[Maintenance] Subpixels: Differentiate between pixel and subpixel types

Which does have the practical effect of communicating more information
via IntelliSense during regular development!

Part of P0112, funded by [Anonymous] and Blue Bolt.

th01/math/subpixel.hpp

@@ -8,48 +8,50 @@ typedef int subpixel_t;
 #define TO_SP(v) \
 	(v << 4)
 
-inline subpixel_t to_sp(float screen_v) {
-	return static_cast<subpixel_t>(screen_v * 16.0f);
+inline subpixel_t to_sp(float pixel_v) {
+	return static_cast<subpixel_t>(pixel_v * 16.0f);
 }
 
-template <class T> class SubpixelBase {
+template <class SubpixelType, class PixelType> class SubpixelBase {
 public:
+	typedef SubpixelBase<SubpixelType, PixelType> SelfType;
+
 	// Code generation will require direct access to v, if performing
 	// arithmetic with a local variable...
-	T v;
+	SubpixelType v;
 
-	subpixel_t operator -(const SubpixelBase<T> &other) {
+	SubpixelType operator -(const SelfType &other) {
 		return (this->v - other.v);
 	}
 
-	void operator +=(float screen_v) {
-		this->v += static_cast<T>(to_sp(screen_v));
+	void operator +=(float pixel_v) {
+		this->v += static_cast<SubpixelType>(to_sp(pixel_v));
 	}
 
-	void operator -=(float screen_v) {
-		this->v -= static_cast<T>(to_sp(screen_v));
+	void operator -=(float pixel_v) {
+		this->v -= static_cast<SubpixelType>(to_sp(pixel_v));
 	}
 
 	// No overloads of `operator =()`, since the class needs to be trivially
 	// copyable.
-	void set(float screen_v) {
-		v = static_cast<T>(to_sp(screen_v));
+	void set(float pixel_v) {
+		v = static_cast<SubpixelType>(to_sp(pixel_v));
 	}
 
-	void set(const T &screen_v) {
-		v = TO_SP(screen_v);
+	void set(const PixelType &pixel_v) {
+		v = static_cast<SubpixelType>(TO_SP(pixel_v));
 	}
 
-	T to_pixel() const {
-		return v >> 4;
+	PixelType to_pixel() const {
+		return static_cast<PixelType>(v >> 4);
 	}
 
-	operator T() const {
+	operator SubpixelType() const {
 		return v;
 	}
 
-	static T None() {
-		return to_sp(PIXEL_NONE);
+	static SubpixelType None() {
+		return static_cast<SubpixelType>(TO_SP(PIXEL_NONE));
 	}
 };
 
@@ -63,9 +65,9 @@ template <class T> struct SPPointBase {
 };
 
 // 16-bit (Q12.4)
-typedef SubpixelBase<subpixel_t> Subpixel;
+typedef SubpixelBase<subpixel_t, pixel_t> Subpixel;
 typedef SPPointBase<Subpixel> SPPoint;
 // 8-bit (Q4.4)
-typedef SubpixelBase<unsigned char> SubpixelLength8;
-typedef SubpixelBase<char> Subpixel8;
+typedef SubpixelBase<unsigned char, unsigned char> SubpixelLength8;
+typedef SubpixelBase<char, char> Subpixel8;
 typedef SPPointBase<Subpixel8> SPPoint8;