Implement the Delta E algorithm to improve color perception (#11095)

- Implements the Delta E algorithm
- Uses the Delta E algorithm to precalculate adjusted foreground values based on possible foreground/background color pairs in the color table
- Adds a setting to use the adjusted foreground values when applicable

## PR Checklist
* [x] Closes #2638
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [ ] Schema updated.
* [x] I work here

## Validation Steps Performed
Before:
<img width="759" alt="color before" src="https://user-images.githubusercontent.com/26824113/131576768-b3b9eebd-5933-45de-8da8-88a985070312.png">

After (note dark blue):
<img width="760" alt="color after" src="https://user-images.githubusercontent.com/26824113/133158807-4e63198f-8a49-4d03-914e-55a5ad57d725.png">

.github/actions/spelling/allow/math.txt

@@ -1,3 +1,10 @@
+atan
+CPrime
+HBar
+HPrime
 isnan
+LPrime
+LStep
 powf
+RSub
 sqrtf

.github/actions/spelling/expect/web.txt

@@ -1,5 +1,7 @@
 http
 www
+easyrgb
+php
 ecma
 rapidtables
 WCAG

NOTICE.md

@@ -252,6 +252,43 @@ DEALINGS IN THE SOFTWARE.
 
 ```
 
+## ConEmu
+**Source**: [https://github.com/Maximus5/ConEmu](https://github.com/Maximus5/ConEmu)
+
+### License
+
+```
+BSD 3-Clause License
+
+Copyright (c) 2009-2017, Maximus5 <ConEmu.Maximus5@gmail.com>
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+* Neither the name of the copyright holder nor the names of its
+  contributors may be used to endorse or promote products derived from
+  this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+```
+
 # Microsoft Open Source
 
 This product also incorporates source code from other Microsoft open source projects, all licensed under the MIT license.

doc/cascadia/profiles.schema.json

@@ -188,6 +188,10 @@
           ],
           "type": "string"
         },
+        "adjustIndistinguishableColors": {
+          "description": "When set to true, we will (when necessary) adjust the foreground color to make it more visible, based on the background color.",
+          "type": "boolean"
+        },
         "experimental.retroTerminalEffect": {
           "description": "When set to true, enable retro terminal effects when unfocused. This is an experimental feature, and its continued existence is not guaranteed.",
           "type": "boolean"
@@ -1973,6 +1977,10 @@
             }
           ]
         },
+        "adjustIndistinguishableColors": {
+          "description": "When set to true, we will (when necessary) adjust the foreground color to make it more visible, based on the background color.",
+          "type": "boolean"
+        },
         "scrollbarState": {
           "default": "visible",
           "description": "Defines the visibility of the scrollbar.",

src/cascadia/TerminalCore/ColorFix.cpp

@@ -0,0 +1,228 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+// A lot of code was taken from
+// https://github.com/Maximus5/ConEmu/blob/master/src/ConEmu/ColorFix.cpp
+// and then adjusted to fit our style guidelines
+
+#include "pch.h"
+
+#include <Windows.h>
+#include "ColorFix.hpp"
+
+static constexpr double gMinThreshold = 12.0;
+static constexpr double gExpThreshold = 20.0;
+static constexpr double gLStep = 5.0;
+
+static constexpr double rad006 = 0.104719755119659774;
+static constexpr double rad025 = 0.436332312998582394;
