diff --git a/src/fart/Cli.cpp b/src/fart/Cli.cpp
index e7a924a..c29d198 100644
--- a/src/fart/Cli.cpp
+++ b/src/fart/Cli.cpp
@@ -139,7 +139,7 @@ int Cli::run(int argc, char * argv[])
     return 0;
 }
 
-static bool parse_size(const char * flag, const char * value, size_t * out)
+static bool parse_size(const char * flag, const char * value, opt<size_t> * out)
 {
     bool valid = value[0] != '\0';
     for (const char * c = value; *c != '\0'; c++)
diff --git a/src/fart/RenderParams.cpp b/src/fart/RenderParams.cpp
index c289a1b..f7d3ed4 100644
--- a/src/fart/RenderParams.cpp
+++ b/src/fart/RenderParams.cpp
@@ -1,7 +1,25 @@
 #include <fart/RenderParams.h>
 
-RenderParams::RenderParams()
+void RenderParams::merge(const RenderParams & other)
 {
-    width = 800;
-    height = 600;
+    if (!width)
+    {
+        width = other.width;
+    }
+    if (!height)
+    {
+        height = other.height;
+    }
+}
+
+void RenderParams::apply_defaults()
+{
+    if (!width)
+    {
+        width = 800u;
+    }
+    if (!height)
+    {
+        height = 600u;
+    }
 }
diff --git a/src/fart/RenderParams.h b/src/fart/RenderParams.h
index 39afac4..0ec22d3 100644
--- a/src/fart/RenderParams.h
+++ b/src/fart/RenderParams.h
@@ -4,8 +4,12 @@
 
 struct RenderParams
 {
-    size_t width = 0u;
-    size_t height = 0u;
+    opt<size_t> width;
+    opt<size_t> height;
 
-    RenderParams();
+    /* Set each field that is not already set to the value from other. */
+    void merge(const RenderParams & other);
+
+    /* Fill in any unset fields with default values. */
+    void apply_defaults();
 };