..7bffe0b 100755
--- a/examples/2048/2048.scm
+++ b/examples/2048/2048.scm
@@ -37,7 +37,6 @@
              (sly render color)
              (sly render context)
              (sly render font)
-             (sly render group)
              (sly render model)
              (sly render sprite)
              (sly render texture)
@@ -324,14 +323,15 @@
          (label (assoc-ref tile-label-cache n))
          (label-color (tile-text-color n))
          (bg-color (tile-bg-color n)))
-    (group-move (vector2 (* x w) (* y h))
-                (make-group
-                 (cons (model-paint bg-color tile-sprite)
-                       (if (zero? n)
-                           '()
-                           (list (group-move (vector2 (/ w 2) (/ h 2))
-                                             (group (model-paint label-color
-                                                                 label))))))))))
+    (move (vector2 (* x w) (* y h))
+          (group*
+           (cons (paint bg-color tile-sprite)
+                 (if (zero? n)
+                     '()
+                     (list (move (vector2 (/ w 2) (/ h 2))
+                                 (group
+                                  (paint label-color
+                                         label))))))))))
 
 (define window-width 640)
 (define window-height 480)
@@ -347,7 +347,7 @@
 
 (define-signal tiles
   (signal-map (lambda (board)
-                (make-group
+                (group*
                  (append-map
                   (match-lambda
                    ((y (row ...))
@@ -361,24 +361,24 @@
 (define play-again-font (load-default-font 16))
 
 (define-signal status-message
-  (let ((play-again (model-paint black (label play-again-font
-                                              "Press N to play again"
-                                              #:anchor 'top-center)))
-        (game-over (model-paint black (label font "GAME OVER"
-                                             #:anchor 'bottom-center)))
-        (you-win (model-paint black (label font "YOU WIN!"
-                                           #:anchor 'bottom-center))))
+  (let ((play-again (paint black (label play-again-font
+                                        "Press N to play again"
+                                        #:anchor 'top-center)))
+        (game-over (paint black (label font "GAME OVER"
+                                       #:anchor 'bottom-center)))
+        (you-win (paint black (label font "YOU WIN!"
+                                     #:anchor 'bottom-center))))
     (signal-map
            (lambda (board)
              (let ((message (cond
                              ((board-lose? board) game-over)
                              ((board-win? board) you-win)
                              (else #f))))
-               (group-move (vector2 (/ board-width 2)
-                                    (/ board-height 2))
-                           (make-group (if message
-                                           (list message play-again)
-                                           '())))))
+               (move (vector2 (/ board-width 2)
+                              (/ board-height 2))
+                     (group* (if message
+                                 (list message play-again)
+                                 '())))))
            board)))
 
 (define instruction-font (load-default-font 16))
@@ -387,11 +387,11 @@
   "Use the arrow keys to join the numbers and get to the 2048 tile!")
 
 (define-signal instructions
-  (group-move (vector2 (/ board-width 2) (- window-height (vy center-pos)))
-              (group
-               (model-paint text-color-1
-                            (label instruction-font instruction-text
-                                   #:anchor 'top-center)))))
+  (move (vector2 (/ board-width 2) (- window-height (vy center-pos)))
+        (group
+         (paint text-color-1
+                (label instruction-font instruction-text
+                       #:anchor 'top-center)))))
 
 (define score-header-font (load-default-font 14))
 (define score-font (load-default-font 22))
@@ -408,13 +408,13 @@
     (signal-map (lambda (score timer)
                   (let ((score (label score-font (number->string score)
                                       #:anchor 'center)))
-                    (group-move (vector2 x (- window-height 28))
-                                (group
-                                 (model-paint text-color-1 header)
-                                 (group-move (position-tween timer)
-                                             (group
-                                              (model-paint (color-tween timer)
-                                                           score)))))))
+                    (move (vector2 x (- window-height 28))
+                          (group
+                           (paint text-color-1 header)
+                           (move (position-tween timer)
+                                 (group
+                                  (paint (color-tween timer)
+                                         score)))))))
                 score
                 (signal-drop (lambda (t) (> t duration))
                              0 (signal-since 1 score)))))
@@ -427,7 +427,7 @@
                (- board-width (/ board-width 4))))
 