+static constexpr double rad030 = 0.523598775598298873;
+static constexpr double rad060 = 1.047197551196597746;
+static constexpr double rad063 = 1.099557428756427633;
+static constexpr double rad180 = 3.141592653589793238;
+static constexpr double rad275 = 4.799655442984406336;
+static constexpr double rad360 = 6.283185307179586476;
+
+ColorFix::ColorFix(COLORREF color)
+{
+    rgb = color;
+    _ToLab();
+}
+
+// Method Description:
+// - Helper function to calculate HPrime
+double ColorFix::_GetHPrimeFn(double x, double y)
+{
+    if (x == 0 && y == 0)
+    {
+        return 0;
+    }
+
+    const auto hueAngle = atan2(x, y);
+    return hueAngle >= 0 ? hueAngle : hueAngle + rad360;
+}
+
+// Method Description:
+// - Given 2 colors, computes the DeltaE value between them
+// Arguments:
+// - x1: the first color
+// - x2: the second color
+// Return Value:
+// - The DeltaE value between x1 and x2
+double ColorFix::_GetDeltaE(ColorFix x1, ColorFix x2)
+{
+    constexpr double kSubL = 1;
+    constexpr double kSubC = 1;
+    constexpr double kSubH = 1;
+
+    // Delta L Prime
+    const double deltaLPrime = x2.L - x1.L;
+
+    // L Bar
+    const double lBar = (x1.L + x2.L) / 2;
+
+    // C1 & C2
+    const double c1 = sqrt(pow(x1.A, 2) + pow(x1.B, 2));
+    const double c2 = sqrt(pow(x2.A, 2) + pow(x2.B, 2));
+
+    // C Bar
+    const double cBar = (c1 + c2) / 2;
+
+    // A Prime 1
+    const double aPrime1 = x1.A + (x1.A / 2) * (1 - sqrt(pow(cBar, 7) / (pow(cBar, 7) + pow(25.0, 7))));
+
+    // A Prime 2
+    const double aPrime2 = x2.A + (x2.A / 2) * (1 - sqrt(pow(cBar, 7) / (pow(cBar, 7) + pow(25.0, 7))));
+
+    // C Prime 1
+    const double cPrime1 = sqrt(pow(aPrime1, 2) + pow(x1.B, 2));
+
+    // C Prime 2
+    const double cPrime2 = sqrt(pow(aPrime2, 2) + pow(x2.B, 2));
+
+    // C Bar Prime
+    const double cBarPrime = (cPrime1 + cPrime2) / 2;
+
+    // Delta C Prime
+    const double deltaCPrime = cPrime2 - cPrime1;
+
+    // S sub L
+    const double sSubL = 1 + ((0.015 * pow(lBar - 50, 2)) / sqrt(20 + pow(lBar - 50, 2)));
+
+    // S sub C
+    const double sSubC = 1 + 0.045 * cBarPrime;
+
+    // h Prime 1
+    const double hPrime1 = _GetHPrimeFn(x1.B, aPrime1);
+
+    // h Prime 2
+    const double hPrime2 = _GetHPrimeFn(x2.B, aPrime2);
+
+    // Delta H Prime
+    const double deltaHPrime = 0 == c1 || 0 == c2 ? 0 : 2 * sqrt(cPrime1 * cPrime2) * sin(abs(hPrime1 - hPrime2) <= rad180 ? hPrime2 - hPrime1 : (hPrime2 <= hPrime1 ? hPrime2 - hPrime1 + rad360 : hPrime2 - hPrime1 - rad360) / 2);
+
+    // H Bar Prime
+    const double hBarPrime = (abs(hPrime1 - hPrime2) > rad180) ? (hPrime1 + hPrime2 + rad360) / 2 : (hPrime1 + hPrime2) / 2;
+
+    // T
+    const double t = 1 - 0.17 * cos(hBarPrime - rad030) + 0.24 * cos(2 * hBarPrime) + 0.32 * cos(3 * hBarPrime + rad006) - 0.20 * cos(4 * hBarPrime - rad063);
+
+    // S sub H
+    const double sSubH = 1 + 0.015 * cBarPrime * t;
+
+    // R sub T
+    const double rSubT = -2 * sqrt(pow(cBarPrime, 7) / (pow(cBarPrime, 7) + pow(25.0, 7))) * sin(rad060 * exp(-pow((hBarPrime - rad275) / rad025, 2)));
+
+    // Put it all together!
+    const double lightness = deltaLPrime / (kSubL * sSubL);
+    const double chroma = deltaCPrime / (kSubC * sSubC);
+    const double hue = deltaHPrime / (kSubH * sSubH);
+
