Add support for using keyframe extrapolator as source of procedural a…

…nimation.

src/engine/animation/animation-curve.cpp

@@ -2,17 +2,11 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 #include <engine/animation/animation-curve.hpp>
-#include <stdexcept>
 
 float animation_curve::evaluate(float time) const
 {
-	if (m_keyframes.empty())
-	{
-		throw std::runtime_error("Failed to evaluate animation curve: no keyframes.");
-	}
-
 	// Check if time is outside keyframe range
-	if (time < m_keyframes.begin()->time || time > m_keyframes.rbegin()->time)
+	if (m_keyframes.empty() || time < m_keyframes.begin()->time || time > m_keyframes.rbegin()->time)
 	{
 		// Extrapolate outside keyframe range
 		return m_extrapolator(m_keyframes, time);

src/engine/animation/animation-curve.hpp

@@ -27,6 +27,8 @@ class animation_curve
 	 * Keyframe extrapolator function type.
 	 *
 	 * An extrapolator function takes two parameters: a reference to the keyframe container, and a time outside the interval formed by the first and last keyframe times. An extrapolator function should return an extrapolated value according to the given time.
+	 *
+	 * @warning Keyframe container may be empty.
 	 */
 	using keyframe_extrapolator_type = std::function<float(const keyframe_container&, float)>;
 
@@ -63,7 +65,11 @@ class animation_curve
 		return m_interpolator;
 	}
 
-	/** Returns a reference to the keyframe extrapolator function object. */
+	/**
+	 * Returns a reference to the keyframe extrapolator function object.
+	 *
+	 * @note The extrapolator may be utilized as a source of procedural animation: if a curve with no keyframes is evaluated, the extrapolator will be called with the empty keyframe container and evaluation time as parameters.
+	 */
 	[[nodiscard]] inline constexpr auto& extrapolator() noexcept
 	{
 		return m_extrapolator;

src/engine/animation/keyframe-extrapolation.cpp

@@ -2,9 +2,15 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 #include <engine/animation/keyframe-extrapolation.hpp>
+#include <stdexcept>
 
 float extrapolate_keyframes_clamp(const keyframe_container& keyframes, float time)
 {
+	if (keyframes.empty())
+	{
+		throw std::runtime_error("Failed clamp extrapolation of keyframes: no keyframes provided.");
+	}
+
 	if (time < keyframes.begin()->time)
 	{
 		return keyframes.begin()->value;

src/engine/animation/keyframe-extrapolation.hpp

@@ -14,7 +14,7 @@
  *
  * @return Extrapolated value.
  *
- * @warning @p keyframes must not be empty!
+ * @exception std::runtime_error Failed clamp extrapolation of keyframes: no keyframes provided.
  */
 [[nodiscard]] float extrapolate_keyframes_clamp(const keyframe_container& keyframes, float time);