diff --git a/src/fart.propane b/src/fart.propane
index f832e9b..5532859 100644
--- a/src/fart.propane
+++ b/src/fart.propane
@@ -1,5 +1,25 @@
 tree;
 tree_prefix P;
+ptype double;
+
+<<header
+/* User terminate codes. */
+#define PARSER_TERMINATE_NUMBER_RANGE 1u
+>>
+
+<<
+#include <charconv>
+
+/* Store the numeric value of the matched text in *out. Returns false if the
+ * value cannot be represented. */
+static bool parse_number(uint8_t const * match_text, size_t match_length, double * out)
+{
+    const char * first = (const char *)match_text;
+    const char * last = first + match_length;
+    std::from_chars_result r = std::from_chars(first, last, *out);
+    return (r.ec == std::errc()) && (r.ptr == last);
+}
+>>
 
 drop /\s+/;
 
@@ -14,8 +34,18 @@ token rcurly /\}/;
 token scene;
 token width;
 
-token uint /\d+/;
-token number /-?\d+(\.\d+)?/;
+token uint /\d+/ <<
+    if (!parse_number(match_text, match_length, &$$))
+    {
+        $terminate(PARSER_TERMINATE_NUMBER_RANGE);
+    }
+>>
+token number /-?\d+(\.\d+)?/ <<
+    if (!parse_number(match_text, match_length, &$$))
+    {
+        $terminate(PARSER_TERMINATE_NUMBER_RANGE);
+    }
+>>
 
 Start -> RenderParams Scene;
 
diff --git a/src/fart/Engine.cpp b/src/fart/Engine.cpp
index c69eca5..b55e6f4 100644
--- a/src/fart/Engine.cpp
+++ b/src/fart/Engine.cpp
@@ -62,5 +62,9 @@ int Engine::run(string_view input_file, opt<string_view> output_file)
         return 1;
     }
 
+    /* CLI params take precedence over scene params; defaults fill the rest. */
+    params.merge(scene->params);
+    params.apply_defaults();
+
     return 0;
 }
diff --git a/src/fart/Parser.cpp b/src/fart/Parser.cpp
index 33bd203..edd50d5 100644
--- a/src/fart/Parser.cpp
+++ b/src/fart/Parser.cpp
@@ -1,6 +1,7 @@
 #include <fart/Parser.h>
 #include <fart/propane.h>
 #include <fart/read_file.h>
+#include <stdint.h>
 #include <stdio.h>
 
 Scene * Parser::load_scene(const string_view & input_file)
@@ -49,6 +50,19 @@ Scene * Parser::load_scene(const string_view & input_file)
         break;
     }
 
+    case P_USER_TERMINATED:
+        if (p_user_terminate_code(context) == PARSER_TERMINATE_NUMBER_RANGE)
+        {
+            fprintf(stderr, "%.*s:%u:%u: error: number out of range\n",
+                fname_len, fname, pos.row, pos.col);
+        }
+        else
+        {
+            fprintf(stderr, "%.*s:%u:%u: internal error: unknown terminate code %zu\n",
+                fname_len, fname, pos.row, pos.col, p_user_terminate_code(context));
+        }
+        break;
+
     default:
         fprintf(stderr, "%.*s:%u:%u: internal error: unknown parser result %zu\n",
             fname_len, fname, pos.row, pos.col, result);
@@ -61,29 +75,67 @@ Scene * Parser::load_scene(const string_view & input_file)
         return nullptr;
     }
 
-    Scene * scene = build_scene(p_result(context));
+    Scene * scene = build_scene(input_file, p_result(context));
     p_context_delete(context);
     return scene;
 }
 
-Scene * Parser::build_scene(PStart start)
+/* Convert a uint token value to a size_t, reporting an error if it does not
+ * fit. */
+static bool token_to_size(const string_view & input_file, PToken token, opt<size_t> * out)
+{
+    /* SIZE_MAX + 1, computed so that it is exactly representable as a double. */
+    const double size_limit = (double)(SIZE_MAX / 2u + 1u) * 2.0;
+    double value = token.pvalue();
+    if (value >= size_limit)
+    {
+        p_position_t pos = token.position();
+        fprintf(stderr, "%.*s:%u:%u: error: value out of range\n",
+            (int)input_file.size(), input_file.data(), pos.row, pos.col);
+        return false;
+    }
+    *out = (size_t)value;
+    return true;
+}
+
+static Color vector_to_color(PVector vector)
+{
+    Color color{
+        vector.pNumber2().pToken().pvalue(),
+        vector.pNumber4().pToken().pvalue(),
+        vector.pNumber6().pToken().pvalue(),
+    };
+    if (PNumber a = vector.pNumber8(); p_node_valid(a))
+    {
+        color.a = a.pToken().pvalue();
+    }
+    return color;
+}
+
+Scene * Parser::build_scene(const string_view & input_file, PStart start)
 {
     Scene * scene = new Scene();
 
     for (PRenderParams rps = start.pRenderParams(); p_node_valid(rps); rps = rps.pRenderParams())
     {
         PRenderParam rp = rps.pRenderParam();
+        bool ok = true;
         if (PBackground background = rp.pBackground(); p_node_valid(background))
         {
-            /* TODO: set scene->params.background from the Vector. */
+            scene->params.background = vector_to_color(background.pVector());
         }
         else if (PHeight height = rp.pHeight(); p_node_valid(height))
         {
-            /* TODO: set scene->params.height from the uint token. */
+            ok = token_to_size(input_file, height.pToken2(), &scene->params.height);
         }
         else if (PWidth width = rp.pWidth(); p_node_valid(width))
         {
-            /* TODO: set scene->params.width from the uint token. */
+            ok = token_to_size(input_file, width.pToken2(), &scene->params.width);
+        }
+        if (!ok)
+        {
+            delete scene;
+            return nullptr;
         }
     }
 
diff --git a/src/fart/Parser.h b/src/fart/Parser.h
index a9427fa..f3bb243 100644
--- a/src/fart/Parser.h
+++ b/src/fart/Parser.h
@@ -10,5 +10,5 @@ public:
     static Scene * load_scene(const string_view & input_file);
 
 private:
-    static Scene * build_scene(PStart start);
+    static Scene * build_scene(const string_view & input_file, PStart start);
 };