+    return sqrt(pow(lightness, 2) + pow(chroma, 2) + pow(hue, 2) + rSubT * chroma * hue);
+}
+
+// Method Description:
+// - Populates our L, A, B values, based on our r, g, b values
+// - Converts a color in rgb format to a color in lab format
+// - Reference: http://www.easyrgb.com/index.php?X=MATH&H=01#text1
+void ColorFix::_ToLab()
+{
+    double var_R = r / 255.0;
+    double var_G = g / 255.0;
+    double var_B = b / 255.0;
+
+    var_R = var_R > 0.04045 ? pow(((var_R + 0.055) / 1.055), 2.4) : var_R / 12.92;
+    var_G = var_G > 0.04045 ? pow(((var_G + 0.055) / 1.055), 2.4) : var_G / 12.92;
+    var_B = var_B > 0.04045 ? pow(((var_B + 0.055) / 1.055), 2.4) : var_B / 12.92;
+
+    var_R = var_R * 100.;
+    var_G = var_G * 100.;
+    var_B = var_B * 100.;
+
+    //Observer. = 2 degrees, Illuminant = D65
+    const double X = var_R * 0.4124 + var_G * 0.3576 + var_B * 0.1805;
+    const double Y = var_R * 0.2126 + var_G * 0.7152 + var_B * 0.0722;
+    const double Z = var_R * 0.0193 + var_G * 0.1192 + var_B * 0.9505;
+
+    double var_X = X / 95.047; //ref_X =  95.047   (Observer= 2 degrees, Illuminant= D65)
+    double var_Y = Y / 100.000; //ref_Y = 100.000
+    double var_Z = Z / 108.883; //ref_Z = 108.883
+
+    var_X = var_X > 0.008856 ? pow(var_X, (1. / 3.)) : (7.787 * var_X) + (16. / 116.);
+    var_Y = var_Y > 0.008856 ? pow(var_Y, (1. / 3.)) : (7.787 * var_Y) + (16. / 116.);
+    var_Z = var_Z > 0.008856 ? pow(var_Z, (1. / 3.)) : (7.787 * var_Z) + (16. / 116.);
+
+    L = (116. * var_Y) - 16.;
+    A = 500. * (var_X - var_Y);
+    B = 200. * (var_Y - var_Z);
+}
+
+// Method Description:
+// - Populates our r, g, b values, based on our L, A, B values
+// - Converts a color in lab format to a color in rgb format
+// - Reference: http://www.easyrgb.com/index.php?X=MATH&H=01#text1
+void ColorFix::_ToRGB()
+{
+    double var_Y = (L + 16.) / 116.;
+    double var_X = A / 500. + var_Y;
+    double var_Z = var_Y - B / 200.;
+
+    var_Y = (pow(var_Y, 3) > 0.008856) ? pow(var_Y, 3) : (var_Y - 16. / 116.) / 7.787;
+    var_X = (pow(var_X, 3) > 0.008856) ? pow(var_X, 3) : (var_X - 16. / 116.) / 7.787;
+    var_Z = (pow(var_Z, 3) > 0.008856) ? pow(var_Z, 3) : (var_Z - 16. / 116.) / 7.787;
+
+    double X = 95.047 * var_X; //ref_X =  95.047     (Observer= 2 degrees, Illuminant= D65)
+    double Y = 100.000 * var_Y; //ref_Y = 100.000
+    double Z = 108.883 * var_Z; //ref_Z = 108.883
+
+    var_X = X / 100.; //X from 0 to  95.047      (Observer = 2 degrees, Illuminant = D65)
+    var_Y = Y / 100.; //Y from 0 to 100.000
+    var_Z = Z / 100.; //Z from 0 to 108.883
+
+    double var_R = var_X * 3.2406 + var_Y * -1.5372 + var_Z * -0.4986;
+    double var_G = var_X * -0.9689 + var_Y * 1.8758 + var_Z * 0.0415;
+    double var_B = var_X * 0.0557 + var_Y * -0.2040 + var_Z * 1.0570;
+
+    var_R = var_R > 0.0031308 ? 1.055 * pow(var_R, (1 / 2.4)) - 0.055 : var_R = 12.92 * var_R;
+    var_G = var_G > 0.0031308 ? 1.055 * pow(var_G, (1 / 2.4)) - 0.055 : var_G = 12.92 * var_G;
+    var_B = var_B > 0.0031308 ? 1.055 * pow(var_B, (1 / 2.4)) - 0.055 : var_B = 12.92 * var_B;
+
+    r = (BYTE)std::clamp(var_R * 255., 0., 255.);
+    g = (BYTE)std::clamp(var_G * 255., 0., 255.);
+    b = (BYTE)std::clamp(var_B * 255., 0., 255.);
+}
+
+// Method Description:
+// - Given foreground and background colors, change the foreground color to