 (define-signal 2048-scene
-  (signal-map (cut group-move center-pos <>)
+  (signal-map (cut move center-pos <>)
               (signal-map group instructions tiles score
                           best-score status-message)))
 
@@ -438,7 +438,7 @@
 
 (define (draw-2048 dt alpha)
   (signal-let ((scene 2048-scene))
-    (draw-group scene camera)))
+    (draw-model scene camera)))
 
 ;;;
 ;;; Initialization
diff --git a/examples/animation.scm b/examples/animation.scm
index cc84c27..ef736d2 100644
--- a/examples/animation.scm
+++ b/examples/animation.scm
@@ -22,7 +22,7 @@
              (sly math tween)
              (sly math vector)
              (sly render camera)
-             (sly render group)
+             (sly render model)
              (sly render sprite)
              (sly render tileset))
 
@@ -45,18 +45,15 @@
     (tween (compose floor lerp) (compose ease-linear ease-loop)
            0 frame-count (* frame-count frame-rate))))
 
-(define-signal timer
-  (signal-fold + 0 (signal-every 1)))
-
 (define-signal scene
   (signal-map (lambda (time)
-                (group-move (position-tween time)
-                            (group (vector-ref walk-cycle (frame-tween time)))))
-              timer))
+                (move (position-tween time)
+                      (vector-ref walk-cycle (frame-tween time))))
+              (signal-timer)))
 
 (define camera (orthographic-camera 640 480))
 
-(add-hook! draw-hook (lambda _ (draw-group (signal-ref scene) camera)))
+(add-hook! draw-hook (lambda _ (draw-model (signal-ref scene) camera)))
 
 (with-window (make-window #:title "Animation")
   (start-game-loop))
diff --git a/examples/font.scm b/examples/font.scm
index 177879e..b3fcfc3 100644
--- a/examples/font.scm
+++ b/examples/font.scm
@@ -24,7 +24,7 @@
              (sly render color)
              (sly render font)
              (sly render model)
-             (sly render group)
+             (sly render model)
              (sly input mouse))
 
 (load "common.scm")
@@ -34,21 +34,20 @@
 (define font (load-default-font 18))
 
 (define-signal message-label
-  (group-move (vector2 320 240)
-              (group
-               (label font "The quick brown fox jumped over the lazy dog."
-                      #:anchor 'center))))
+  (move (vector2 320 240)
+        (label font "The quick brown fox jumped over the lazy dog."
+               #:anchor 'center)))
 
 (define-signal fps-label
   (signal-map (lambda (fps)
                 (let ((text (format #f "FPS: ~d" fps)))
-                  (group-move (vector2 0 480) (group (label font text)))))
+                  (move (vector2 0 480) (label font text))))
               fps))
 
 (define-signal mouse-label
   (signal-map (lambda (p)
                 (let ((text (format #f "Mouse: (~d, ~d)" (vx p) (vy p))))
-                  (group-move (vector2 0 460) (group (label font text)))))
+                  (move (vector2 0 460) (label font text))))
               (signal-throttle 5 mouse-position)))
 
 (define-signal scene
@@ -56,7 +55,7 @@
 
 (define camera (orthographic-camera 640 480))
 
-(add-hook! draw-hook (lambda _ (draw-group (signal-ref scene) camera)))
+(add-hook! draw-hook (lambda _ (draw-model (signal-ref scene) camera)))
 
 (with-window (make-window #:title "Fonts")
   (start-game-loop))
diff --git a/examples/joystick.scm b/examples/joystick.scm
index 57955c9..7aa83c7 100644
--- a/examples/joystick.scm
+++ b/examples/joystick.scm
@@ -28,7 +28,6 @@
              (sly input joystick)
              (sly render camera)
              (sly render model)
-             (sly render group)
              (sly render sprite)
              (sly render texture)
              (sly render font))
@@ -56,8 +55,7 @@
 
