mirror of https://github.com/SixLabors/ImageSharp
James Jackson-South
1 week ago
3 changed files with 601 additions and 4 deletions
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// Copyright (c) Six Labors.
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// Licensed under the Six Labors Split License.
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using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit; |
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namespace SixLabors.ImageSharp.Formats.Heif.Av1; |
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/// <summary>
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/// Applies the H.273 transfer characteristics used by AV1 color conversion.
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/// </summary>
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internal static class Av1TransferFunctions |
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{ |
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/// <summary>
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/// The BT.709 and BT.2020 nonlinear scale factor.
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/// </summary>
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private const float Bt709Alpha = 1.09929682680944F; |
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/// <summary>
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/// The BT.709 and BT.2020 linear-domain transition point.
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/// </summary>
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private const float Bt709Beta = 0.018053968510807F; |
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/// <summary>
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/// The SMPTE ST 240 nonlinear scale factor.
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/// </summary>
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private const float Smpte240Alpha = 1.111572195921731F; |
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/// <summary>
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/// The SMPTE ST 240 linear-domain transition point.
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/// </summary>
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private const float Smpte240Beta = 0.022821585529445F; |
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/// <summary>
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/// The sRGB nonlinear scale factor.
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/// </summary>
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private const float SrgbAlpha = 1.0550107189475866F; |
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/// <summary>
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/// The sRGB linear-domain transition point.
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/// </summary>
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private const float SrgbBeta = 0.0030412825601275209F; |
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/// <summary>
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/// The SMPTE ST 2084 first rational constant.
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/// </summary>
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private const float PqC1 = 0.8359375F; |
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/// <summary>
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/// The SMPTE ST 2084 numerator scale.
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/// </summary>
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private const float PqC2 = 18.8515625F; |
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/// <summary>
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/// The SMPTE ST 2084 denominator scale.
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/// </summary>
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private const float PqC3 = 18.6875F; |
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/// <summary>
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/// The SMPTE ST 2084 outer exponent.
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/// </summary>
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private const float PqM = 78.84375F; |
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/// <summary>
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/// The SMPTE ST 2084 inner exponent.
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/// </summary>
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private const float PqN = 0.1593017578125F; |
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/// <summary>
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/// The SMPTE ST 428 luminance normalization factor.
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/// </summary>
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private const float Smpte428Scale = 0.91655527974030934F; |
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/// <summary>
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/// The HLG logarithmic scale.
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/// </summary>
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private const float HlgA = 0.17883277F; |
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/// <summary>
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/// The HLG logarithmic offset.
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/// </summary>
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private const float HlgB = 0.28466892F; |
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/// <summary>
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/// The HLG output offset.
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/// </summary>
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private const float HlgC = 0.55991073F; |
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/// <summary>
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/// Converts a nonlinear signal value to its H.273 linear-domain value.
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/// </summary>
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/// <param name="transferCharacteristics">The signaled transfer characteristics.</param>
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/// <param name="value">The nonlinear signal value.</param>
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/// <returns>The corresponding linear-domain value.</returns>
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public static float ToLinear(ObuTransferCharacteristics transferCharacteristics, float value) |
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{ |
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switch (transferCharacteristics) |
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{ |
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case ObuTransferCharacteristics.Bt709: |
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case ObuTransferCharacteristics.Bt601: |
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case ObuTransferCharacteristics.Bt202010Bit: |
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case ObuTransferCharacteristics.Bt202012Bit: |
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return ToLinearBt709(value); |
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case ObuTransferCharacteristics.Bt470M: |
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return MathF.Pow(Math.Clamp(value, 0F, 1F), 2.2F); |
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case ObuTransferCharacteristics.Bt470BG: |
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return MathF.Pow(Math.Clamp(value, 0F, 1F), 2.8F); |
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case ObuTransferCharacteristics.Smpte240: |
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return ToLinearSmpte240(value); |
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case ObuTransferCharacteristics.Linear: |
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return Math.Clamp(value, 0F, 1F); |
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case ObuTransferCharacteristics.Log100: |
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// Zero represents an interval rather than one linear value. The midpoint matches libavif and
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// minimizes the worst-case round-trip error when constant-luminance content is decoded.
