tone_mapping_histogram.comp

/*
Copyright (C) 2019, NVIDIA CORPORATION. All rights reserved.

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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This program is distributed in the hope that it will be useful,
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

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with this program; if not, write to the Free Software Foundation, Inc.,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/

// ========================================================================= //
//
// This is the first part of the shader code for the tone mapper, which is
// based on part of Eilertsen, Mantiuk, and Unger's paper *Real-time noise-
// aware tone mapping*, with some additional modifications that we found useful.
//
// This compute shader computes a binned histogram over the log2 luminance
// values (i.e. photographic stops) of the IMG_TAA_OUTPUT image, and stores
// it in tonemap_buffer.accumulator[]. It assumes that the accumulator values
// were cleared to 0 before dispatching this shader.
//
// The tone mapper consists of three compute shaders, a utilities file, and
// a CPU-side code file. See below for an overview of the tone mapper.
//
// ========================================================================= //
//
// As a fully path traced game, Quake 2 RTX faces several interesting
// challenges with regards to tone and color reproduction. Since all of our
// lighting is dynamic and physically based and we implement a full HDR
// pipeline while rendering a wide variety of different scenes and environments,
// individual frames can have contrast ratios exceeding both typical display
// ranges and the scope of what tone mappers usually work with. Differences in
// luminance of 100,000:1 or larger are not uncommon. 
// At the same time, we would like to make images look as photoreal as
// possible, on standard monitors, and under varying viewing and environmental
// conditions. In bright surrounds, for instance, the visible luminance ratio
// can be as low as 100:1.
//
// We'd also like to let the player control the look of the game, including
// exposure and contrast, which can be set in the video options from the
// main menu.
//
// To solve these challenges, we combine two tone mappers. The first tone
// mapper is based in part on some new research by Eilertsen, Mantiuk, and Unger
// on HDR tone mapping, which phrases tone mapping as a minimization problem
// (minimize contrast distortion, or how much the slope of the tone mapper's
// curve varies from 1 - i.e. the identity - for each pixel on screen, while
// making sure the luminance of the result fits in the visible display range),
// and includes modifications to avoid amplifying noise levels, which is
// especially useful for a ray traced game. (We don't implement some things,
// such as the detail extraction diffusion filter described in the paper,
// though it is interesting to note that one could also emulate this by
// tone mapping the rendered lighting buffer.)
// In addition, we've implemented a number of new modifications that are
// useful for real-time rendered media such as games and professional
// applications. These include:
//   - Blurring slopes to avoid artifacts around shadows
//   - Post control over exposure and contrast
//   - New controls for fading down values below the noise threshold
//   - Pixel importance based on screen position instead of edges
//   - Some good tools for cross-workgroup arithmetic using the GL_KHR_shader_subgroup_arithmetic extension
//   - and more.
//
// The second tone mapper, which conceptually runs in parallel (though we
// combine the two to share results between the tone mappers) looks at the
// histogram to automatically expose the image, but does not reduce the contrast
// in the image. Since the first tone mapper both performs autoexposure
// (in some sense) and fits the image luminance within [0,1], we can blend the
// results of these two tone mappers to vary image contrast as desired. We also
// support blending with the original (un-tone mapped) frame.
//
// Finally, since individual RGB channel values can lie outside of [0,1] after
// this process (although pixel luminances mostly lie inside [0,1]) and because
// we want to slightly shift the hue of the brightest values in the scene, we
// apply a subtle knee at the end of the tone mapping process to sufficently
// bright values (see the tm_knee and tm_white_point CVARs and tone_mapping.c
// for more on this).
//
// One good way to see the tone mapper in action is to stand in the corner
// of the hallway between the area with the fan and the first reachable outdoors
// area in the first map. With the tone mapper disabled, it's nearly impossible
// to see both the scene outside and the scene inside at the same time - part
// of the image is either overexposed or underexposed regardless of brightness
// settings, which fails to mimic how the human eye works. With this tone
// mapper, we're able to dynamically adjust to handle the wide physically-
// based contrast range in this scene and provide a fluid gameplay experience
// while providing control over exposure, contrast, eye adaptation simulation,
// and more.
// 
// Here's a short outline of how this tone mapper works:
// - Compute histogram (this file)
//   - Each workgroup computes its histogram in shared memory, and then adds
//     the workgroup's result to the histogram result in tonemap_buffer.accumulator.
//   - Pixel saliency is measured by some metric; this roughly measures how
//     important it is for the tone curve to preserve contrast at this luminance.
//
// - Compute curve (tone_mapping_curve.comp)
//   - Normalize the histogram
//   - Compute tone mapper statistics for the autoexposure tonemapper
//   - Solve the contrast distortion optimization problem (equations 11-12),
//       min
//           sum( p_k (1-s_k)^2 )    // weighted contrast distortion
//       subject to
//           delta * sum(s_k) <= r   // tone curve fits in dynamic range
//           s_k >= 0,               // no slopes are negative
//       where p_k is the importance of each histogram bin, s_k is the slope
// of the tone curve for each histogram bin, delta is the width of each bin,
// and r is the log of the dynamic range of the display.
//     - We have an analytic solution for the problem with only the first two
// conditions. However, just using this alone results in negative slopes (i.e.
// a tone curve that doesn't always preserve luminance order - the basic idea
// is that if your priority is to assign slopes of 1 to the most important bins
// and the image covers a wider luminance range than the dynamic range of the
// display, then your tone curve might agree with the dynamic range of the
// display at the endpoints, but zig-zag wider than the dynamic range of the
// display in the middle). To solve this, we conceptually set negative slopes
// to zero, and then iterate the process until we have satisfied the constraints
// of the minimization problem. (See equations (14-17) for more details.)
//   - Apply a low-pass filter to the slopes.
//   - Prefix sum the array of slopes to get the tone curve for the image.
//   - Below global_ubo.tm_noise_stops, ramp mapped values slowly towards 0.
//   - Blend results with the previous frame. This ensures that the tone mapping
// curve does not abruptly between frames, and also helps to emulate
// eye adaptation.
//
// - Apply curve (tone_mapping_apply.comp)
//   - For each pixel, compute its luminance value and look up its mapped
// luminance using linear interpolation.
//   - Map sufficiently bright values to visible range using another post-tone
// map Reinhard-like tone mapper.
//   - Mix results of noise-aware and autoexposure tone mappers
//   - Additional SDR color grading would be applied here
//   - Finally, apply dithering and write image.
//
// We've separated the tone mapping logic into three dispatches here to
// improve the experience for people who are reading this for the first time.
// However, if the logic to apply the tone map uses the tone curve from the
// previous frame, the shader that computes the histogram from the image can
// also apply the tone curve from the previous frame to the image in the
// same dispatch. This can save up to 0.05ms.
//
// ========================================================================= //

