types_rgbe.h

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/* SPDX-FileCopyrightText: 2025 Blender Foundation
 *
 * SPDX-License-Identifier: Apache-2.0 */
 
#pragma once
 
#include "util/math_fast.h"
#include "util/math_float3.h"
#include "util/types_base.h"
 
CCL_NAMESPACE_BEGIN
 
struct RGBE {
  union {
    struct {
      uint8_t r, g, b, e;
    };
    float f;
  };
 
  RGBE() = default;
 
  ccl_device_inline_method RGBE(const float f_) : f(f_) {}
};
 
static_assert(sizeof(RGBE) == 4, "RGBE expected to be exactly 4 bytes");

 
#define RGBE_EXP_BIAS 15
#define RGBE_MANTISSA_BITS 8
#define RGBE_EXPONENT_BITS 5
#define RGBE_MAX 65280.0f
 
ccl_device RGBE rgb_to_rgbe(float3 rgb)
{
  const float max_v = min(reduce_max(fabs(rgb)), RGBE_MAX);
  if (max_v < ldexpf(0.5f, -RGBE_EXP_BIAS - RGBE_MANTISSA_BITS)) {
    return RGBE(0.0f);
  }
 
  int e = max(-RGBE_EXP_BIAS - 1, floor_log2f(max_v)) + 1;
  float v = ldexpf(1.0f, RGBE_MANTISSA_BITS - e);
 
  /* The original implementation by Greg Ward uses `floor`, causing systematic bias when
   * accumulated in a buffer.
   * We use `round` instead, but need to deal with overflow. */
  if (int(roundf(max_v * v)) == power_of_2(RGBE_MANTISSA_BITS)) {
    e += 1;
    v *= 0.5f;
  }
 
  /* Get sign bits. */
  const uint sign_bits = ((__float_as_uint(rgb.x) >> 31) << 7) |
                         ((__float_as_uint(rgb.y) >> 31) << 6) |
                         ((__float_as_uint(rgb.z) >> 31) << 5);
 
  RGBE rgbe;
  rgb = min(round(fabs(rgb) * v), make_float3(255.0f));
  rgbe.r = uint8_t(rgb.x);
  rgbe.g = uint8_t(rgb.y);
  rgbe.b = uint8_t(rgb.z);
  rgbe.e = uint8_t(((e + RGBE_EXP_BIAS) & 0x1Fu) | sign_bits);
  return rgbe;
}
 
ccl_device_inline float3 rgbe_to_rgb(const RGBE rgbe)
{
  if (rgbe.f == 0.0f) {
    return zero_float3();
  }
 
  const int e = rgbe.e & 0x1Fu;
  const float f = ldexpf(1.0f, e - (int)(RGBE_EXP_BIAS + RGBE_MANTISSA_BITS));
  float3 result = make_float3(rgbe.r, rgbe.g, rgbe.b) * f;
 
  /* Set sign bits. */
  result.x = or_mask(result.x, (uint(rgbe.e) & 0x80u) << 24);
  result.y = or_mask(result.y, (uint(rgbe.e) & 0x40u) << 25);
  result.z = or_mask(result.z, (uint(rgbe.e) & 0x20u) << 26);
  return result;
}
 
CCL_NAMESPACE_END