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return value <= 0F ? 0.005F : MathF.Pow(10F, 2F * (MathF.Min(value, 1F) - 1F)); |
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case ObuTransferCharacteristics.Log100Sqrt10: |
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return value <= 0F ? 0.00158113883F : MathF.Pow(10F, 2.5F * (MathF.Min(value, 1F) - 1F)); |
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case ObuTransferCharacteristics.Iec61966: |
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return ToLinearIec61966(value); |
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case ObuTransferCharacteristics.Bt1361: |
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return ToLinearBt1361(value); |
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case ObuTransferCharacteristics.Srgb: |
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return ToLinearSrgb(value); |
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case ObuTransferCharacteristics.Smpte2084: |
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return ToLinearPq(value); |
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case ObuTransferCharacteristics.Smpte428: |
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return MathF.Pow(MathF.Max(value, 0F), 2.6F) / Smpte428Scale; |
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case ObuTransferCharacteristics.Hlg: |
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return ToLinearHlg(value); |
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default: |
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// H.273 leaves unspecified and reserved transfer values to the application. Match libavif's
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// deterministic BT.709 fallback for still-image conversion.
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return ToLinearBt709(value); |
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} |
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} |
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/// <summary>
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/// Converts a linear signal value to its H.273 nonlinear-domain value.
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/// </summary>
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/// <param name="transferCharacteristics">The signaled transfer characteristics.</param>
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/// <param name="value">The linear signal value.</param>
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/// <returns>The corresponding nonlinear-domain value.</returns>
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public static float ToGamma(ObuTransferCharacteristics transferCharacteristics, float value) |
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{ |
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switch (transferCharacteristics) |
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{ |
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case ObuTransferCharacteristics.Bt709: |
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case ObuTransferCharacteristics.Bt601: |
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case ObuTransferCharacteristics.Bt202010Bit: |
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case ObuTransferCharacteristics.Bt202012Bit: |
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return ToGammaBt709(value); |
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case ObuTransferCharacteristics.Bt470M: |
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return MathF.Pow(Math.Clamp(value, 0F, 1F), 1F / 2.2F); |
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case ObuTransferCharacteristics.Bt470BG: |
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return MathF.Pow(Math.Clamp(value, 0F, 1F), 1F / 2.8F); |
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case ObuTransferCharacteristics.Smpte240: |
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return ToGammaSmpte240(value); |
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case ObuTransferCharacteristics.Linear: |
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return Math.Clamp(value, 0F, 1F); |
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case ObuTransferCharacteristics.Log100: |
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return value <= 0.01F ? 0F : 1F + (MathF.Log10(MathF.Min(value, 1F)) / 2F); |
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case ObuTransferCharacteristics.Log100Sqrt10: |
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return value <= 0.00316227766F ? 0F : 1F + (MathF.Log10(MathF.Min(value, 1F)) / 2.5F); |
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case ObuTransferCharacteristics.Iec61966: |
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return ToGammaIec61966(value); |
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case ObuTransferCharacteristics.Bt1361: |
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return ToGammaBt1361(value); |
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case ObuTransferCharacteristics.Srgb: |
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return ToGammaSrgb(value); |
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case ObuTransferCharacteristics.Smpte2084: |
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return ToGammaPq(value); |
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case ObuTransferCharacteristics.Smpte428: |
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return MathF.Pow(Smpte428Scale * MathF.Max(value, 0F), 1F / 2.6F); |
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case ObuTransferCharacteristics.Hlg: |
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return ToGammaHlg(value); |
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default: |
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return ToGammaBt709(value); |
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} |
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} |
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/// <summary>
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/// Applies the inverse BT.709-family opto-electronic transfer function.
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/// </summary>
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/// <param name="value">The nonlinear signal value.</param>
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/// <returns>The linear signal value.</returns>
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private static float ToLinearBt709(float value) |
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{ |
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if (value < 0F) |
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{ |
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return 0F; |
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} |
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if (value < 4.5F * Bt709Beta) |
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{ |
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return value / 4.5F; |
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} |
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return value < 1F |
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? MathF.Pow((value + (Bt709Alpha - 1F)) / Bt709Alpha, 1F / 0.45F) |
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: 1F; |
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} |
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/// <summary>
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/// Applies the BT.709-family opto-electronic transfer function.
