From 9c3e36e72cdd569f866aa7477a0f2e8dee901081 Mon Sep 17 00:00:00 2001
From: Nico Weber <thakis@chromium.org>
Date: Mon, 1 May 2023 20:52:21 -0400
Subject: [PATCH] ICC: Implement TRC inversion in from_pcs for point curves

This allows converting to a color space that uses a non-parametric
curve, for example:

    Build/lagom/image -o foo.png \
        --convert-to-color-profile .../profiles/sRGB-v2-micro.icc \
        input.jpg

...where profiles/sRGB-v2-micro.icc is from
https://github.com/saucecontrol/Compact-ICC-Profiles/

(Parametric curves are new in ICC v4, which means all v2 profiles
use point curves.)
---
 Userland/Libraries/LibGfx/ICC/Profile.cpp |  6 ++--
 Userland/Libraries/LibGfx/ICC/TagTypes.h  | 35 +++++++++++++++++++++++
 2 files changed, 37 insertions(+), 4 deletions(-)

diff --git a/Userland/Libraries/LibGfx/ICC/Profile.cpp b/Userland/Libraries/LibGfx/ICC/Profile.cpp
index 5fa6b66621..94c337315d 100644
--- a/Userland/Libraries/LibGfx/ICC/Profile.cpp
+++ b/Userland/Libraries/LibGfx/ICC/Profile.cpp
@@ -1570,10 +1570,8 @@ ErrorOr<void> Profile::from_pcs(FloatVector3 const& pcs, Bytes color) const
             auto evaluate_curve_inverse = [this](TagSignature curve_tag, float f) {
                 auto const& trc = *m_tag_table.get(curve_tag).value();
                 VERIFY(trc.type() == CurveTagData::Type || trc.type() == ParametricCurveTagData::Type);
-                if (trc.type() == CurveTagData::Type) {
-                    TODO();
-                    return 0.f;
-                }
+                if (trc.type() == CurveTagData::Type)
+                    return static_cast<CurveTagData const&>(trc).evaluate_inverse(f);
                 return static_cast<ParametricCurveTagData const&>(trc).evaluate_inverse(f);
             };
 
diff --git a/Userland/Libraries/LibGfx/ICC/TagTypes.h b/Userland/Libraries/LibGfx/ICC/TagTypes.h
index 8c57f0075a..751207f8cf 100644
--- a/Userland/Libraries/LibGfx/ICC/TagTypes.h
+++ b/Userland/Libraries/LibGfx/ICC/TagTypes.h
@@ -179,6 +179,41 @@ public:
         return mix(values()[i] / 65535.f, values()[i + 1] / 65535.f, f);
     }
 
+    // y must be in [0..1].
+    float evaluate_inverse(float y) const
+    {
+        VERIFY(0.f <= y && y <= 1.f);
+
+        if (values().is_empty())
+            return y;
+
+        if (values().size() == 1)
+            return powf(y, 1.f / (values()[0] / (float)0x100));
+
+        // FIXME: Verify somewhere that:
+        // * values() is non-decreasing
+        // * values()[0] is 0, values()[values().size() - 1] is 65535
+
+        // FIXME: Use binary search.
+        size_t n = values().size() - 1;
+        size_t i = 0;
+        for (; i < n; ++i) {
+            if (values()[i] / 65535.f <= y && y <= values()[i + 1] / 65535.f)
+                break;
+        }
+
+        float x1 = i / (float)n;
+        float y1 = values()[i] / 65535.f;
+        float x2 = (i + 1) / (float)n;
+        float y2 = values()[i + 1] / 65535.f;
+
+        // Flat line segment?
+        if (y1 == y2)
+            return (x1 + x2) / 2;
+
+        return (y - y1) / (y2 - y1) * (x2 - x1) + x1; // Same as `((y - y1) / (y2 - y1) + i) / (float)n`
+    }
+
 private:
     Vector<u16> m_values;
 };