 (define-signal caption
   (signal-map (lambda (text)
-                (group-move (vector2 -76 -90)
-                            (group (label font text))))
+                (move (vector2 -76 -90) (label font text)))
               (signal-merge
                (make-signal "Press a button")
                (button-caption-signal "Hello there" 0)
@@ -67,13 +65,12 @@
 
 (define-signal scene
   (signal-map (lambda (position caption)
-                (group-move position
-                            (group player caption)))
+                (move position (group player caption)))
               player-position caption))
 
 (define camera (orthographic-camera (vx resolution) (vy resolution)))
 
-(add-hook! draw-hook (lambda _ (draw-group (signal-ref scene) camera)))
+(add-hook! draw-hook (lambda _ (draw-model (signal-ref scene) camera)))
 
 (add-hook! joystick-axis-hook
            (lambda (which axis value)
diff --git a/examples/mines/mines.scm b/examples/mines/mines.scm
index 97d77b0..663e146 100644
--- a/examples/mines/mines.scm
+++ b/examples/mines/mines.scm
@@ -37,7 +37,6 @@
              (sly render camera)
              (sly render color)
              (sly render font)
-             (sly render group)
              (sly render model)
              (sly render sprite)
              (sly input keyboard)
@@ -332,24 +331,24 @@
     (lambda (tile)
       ;; A tile may or may not have an overlay, so we do a little
       ;; quasiquoting magic to build the right list.
-      (make-group
+      (group*
        `(,(tile-base-sprite tile)
          ,@(let ((overlay (tile-overlay-sprite tile)))
             (if overlay
-                (list (group-place offset (group overlay)))
+                (list (place offset (group overlay)))
                 '())))))))
 
 (define-signal board-view
   (signal-map (lambda (board)
                 (define (draw-column tile x)
-                  (group-move (vector2 (* x tile-size) 0)
-                              (draw-tile tile)))
+                  (move (vector2 (* x tile-size) 0)
+                        (draw-tile tile)))
 
                 (define (draw-row row y)
-                  (group-move (vector2 0 (* y tile-size))
-                              (make-group (enumerate-map draw-column row))))
+                  (move (vector2 0 (* y tile-size))
+                        (group* (enumerate-map draw-column row))))
 
-                (make-group (enumerate-map draw-row board)))
+                (group* (enumerate-map draw-row board)))
               board))
 
 (define-signal status-message
@@ -357,9 +356,9 @@
                 (define (make-message message)
                   (label font message #:anchor 'center))
 
-                (group-move
+                (move
                  (vector2 (/ (vx resolution) 2) (- (vy resolution) 64))
-                 (make-group
+                 (group*
                   (cond
                    ((board-lose? board)
                     (list (make-message "GAME OVER - Press N to play again")))
@@ -372,7 +371,7 @@
   (signal-map (lambda (board-view status center-position)
                 (group
                  status
-                 (group-move center-position board-view)))
+                 (move center-position board-view)))
               board-view status-message center-position))
 
 (define camera
@@ -382,7 +381,7 @@
                              #:clear-color tango-dark-plum)))
 
 (define (draw-scene dt alpha)
-  (draw-group (signal-ref scene) camera))
+  (draw-model (signal-ref scene) camera))
 
 ;;;
 ;;; Initialization
diff --git a/examples/simple.scm b/examples/simple.scm
index 9560f6f..1596cfe 100644
--- a/examples/simple.scm
+++ b/examples/simple.scm
@@ -17,20 +17,23 @@
 
 (use-modules (sly game)
              (sly window)
+             (sly utils)
              (sly math vector)
              (sly render camera)
-             (sly render group)
-             (sly render sprite))
+             (sly render model)
+             (sly render sprite)
+             (sly render color))
 
 (load "common.scm")
 
 (define scene
-  (group-move (vector2 320 240)
-              (group (load-sprite "images/p1_front.png"))))
+  (chain (load-sprite "images/p1_front.png")
+    (paint red)
+    (move (vector2 320 240))))
 