#version 460
#extension GL_GOOGLE_include_directive    : enable
#extension GL_EXT_nonuniform_qualifier    : enable

#include "utils.glsl"

#define GLOBAL_UBO_DESC_SET_IDX 0
#include "global_ubo.h"

#define GLOBAL_TEXTURES_DESC_SET_IDX 1
#include "global_textures.h"

#define VERTEX_BUFFER_DESC_SET_IDX 2
#include "vertex_buffer.h"

#include "tone_mapping_utils.glsl"

layout(local_size_x = 16, local_size_y = 16, local_size_z = 1) in;

shared uint s_Histogram[HISTOGRAM_BINS];

void main()
{
    const ivec2 ipos = ivec2(gl_GlobalInvocationID);
    const ivec2 screenSize = ivec2(global_ubo.taa_output_width, global_ubo.taa_output_height);

    if(any(greaterThanEqual(ipos, screenSize)))
        return;

    vec3 input_color = imageLoad(IMG_TAA_OUTPUT, ipos).rgb;
    input_color /= STORAGE_SCALE_HDR;

    const uint linear_idx = gl_LocalInvocationIndex;

    // Compute and write histogram value
    // Initialize local memory
    if(linear_idx < HISTOGRAM_BINS)
    {
        s_Histogram[linear_idx] = 0;
    }

    // Thread sync
    barrier();

    // Ignore completely black pixels
    if(luminance(input_color) > 0)
    {
        // Compute histogram bin (based on photographic stops)
        // This is (log2(lum) - min_log_luminance)/(max_log_luminance - min_log_luminance),
        // but phrased as a fused multiply-add:
        const float lum = max(luminance(input_color), exp2(min_log_luminance));
        const float log_luminance = log2(lum);
        const float biased_log_luminance = log_luminance * log_luminance_scale + log_luminance_bias;
        const float histogram_bin = clamp(biased_log_luminance * HISTOGRAM_BINS, 0.0, HISTOGRAM_BINS - 1.0);

        // Distribute our luminance between two bins using a linear (tent) filter:
        // Since histogram_bin >= 0.0, this is the same as uint(floor(histogram_bin)):
        const uint left_bin = uint(histogram_bin);
        const uint right_bin = left_bin + 1;

        // Compute pixel importance based on distance to the center of the screen.
        float weight = clamp(1.0 - length(vec2(ipos) / vec2(screenSize) - vec2(0.5)) * 1.5, 0.01, 1.0); // Spatial weight

        const float right_weight_F = fract(histogram_bin) * weight;
        const float left_weight_F = weight - right_weight_F;

        // Convert and store to fixed-point for atomic addition:
        uint right_weight_U = uint(right_weight_F * FIXED_POINT_FRAC_MULTIPLIER);
        uint left_weight_U = uint(left_weight_F * FIXED_POINT_FRAC_MULTIPLIER);

        atomicAdd(s_Histogram[left_bin], left_weight_U); // since 0 <= left_bin < HISTOGRAM_BINS
        if(right_bin < HISTOGRAM_BINS)
        {
            atomicAdd(s_Histogram[right_bin], right_weight_U);
        }
    }

    // Wait for all threads in warp to finish.
    barrier();

    // Add warp histogram to global histogram.
    if(linear_idx < HISTOGRAM_BINS)
    {
        int localBinValue = int(s_Histogram[linear_idx]);
        if (localBinValue != 0)
        {
            atomicAdd(tonemap_buffer.accumulator[linear_idx], localBinValue);
        }
    }
}