+//   make it more perceivable if necessary
+// - Arguments:
+// - fg: the foreground color
+// - bg: the background color
+// - Return Value:
+// - The foreground color after performing any necessary changes to make it more perceivable
+COLORREF ColorFix::GetPerceivableColor(COLORREF fg, COLORREF bg)
+{
+    ColorFix backLab(bg);
+    ColorFix frontLab(fg);
+    const double de1 = _GetDeltaE(frontLab, backLab);
+    if (de1 < gMinThreshold)
+    {
+        for (int i = 0; i <= 1; i++)
+        {
+            const double step = (i == 0) ? gLStep : -gLStep;
+            frontLab.L += step;
+
+            while (((i == 0) && (frontLab.L <= 100)) || ((i == 1) && (frontLab.L >= 0)))
+            {
+                const double de2 = _GetDeltaE(frontLab, backLab);
+                if (de2 >= gExpThreshold)
+                {
+                    frontLab._ToRGB();
+                    return frontLab.rgb;
+                }
+                frontLab.L += step;
+            }
+        }
+    }
+    return frontLab.rgb;
+}

src/cascadia/TerminalCore/ColorFix.hpp

@@ -0,0 +1,49 @@
+/*++
+Copyright (c) Microsoft Corporation
+Licensed under the MIT license.
+
+Module Name:
+- ColorFix
+
+Abstract:
+- Implementation of perceptual color nudging, which allows the Terminal
+  to slightly shift the foreground color to make it more perceivable on
+  the current background (for cases where the foreground is very close
+  to being imperceivable on the background).
+
+Author(s):
+- Pankaj Bhojwani - Sep 2021
+
+--*/
+
+#pragma once
+
+struct ColorFix
+{
+public:
+    ColorFix(COLORREF color);
+
+    static COLORREF GetPerceivableColor(COLORREF fg, COLORREF bg);
+
+    // RGB
+    union
+    {
+        struct
+        {
+            BYTE r, g, b, dummy;
+        };
+        COLORREF rgb;
+    };
+
+    // Lab
+    struct
+    {
+        double L, A, B;
+    };
+
+private:
+    static double _GetHPrimeFn(double x, double y);
+    static double _GetDeltaE(ColorFix x1, ColorFix x2);
+    void _ToLab();
+    void _ToRGB();
+};

src/cascadia/TerminalCore/ICoreAppearance.idl

@@ -67,5 +67,6 @@ namespace Microsoft.Terminal.Core
         CursorStyle CursorShape;
         UInt32 CursorHeight;
         Boolean IntenseIsBright;
+        Boolean AdjustIndistinguishableColors;
     };
 }

src/cascadia/TerminalCore/Terminal.cpp

@@ -53,7 +53,8 @@ Terminal::Terminal() :
     _taskbarState{ 0 },
     _taskbarProgress{ 0 },
     _trimBlockSelection{ false },
-    _intenseIsBright{ true }
+    _intenseIsBright{ true },
+    _adjustIndistinguishableColors{ true }
 {
     auto dispatch = std::make_unique<TerminalDispatch>(*this);
     auto engine = std::make_unique<OutputStateMachineEngine>(std::move(dispatch));
@@ -175,11 +176,16 @@ void Terminal::UpdateAppearance(const ICoreAppearance& appearance)
     _defaultBg = newBackgroundColor.with_alpha(0);
     _defaultFg = appearance.DefaultForeground();
     _intenseIsBright = appearance.IntenseIsBright();
+    _adjustIndistinguishableColors = appearance.AdjustIndistinguishableColors();
 
     for (int i = 0; i < 16; i++)
     {
         _colorTable.at(i) = til::color{ appearance.GetColorTableEntry(i) };
     }
+    if (_adjustIndistinguishableColors)
+    {
+        _MakeAdjustedColorArray();
+    }
 
     CursorType cursorShape = CursorType::VerticalBar;
     switch (appearance.CursorShape())