 (define camera (orthographic-camera 640 480))
 
-(add-hook! draw-hook (lambda _ (draw-group scene camera)))
+(add-hook! draw-hook (lambda _ (draw-model scene camera)))
 
 (with-window (make-window #:title "Simple Sprite Demo")
   (start-game-loop))
diff --git a/examples/tilemap.scm b/examples/tilemap.scm
index 1615c88..5e54bde 100644
--- a/examples/tilemap.scm
+++ b/examples/tilemap.scm
@@ -26,7 +26,6 @@
              (sly utils)
              (sly render camera)
              (sly render color)
-             (sly render group)
              (sly render model)
              (sly render mesh)
              (sly render shader)
@@ -91,15 +90,15 @@
      (225 225 176 242 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65))))
 
 (define scene
-  (group-move (v- (vector2 320 240)
-                  (v* (vector2 tile-width tile-height)
-                      (vector2 10 15/2)))
-              (apply group (compile-tile-layer map-tiles 32 32))))
+  (move (v- (vector2 320 240)
+            (v* (vector2 tile-width tile-height)
+                (vector2 10 15/2)))
+        (group* (compile-tile-layer map-tiles 32 32))))
 
 (define camera
   (orthographic-camera 640 480))
 
-(add-hook! draw-hook (lambda _ (draw-group scene camera)))
+(add-hook! draw-hook (lambda _ (draw-model scene camera)))
 
 (with-window (make-window #:title "Tilemap")
   (start-game-loop))
diff --git a/sly/render/group.scm b/sly/render/group.scm
deleted file mode 100644
index 9cfc062..0000000
--- a/sly/render/group.scm
+++ /dev/null
@@ -1,137 +0,0 @@
-;;; Sly
-;;; Copyright (C) 2014 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:
-;;
-;; Hierarchy of renderable objects using a directed acyclic graph
-;; structure.
-;;
-;;; Code:
-
-(define-module (sly render group)
-  #:use-module (ice-9 match)
-  #:use-module (srfi srfi-9)
-  #:use-module (sly math transform)
-  #:use-module (sly render shader)
-  #:use-module (sly render texture)
-  #:use-module (sly render utils)
-  #:use-module (sly render camera)
-  #:use-module (sly render context)
-  #:use-module (sly render model)
-  #:export (make-group group
-            group?
-            group-transform group-visible? group-children
-            draw-group
-            group-move group-place group-show))
-
-;;;
-;;; Group
-;;;
-
-;; The composite object of the scene graph.  Groups have zero or more
-;; children, which may be groups or models.
-(define-record-type <group>
-  (%make-group transform visible? children)
-  group?
-  (transform group-transform)
-  (visible? group-visible?)
-  (children group-children))
-
-(define* (make-group children #:optional #:key (transform identity-transform)
-                     (visible? #t))
-  "Create a new group containing CHILDREN in which each child is
-rendered relative to TRANSFORM.  The VISIBLE? flag determines whether
-or not to render child nodes."
-  (%make-group transform visible? children))
-
-(define (%draw-group group parent-transform view context)
-  (match group
-    (($ <group> transform visible? children)
-     (when visible?
-       (with-temp-transform context world-transform
-         (transform*! world-transform transform parent-transform)
-         (for-each (lambda (child)
-                     (let ((draw (match child
-                                   ((? group? group) %draw-group)
-                                   ((? model? model) draw-model))))
-                       (draw child world-transform view context)))
-                   children))))))
-
-(define (set-transform-identity! t)
-  (let ((matrix (transform-matrix t)))
-    (array-set! matrix 1 0 0)
-    (array-set! matrix 0 0 1)
-    (array-set! matrix 0 0 2)
-    (array-set! matrix 0 0 3)
-    (array-set! matrix 0 1 0)
-    (array-set! matrix 1 1 1)
-    (array-set! matrix 0 1 2)
-    (array-set! matrix 0 1 3)
-    (array-set! matrix 0 2 0)
-    (array-set! matrix 0 2 1)
-    (array-set! matrix 1 2 2)
-    (array-set! matrix 0 2 3)
-    (array-set! matrix 0 3 0)
-    (array-set! matrix 0 3 1)
-    (array-set! matrix 0 3 2)
-    (array-set! matrix 1 3 3)))
-
-(define draw-group
-  (let ((context (make-render-context)))
-    (lambda* (group camera #:optional (context context))
-      "Draw the scene defined by GROUP as viewed by CAMERA, with the given
-render CONTEXT."
-      (with-render-context context
-        (with-temp-transform context view
-          (transform*! view
-                       (camera-location camera)
-                       (camera-projection camera))
-          (with-temp-transform context base-transform
-            (set-transform-identity! base-transform)
-            (apply-viewport (camera-viewport camera))
-            (%draw-group group base-transform view context)))))))
-
-;;;
-;;; Utility Procedures
-;;;
-
-(define (group . children)
-  "Create a new group containing the list of CHILDREN."
-  (make-group children))
-
-(define (group-move position group)
-  "Create a new group in which the list of CHILDREN are translated by
-the vector POSITION."
-  (match group
-    (($ <group> transform visible? children)
-     (%make-group (transform* transform (translate position))
-                  visible? children))))
-
-(define (group-place transform group)
-  "Create a new group in which the tranformation matrices of the
-CHILDREN are multiplied by TRANSFORM."
-  (match group
-    (($ <group> original-transform visible? children)
-     (%make-group (transform* original-transform transform)
-                  visible? children))))
-
-(define (group-show visible? group)
-  "Create a new group in which the visibility of the list of
-CHILDREN is determined by the VISIBLE? flag."
-  (match group
-    (($ <group> transform _ children)
-     (%make-group transform visible? children))))
diff --git a/sly/render/model.scm b/sly/render/model.scm
index b42dd64..01c1223 100644
--- a/sly/render/model.scm
+++ b/sly/render/model.scm
@@ -40,29 +40,35 @@
   #:use-module (sly render mesh)
   #:export (make-model model model-inherit
             model?
-            model-mesh model-texture model-shader model-color
-            model-blend-mode model-depth-test?
+            model-mesh model-transform model-texture model-shader model-color
+            model-blend-mode model-depth-test? model-children
             draw-model
-            model-paint model-blend))
+            paint blend group group* move place))
 
