AgX tonemapping for Unreal Engine 5

Unreal-AgX-Tonemapper.usf

// See image comparison https://imgur.com/a/9L2P7GJ
// Read details https://iolite-engine.com/blog_posts/minimal_agx_implementation

// Usage:
// 1. Open "Project Settings" and change "Working Color Space" to "sRGB / Rec709"
// 2. Open `Engine\Shaders\Private\PostProcessTonemap.usf` file
// 3. Find `half3 OutDeviceColor = ColorLookupTable(FinalLinearColor);` line
// 4. Replace it with `half3 OutDeviceColor = ApplyAgX(FinalLinearColor);` line
// 5. Find `half3 ColorLookupTable( half3 LinearColor )` function
// 6. After the scope of the function, add the code below and run `RecompileShaders Changed` from console
// 7. Restart the editor
// 8. Adjust "Exposure Compensation" under "Lens \ Exposure" in "Post Process Volume"

// AGX BEGIN
//
// MIT License
//
// Copyright (c) 2024 Missing Deadlines (Benjamin Wrensch)
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

// All values used to derive this implementation are sourced from Troy’s initial AgX implementation/OCIO config file available here:
//   https://github.com/sobotka/AgX

// 0: Default, 1: Golden, 2: Punchy
#define AGX_LOOK 2

// Mean error^2: 3.6705141e-06
float3 AgxDefaultContrastApprox(float3 x) 
{
	const float3 x2 = x * x;
	const float3 x4 = x2 * x2;
 
	return + 15.5   * x4 * x2
			- 40.14    * x4 * x
			+ 31.96    * x4
			- 6.868    * x2 * x
			+ 0.4298   * x2
			+ 0.1191   * x
			- 0.00232;
}

float3 Agx(float3 val) 
{
	const float3x3 agx_mat = float3x3(
		0.842479062253094, 0.0423282422610123, 0.0423756549057051,
		0.0784335999999992,  0.878468636469772,  0.0784336,
		0.0792237451477643, 0.0791661274605434, 0.879142973793104);

	// DEFAULT_LOG2_MIN      = -10.0
	// DEFAULT_LOG2_MAX      =  +6.5
	// MIDDLE_GRAY           =  0.18
	// log2(pow(2, VALUE) * MIDDLE_GRAY)
	// Adjusted for Unreal's zero exposure compensation
	const float min_ev = -12.47393f; // Default: -12.47393f;
	const float max_ev = 0.526069f;  // Default:  4.026069f;

	// Input transform (inset)
	val = mul(val, agx_mat);
  
	// Log2 space encoding
	val = clamp(log2(val), min_ev, max_ev);
	val = (val - min_ev) / (max_ev - min_ev);
  
	// Apply sigmoid function approximation
	val = AgxDefaultContrastApprox(val);

	return val;
}

float3 AgxEotf(float3 val) 
{
	const float3x3 agx_mat_inv = float3x3(
		1.19687900512017, -0.0528968517574562, -0.0529716355144438,
		-0.0980208811401368, 1.15190312990417, -0.0980434501171241,
		-0.0990297440797205, -0.0989611768448433, 1.15107367264116);
	
	// Inverse input transform (outset)
	val = mul(val, agx_mat_inv);
  
	// sRGB IEC 61966-2-1 2.2 Exponent Reference EOTF Display
	// NOTE: We're linearizing the output here. Comment/adjust when
	// *not* using a sRGB render target
	val = pow(val, 2.2);

	return val;
}

float3 AgxLook(float3 val) 
{
	const float3 lw = float3(0.2126, 0.7152, 0.0722);
	const float luma = dot(val, lw);
  
	// Default
	const float3 offset = float3(0.0, 0.0, 0.0);
	float3 slope = float3(1.0, 1.0, 1.0);
	float3 power = float3(1.0, 1.0, 1.0);
	float sat = 1.0;
 
#if AGX_LOOK == 1
	// Golden
	slope = float3(1.0, 0.9, 0.5);
	power = float3(0.8, 0.8, 0.8);
	sat = 0.8;
#elif AGX_LOOK == 2
	// Punchy
	slope = float3(1.0, 1.0, 1.0);
	power = float3(1.35, 1.35, 1.35);
	sat = 1.4;
#endif
  
	// ASC CDL
	val = pow(val * slope + offset, power);
	return luma + sat * (val - luma);
}

half3 ApplyAgX(half3 LinearColorRec709)
{
	LinearColorRec709 = Agx(LinearColorRec709);
	LinearColorRec709 = AgxLook(LinearColorRec709);
	LinearColorRec709 = AgxEotf(LinearColorRec709);
	
	return LinearColorRec709;
}
// AGX END