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/// </summary>
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/// <param name="value">The linear signal value.</param>
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/// <returns>The nonlinear signal value.</returns>
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private static float ToGammaBt709(float value) |
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{ |
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if (value < 0F) |
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{ |
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return 0F; |
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} |
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if (value < Bt709Beta) |
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{ |
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return value * 4.5F; |
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} |
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return value < 1F |
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? (Bt709Alpha * MathF.Pow(value, 0.45F)) - (Bt709Alpha - 1F) |
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: 1F; |
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} |
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/// <summary>
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/// Applies the inverse SMPTE ST 240 opto-electronic transfer function.
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/// </summary>
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/// <param name="value">The nonlinear signal value.</param>
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/// <returns>The linear signal value.</returns>
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private static float ToLinearSmpte240(float value) |
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{ |
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if (value < 0F) |
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{ |
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return 0F; |
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} |
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if (value < 4F * Smpte240Beta) |
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{ |
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return value / 4F; |
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} |
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return value < 1F |
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? MathF.Pow((value + (Smpte240Alpha - 1F)) / Smpte240Alpha, 1F / 0.45F) |
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: 1F; |
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} |
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/// <summary>
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/// Applies the SMPTE ST 240 opto-electronic transfer function.
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/// </summary>
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/// <param name="value">The linear signal value.</param>
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/// <returns>The nonlinear signal value.</returns>
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private static float ToGammaSmpte240(float value) |
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{ |
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if (value < 0F) |
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{ |
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return 0F; |
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} |
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if (value < Smpte240Beta) |
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{ |
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return value * 4F; |
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} |
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return value < 1F |
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? (Smpte240Alpha * MathF.Pow(value, 0.45F)) - (Smpte240Alpha - 1F) |
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: 1F; |
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} |
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/// <summary>
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/// Applies the inverse extended IEC 61966-2-4 transfer function.
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/// </summary>
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/// <param name="value">The nonlinear signal value.</param>
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/// <returns>The linear signal value.</returns>
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private static float ToLinearIec61966(float value) |
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{ |
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if (value < -4.5F * Bt709Beta) |
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{ |
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return -MathF.Pow((value - (Bt709Alpha - 1F)) / -Bt709Alpha, 1F / 0.45F); |
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} |
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return value < 4.5F * Bt709Beta |
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? value / 4.5F |
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: MathF.Pow((value + (Bt709Alpha - 1F)) / Bt709Alpha, 1F / 0.45F); |
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} |
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/// <summary>
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/// Applies the extended IEC 61966-2-4 transfer function.
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/// </summary>
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/// <param name="value">The linear signal value.</param>
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/// <returns>The nonlinear signal value.</returns>
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private static float ToGammaIec61966(float value) |
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{ |
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if (value < -Bt709Beta) |
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{ |
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return (-Bt709Alpha * MathF.Pow(-value, 0.45F)) + (Bt709Alpha - 1F); |
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} |
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return value < Bt709Beta |
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? value * 4.5F |
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: (Bt709Alpha * MathF.Pow(value, 0.45F)) - (Bt709Alpha - 1F); |
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} |
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/// <summary>
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/// Applies the inverse extended BT.1361 transfer function.
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/// </summary>
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/// <param name="value">The nonlinear signal value.</param>
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/// <returns>The linear signal value.</returns>
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private static float ToLinearBt1361(float value) |
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{ |
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if (value < -0.25F) |
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{ |
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return -0.25F; |
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} |
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if (value < 0F) |
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{ |
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return MathF.Pow((value - 0.02482420670236F) / -0.27482420670236F, 1F / 0.45F) / -4F; |
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} |
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return ToLinearBt709(value); |
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} |
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/// <summary>
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/// Applies the extended BT.1361 transfer function.