 ;; Representation of a single OpenGL render call.
 (define-record-type <model>
-  (%make-model mesh texture shader color blend-mode depth-test?)
+  (%make-model mesh transform texture shader color blend-mode
+               depth-test? children)
   model?
   (mesh model-mesh)
+  (transform model-transform)
   (texture model-texture)
   (shader model-shader)
   (color model-color)
   (blend-mode model-blend-mode)
-  (depth-test? model-depth-test?))
+  (depth-test? model-depth-test?)
+  (children model-children))
 
-(define* (make-model #:optional #:key (mesh #f) (texture #f) (shader #f)
-                     (color white) (blend-mode default-blend-mode)
-                     (depth-test? #t))
+(define* (make-model #:optional #:key (mesh null-mesh)
+                     (transform identity-transform) (texture null-texture)
+                     (shader (load-default-shader)) (color white)
+                     (blend-mode default-blend-mode) (depth-test? #t)
+                     (children '()))
   "Create a new model from MESH and the given rendering state.  When
 rendering, TEXTURE and SHADER are bound, BLEND-MODE and DEPTH-TEST?
 are set, and the COLOR uniform variable is set."
-  (%make-model mesh texture shader color blend-mode depth-test?))
+  (%make-model mesh transform texture shader color blend-mode
+               depth-test? children))
 
 (define model make-model)
 
@@ -90,35 +96,93 @@ changing the fields specified in KWARGS."
                             (struct-ref original index))))
                     fields field-indices))))))
 
