ICC: Implement TRC inversion in from_pcs for parametric curves

Tests/LibGfx/TestICCProfile.cpp

@@ -158,6 +158,10 @@ TEST_CASE(from_pcs)
 {
     auto sRGB = MUST(Gfx::ICC::sRGB());
 
+    auto sRGB_curve_pointer = MUST(Gfx::ICC::sRGB_curve());
+    VERIFY(sRGB_curve_pointer->type() == Gfx::ICC::ParametricCurveTagData::Type);
+    auto const& sRGB_curve = static_cast<Gfx::ICC::ParametricCurveTagData const&>(*sRGB_curve_pointer);
+
     auto sRGB_from_xyz = [&sRGB](FloatVector3 const& XYZ) {
         u8 rgb[3];
         MUST(sRGB->from_pcs(XYZ, rgb));
@@ -185,7 +189,16 @@ TEST_CASE(from_pcs)
     EXPECT_EQ(sRGB_from_xyz(g_xyz + b_xyz), Color(0, 255, 255));
     EXPECT_EQ(sRGB_from_xyz(r_xyz + g_xyz + b_xyz), Color(255, 255, 255));
 
-    // FIXME: Implement and test the inverse curve transform.
+    // Test the inverse curve transform.
+    float f64 = sRGB_curve.evaluate(64 / 255.f);
+    EXPECT_EQ(sRGB_from_xyz((r_xyz + g_xyz + b_xyz) * f64), Color(64, 64, 64));
+
+    float f128 = sRGB_curve.evaluate(128 / 255.f);
+    EXPECT_EQ(sRGB_from_xyz((r_xyz + g_xyz + b_xyz) * f128), Color(128, 128, 128));
+
+    // Test for curve and matrix combined.
+    float f192 = sRGB_curve.evaluate(192 / 255.f);
+    EXPECT_EQ(sRGB_from_xyz(r_xyz * f64 + g_xyz * f128 + b_xyz * f192), Color(64, 128, 192));
 }
 
 TEST_CASE(to_lab)

Userland/Libraries/LibGfx/ICC/Profile.cpp

@@ -1551,6 +1551,7 @@ ErrorOr<void> Profile::from_pcs(FloatVector3 const& pcs, Bytes color) const
             // greenTRC^-1, and blueTRC^-1 function is undefined. If a one-dimensional curve is constant, the curve cannot be
             // inverted."
 
+            // Convert from XYZ to linear rgb.
             // FIXME: Inverting matrix and curve on every call to this function is very inefficient.
             auto const& red_matrix_column = this->red_matrix_column();
             auto const& green_matrix_column = this->green_matrix_column();
@@ -1561,13 +1562,22 @@ ErrorOr<void> Profile::from_pcs(FloatVector3 const& pcs, Bytes color) const
                 red_matrix_column.Y, green_matrix_column.Y, blue_matrix_column.Y,
                 red_matrix_column.Z, green_matrix_column.Z, blue_matrix_column.Z
             };
-
             if (!forward_matrix.is_invertible())
                 return Error::from_string_literal("ICC::Profile::from_pcs: matrix not invertible");
             auto matrix = forward_matrix.inverse();
-
             FloatVector3 linear_rgb = matrix * pcs;
 
+            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;
+                }
+                return static_cast<ParametricCurveTagData const&>(trc).evaluate_inverse(f);
+            };
+
+            // Convert from linear rgb to device rgb.
             // See equations (F.8) - (F.16) above.
             // FIXME: The spec says to do this, but it loses information. Color.js returns unclamped
             //        values instead (...but how do those make it through the TRC?) and has a separate
@@ -1576,12 +1586,13 @@ ErrorOr<void> Profile::from_pcs(FloatVector3 const& pcs, Bytes color) const
             //        (For LUT profiles, I think the gamut mapping is baked into the BToA* data in the profile (?).
             //        But for matrix profiles, it'd have to be done in code.)
             linear_rgb.clamp(0.f, 1.f);
+            float device_r = evaluate_curve_inverse(redTRCTag, linear_rgb[0]);
+            float device_g = evaluate_curve_inverse(greenTRCTag, linear_rgb[1]);
+            float device_b = evaluate_curve_inverse(blueTRCTag, linear_rgb[2]);
 
-            // FIXME: Implement curve inversion and apply inverse curve transform here.
+            color[0] = round(255 * device_r);
-
+            color[1] = round(255 * device_g);
-            color[0] = round(255 * linear_rgb[0]);
+            color[2] = round(255 * device_b);
-            color[1] = round(255 * linear_rgb[1]);
-            color[2] = round(255 * linear_rgb[2]);
             return {};
         }
 

Userland/Libraries/LibGfx/ICC/TagTypes.h

@@ -739,6 +739,45 @@ public:
         VERIFY_NOT_REACHED();
     }
 
+    // y must be in [0..1].
+    float evaluate_inverse(float y) const
+    {
+        VERIFY(0.f <= y && y <= 1.f);
+
+        // See "Recommendations" section in https://www.color.org/whitepapers/ICC_White_Paper35-Use_of_the_parametricCurveType.pdf
+        // Requirements for the curve to be non-decreasing:
+        // * γ > 0
+        // * a > 0 for types 1-4
+        // * c ≥ 0 for types 3 and 4
+        //
+        // Types 3 and 4 additionally require:
+        // To prevent negative discontinuities:
+        // * cd ≤ (ad + b) for type 3
+        // * cd + f ≤ (ad + b)^γ + e for type 4
+        // To prevent complex numbers:
+        // * ad + b ≥ 0
+        // FIXME: Check these requirements somewhere.
+
+        switch (function_type()) {
+        case FunctionType::Type0:
+            return powf(y, 1.f / (float)g());
+        case FunctionType::Type1:
+            return (powf(y, 1.f / (float)g()) - (float)b()) / (float)a();
+        case FunctionType::Type2:
+            // Only defined for Y >= c, so I suppose this requires c <= 0 in practice (?).
+            return (powf(y - (float)c(), 1.f / (float)g()) - (float)b()) / (float)a();
+        case FunctionType::Type3:
+            if (y >= (float)c() * (float)d())
+                return (powf(y, 1.f / (float)g()) - (float)b()) / (float)a();
+            return y / (float)c();
+        case FunctionType::Type4:
+            if (y >= (float)c() * (float)d())
+                return (powf(y - (float)e(), 1.f / (float)g()) - (float)b()) / (float)a();
+            return (y - (float)f()) / (float)c();
+        }
+        VERIFY_NOT_REACHED();
+    }
+
 private:
     FunctionType m_function_type;