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/// </summary>
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/// <param name="value">The linear signal value.</param>
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/// <returns>The nonlinear signal value.</returns>
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private static float ToGammaBt1361(float value) |
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{ |
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if (value < -0.25F) |
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{ |
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return -0.25F; |
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} |
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if (value < 0F) |
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{ |
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return (-0.27482420670236F * MathF.Pow(-4F * value, 0.45F)) + 0.02482420670236F; |
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} |
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return ToGammaBt709(value); |
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} |
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/// <summary>
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/// Applies the inverse extended IEC 61966-2-1 transfer function.
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/// </summary>
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/// <param name="value">The nonlinear signal value.</param>
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/// <returns>The linear signal value.</returns>
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private static float ToLinearSrgb(float value) |
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{ |
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if (value < -12.92F * SrgbBeta) |
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{ |
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return -MathF.Pow((value - (SrgbAlpha - 1F)) / -SrgbAlpha, 2.4F); |
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} |
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return value < 12.92F * SrgbBeta |
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? value / 12.92F |
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: MathF.Pow((value + (SrgbAlpha - 1F)) / SrgbAlpha, 2.4F); |
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} |
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/// <summary>
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/// Applies the extended IEC 61966-2-1 transfer function.
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/// </summary>
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/// <param name="value">The linear signal value.</param>
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/// <returns>The nonlinear signal value.</returns>
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private static float ToGammaSrgb(float value) |
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{ |
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if (value < -SrgbBeta) |
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{ |
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return (-SrgbAlpha * MathF.Pow(-value, 1F / 2.4F)) + (SrgbAlpha - 1F); |
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} |
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return value < SrgbBeta |
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? value * 12.92F |
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: (SrgbAlpha * MathF.Pow(value, 1F / 2.4F)) - (SrgbAlpha - 1F); |
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} |
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/// <summary>
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/// Applies the inverse SMPTE ST 2084 perceptual-quantizer transfer function.
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/// </summary>
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/// <param name="value">The nonlinear signal value.</param>
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/// <returns>The normalized linear signal value.</returns>
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private static float ToLinearPq(float value) |
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{ |
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if (value <= 0F) |
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{ |
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return 0F; |
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} |
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float nonlinearPower = MathF.Pow(MathF.Min(value, 1F), 1F / PqM); |
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float numerator = MathF.Max(nonlinearPower - PqC1, 0F); |
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float denominator = PqC2 - (PqC3 * nonlinearPower); |
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return MathF.Pow(numerator / denominator, 1F / PqN); |
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} |
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/// <summary>
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/// Applies the SMPTE ST 2084 perceptual-quantizer transfer function.
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/// </summary>
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/// <param name="value">The normalized linear signal value.</param>
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/// <returns>The nonlinear signal value.</returns>
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private static float ToGammaPq(float value) |
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{ |
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if (value <= 0F) |
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{ |
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return 0F; |
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} |
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float linearPower = MathF.Pow(MathF.Min(value, 1F), PqN); |
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return MathF.Pow((PqC1 + (PqC2 * linearPower)) / (1F + (PqC3 * linearPower)), PqM); |
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} |
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/// <summary>
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/// Applies the inverse HLG opto-electronic transfer function.
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/// </summary>
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/// <param name="value">The nonlinear signal value.</param>
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/// <returns>The normalized linear signal value.</returns>
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private static float ToLinearHlg(float value) |
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{ |
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if (value <= 0F) |
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{ |
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return 0F; |
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} |
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return value <= 0.5F |
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? (value * value) / 3F |
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: (MathF.Exp((MathF.Min(value, 1F) - HlgC) / HlgA) + HlgB) / 12F; |
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} |
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/// <summary>
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/// Applies the HLG opto-electronic transfer function.
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/// </summary>
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/// <param name="value">The normalized linear signal value.</param>
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/// <returns>The nonlinear signal value.</returns>
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private static float ToGammaHlg(float value) |
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{ |
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if (value <= 0F) |
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{ |
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return 0F; |
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} |
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float bounded = MathF.Min(value, 1F); |
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return bounded <= 1F / 12F |
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? MathF.Sqrt(3F * bounded) |
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: (HlgA * MathF.Log((12F * bounded) - HlgB)) + HlgC; |
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} |
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} |
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