/**
 ******************************************************************************
 * Xenia : Xbox 360 Emulator Research Project                                 *
 ******************************************************************************
 * Copyright 2022 Ben Vanik. All rights reserved.                             *
 * Released under the BSD license - see LICENSE in the root for more details. *
 ******************************************************************************
 */

#ifndef XENIA_UI_SHADERS_DITHER_8BPC_XESLI_
#define XENIA_UI_SHADERS_DITHER_8BPC_XESLI_

#include "xesl.xesli"

#include "noise.xesli"

xesl_staticConst float xe_dither_8bpc_noise[] = {
  // The conversion to 8bpc in the fixed-function output-merger is done as
  // floor(saturate(color) * 255.0 + 0.5). This dithering function effectively
  // replaces that 0.5 offset, done for rounding to the nearest, with the noise.
  // To do that, first, 0.5/255 needs to be subtracted from the color, and
  // then the noise value `[0, 255/256] / 255` from xe_dither_8bpc_noise needs
  // to be added. However, due to rounding (because division by 255.0 is
  // inexact), subtracting 0.5/255 from exact 8-bit integer color values
  // (including, for instance, 1.0, or 255/255) may result in the result being
  // smaller than the original exact 8-bit value where the noise value is 0. So,
  // remapping the noise to the `[0.5/256, 255.5/256] / 255` range. Another way
  // of preventing rounding issues is doing it manually and returning only exact
  // integer 8-bit color values divided by 255, but that would assume that the
  // render target is specifically 8bpc, so it's better to avoid that in case
  // the window system, for instance, provides a 10bpc render target (for which
  // dithering is still done here for more consistency between displays, but
  // some addition precision would still be desirable).
  XeBlueNoise16x16Values0Until256(1.0 / 256.0 / 255.0,
                                  (-0.5 + 0.5 / 256.0) / 255.0)
};

float XeDitherOffset8bpc(xesl_uint2 pixel_coord) {
  pixel_coord &= 15u;
  return xe_dither_8bpc_noise[pixel_coord.y * 16u + pixel_coord.x];
}

#endif  // XENIA_UI_SHADERS_DITHER_8BPC_XESLI_