Merge pull request #95291 from BlueCube3310/hdr-optimizations

Optimize .hdr loading and RGB9E5 conversion

1 files changed, 40 insertions, 27 deletions

diff --git a/core/math/color.h b/core/math/color.h
index e17b8c9fd7..70fad78acb 100644
--- a/core/math/color.h
+++ b/core/math/color.h
@@ -129,33 +129,46 @@ struct [[nodiscard]] Color {
 	}
 
 	_FORCE_INLINE_ uint32_t to_rgbe9995() const {
-		const float pow2to9 = 512.0f;
-		const float B = 15.0f;
-		const float N = 9.0f;
-
-		float sharedexp = 65408.000f; // Result of: ((pow2to9 - 1.0f) / pow2to9) * powf(2.0f, 31.0f - 15.0f)
-
-		float cRed = MAX(0.0f, MIN(sharedexp, r));
-		float cGreen = MAX(0.0f, MIN(sharedexp, g));
-		float cBlue = MAX(0.0f, MIN(sharedexp, b));
-
-		float cMax = MAX(cRed, MAX(cGreen, cBlue));
-
-		float expp = MAX(-B - 1.0f, floor(Math::log(cMax) / (real_t)Math_LN2)) + 1.0f + B;
-
-		float sMax = (float)floor((cMax / Math::pow(2.0f, expp - B - N)) + 0.5f);
-
-		float exps = expp + 1.0f;
-
-		if (0.0f <= sMax && sMax < pow2to9) {
-			exps = expp;
-		}
-
-		float sRed = Math::floor((cRed / pow(2.0f, exps - B - N)) + 0.5f);
-		float sGreen = Math::floor((cGreen / pow(2.0f, exps - B - N)) + 0.5f);
-		float sBlue = Math::floor((cBlue / pow(2.0f, exps - B - N)) + 0.5f);
-
-		return (uint32_t(Math::fast_ftoi(sRed)) & 0x1FF) | ((uint32_t(Math::fast_ftoi(sGreen)) & 0x1FF) << 9) | ((uint32_t(Math::fast_ftoi(sBlue)) & 0x1FF) << 18) | ((uint32_t(Math::fast_ftoi(exps)) & 0x1F) << 27);
+		// https://github.com/microsoft/DirectX-Graphics-Samples/blob/v10.0.19041.0/MiniEngine/Core/Color.cpp
+		static const float kMaxVal = float(0x1FF << 7);
+		static const float kMinVal = float(1.f / (1 << 16));
+
+		// Clamp RGB to [0, 1.FF*2^16]
+		const float _r = CLAMP(r, 0.0f, kMaxVal);
+		const float _g = CLAMP(g, 0.0f, kMaxVal);
+		const float _b = CLAMP(b, 0.0f, kMaxVal);
+
+		// Compute the maximum channel, no less than 1.0*2^-15
+		const float MaxChannel = MAX(MAX(_r, _g), MAX(_b, kMinVal));
+
+		// Take the exponent of the maximum channel (rounding up the 9th bit) and
+		// add 15 to it.  When added to the channels, it causes the implicit '1.0'
+		// bit and the first 8 mantissa bits to be shifted down to the low 9 bits
+		// of the mantissa, rounding the truncated bits.
+		union {
+			float f;
+			int32_t i;
+		} R, G, B, E;
+
+		E.f = MaxChannel;
+		E.i += 0x07804000; // Add 15 to the exponent and 0x4000 to the mantissa
+		E.i &= 0x7F800000; // Zero the mantissa
+
+		// This shifts the 9-bit values we need into the lowest bits, rounding as
+		// needed. Note that if the channel has a smaller exponent than the max
+		// channel, it will shift even more.  This is intentional.
+		R.f = _r + E.f;
+		G.f = _g + E.f;
+		B.f = _b + E.f;
+
+		// Convert the Bias to the correct exponent in the upper 5 bits.
+		E.i <<= 4;
+		E.i += 0x10000000;
+
+		// Combine the fields. RGB floats have unwanted data in the upper 9
+		// bits. Only red needs to mask them off because green and blue shift
+		// it out to the left.
+		return E.i | (B.i << 18) | (G.i << 9) | (R.i & 511);
 	}
 
 	_FORCE_INLINE_ Color blend(const Color &p_over) const {