-(define (draw-model model world-transform view context)
-  "Render MODEL by applying its transform (multiplied by VIEW), texture,
+(define (set-transform-identity! t)
+  (let ((matrix (transform-matrix t)))
+    (array-set! matrix 1 0 0)
+    (array-set! matrix 0 0 1)
+    (array-set! matrix 0 0 2)
+    (array-set! matrix 0 0 3)
+    (array-set! matrix 0 1 0)
+    (array-set! matrix 1 1 1)
+    (array-set! matrix 0 1 2)
+    (array-set! matrix 0 1 3)
+    (array-set! matrix 0 2 0)
+    (array-set! matrix 0 2 1)
+    (array-set! matrix 1 2 2)
+    (array-set! matrix 0 2 3)
+    (array-set! matrix 0 3 0)
+    (array-set! matrix 0 3 1)
+    (array-set! matrix 0 3 2)
+    (array-set! matrix 1 3 3)))
+
+(define draw-model
+  (let ((context (make-render-context)))
+    (lambda* (model camera #:optional (context context))
+      "Render MODEL by applying its transform (multiplied by VIEW), texture,
 shader, vertex array, uniforms, blend mode, etc. to the render
 CONTEXT."
-  (match model
-    (($ <model> mesh texture shader color blend-mode depth-test?)
-     (with-temp-transform context mvp
-       (transform*! mvp world-transform view)
-       (set-render-context-depth-test?! context depth-test?)
-       (set-render-context-blend-mode! context blend-mode)
-       (set-render-context-shader! context shader)
-       (set-render-context-mesh! context mesh)
-       (set-render-context-texture! context texture)
-       ;; TODO: Support user-defined uniforms.
-       (uniform-set! shader "mvp" mvp)
-       (uniform-set! shader "color" color)
-       (glDrawElements (begin-mode triangles)
-                       (mesh-length mesh)
-                       (data-type unsigned-int)
-                       %null-pointer)))))
+      (define (iter model world-transform view context)
+        (match model
+          (($ <model> mesh transform texture shader color blend-mode
+                      depth-test? children)
+           (with-temp-transform context new-transform
+             (transform*! new-transform transform world-transform)
+             (with-temp-transform context mvp
+              (transform*! mvp new-transform view)
+              (set-render-context-depth-test?! context depth-test?)
+              (set-render-context-blend-mode! context blend-mode)
+              (set-render-context-shader! context shader)
+              (set-render-context-mesh! context mesh)
+              (set-render-context-texture! context texture)
+              ;; TODO: Support user-defined uniforms.
+              (uniform-set! shader "mvp" mvp)
+              (uniform-set! shader "color" color)
+              (glDrawElements (begin-mode triangles)
+                              (mesh-length mesh)
+                              (data-type unsigned-int)
+                              %null-pointer))
+             (for-each (lambda (child)
+                         (iter child new-transform view context))
+                       children)))))
+
+      (with-render-context context
+        (with-temp-transform context view
+          (transform*! view
+                       (camera-location camera)
+                       (camera-projection camera))
+          (with-temp-transform context base-transform
+            (set-transform-identity! base-transform)
+            (apply-viewport (camera-viewport camera))
+            (iter model base-transform view context)))))))
 
 ;;;
 ;;; Utility Procedures
 ;;;
 
-(define (model-paint color model)
+(define (paint color model)
   "Create a copy of MODEL, but with a new COLOR."
   (model-inherit model #:color color))
 
-(define (model-blend blend-mode model)
+(define (blend blend-mode model)
   "Create a copy of MODEL, but with a new BLEND-MODE."
   (model-inherit model #:blend-mode blend-mode))
+
+(define (group . children)
+  "Create a new group containing the list of CHILDREN."
+  (make-model #:children children))
+
+(define (group* children)
+  "Create a new group containing the list of CHILDREN."
+  (make-model #:children children))
+
+(define (move position model)
+  "Create a new group in which the list of CHILDREN are translated by
+the vector POSITION."
+  (model-inherit model #:transform (transform* (model-transform model)
+                                               (translate position))))
+
+(define (place transform model)
+  "Create a new group in which the tranformation matrices of the
+CHILDREN are multiplied by TRANSFORM."
+  (model-inherit model #:transform (transform* (model-transform model)
+                                               transform)))