Add explanation of converting between floating and fixed point

content/fixed-point-math.md

@@ -111,13 +111,13 @@ Let's say we have a player sprite, with an X and Y position. What speed should w
 
 ```c
 // a function elsewhere in code, which needs the ONSCREEN x and y position
-extern void display_player(int x, int y);
+void display_player(int x, int y);
 
 // player coordinates use signed 28.3f fixed-point format
-int player_fp = 3;
+const int player_fp = 3;
 
 // 1 and 5/8, written verbosely for demonstration purposes
-int player_speed = 8 + 5;
+int player_speed = (1 << player_fp) + ((5 << player_fp) / 8);
 
 // let's start the player at (5, 6) on the screen
 // shift both of these values left to account for the fixed-point
@@ -163,10 +163,11 @@ Before adding or subtracting fixed-point numbers, you need to make the denominat
 
 ```c
 // variable 'a' uses an unsigned 27.5f fixed-point format
-int a_fp = 5;
+const int a_fp = 5;
 unsigned int a = 3 << a_fp;
+
 // variable 'b' uses an unsigned 16.16f fixed-point format
-int b_fp = 16;
+const int b_fp = 16;
 unsigned int b = 4 << b_fp;
 ```
 
@@ -206,6 +207,54 @@ Even though you can't do hardware-level division, there are usually creative wor
 
 If you really must divide, you would multiply the numerator by the fixed-point first, *before* dividing.
 
+## Converting between fixed and floating point
+
+Okay, so now you have a way to do mathematical operations efficiently. How do you set the initial values in a convenient way? How do you print the values in a way that is easier to understand than very big integer values?
+
+Well, you can convert between fixed and floating point easily:
+
+```c
+const int player_fp = 3;
+
+static inline int player_float2fixed(float value)
+{
+	return (int)(value * (1 << player_fp));
+}
+
+static inline float player_fixed2float(int value)
+{
+	return value / (float)(1 << player_fp);
+}
+
+// Macro version of the functions, for situations where you can't use functions
+#define PLAYER_FLOAT2FIXED(value)     (int)((value) * (1 << (player_fp)))
+#define PLAYER_FIXED2FLOAT(value)     ((value) / (float)(1 << (player_fp)))
+
+int setup(void)
+{
+	int player_x = player_float2fixed(1.23);
+	int player_y = PLAYER_FLOAT2FIXED(2.35);
+
+	printf("Player X: %f\n", player_fixed2float(player_x);
+	printf("Player Y: %f\n", PLAYER_FIXED2FLOAT(player_y);
+}
+```
+
+Remember that the operations are floating point operations, so they will be slow. There is an exception. If you use `constexpr` or if the compiler detects that an expression is constant, it will calculate it at compile time automatically. This is very useful for setting initial fixed point values from floating point values.
+
+```c
+int player_x, player_y;
+
+constexpr int player_start_x = player_float2fixed(1.23); // Only in C++
+const int player_start_y = PLAYER_FLOAT2FIXED(2.35);
+
+int setup(void)
+{
+	player_x = player_start_x;
+	player_y = player_start_y;
+}
+```
+
 And there you go! You now know everything needed to do fixed-point math. Good luck!
 
 ## FAQ