From f2c60b77163e40d191f57c9f0307125857367016 Mon Sep 17 00:00:00 2001
From: Lucas CHOLLET <lucas.chollet@free.fr>
Date: Thu, 10 Aug 2023 23:51:11 -0400
Subject: [PATCH] LibGfx/JPEGXL: Support images encoded with the YCbCr color
 space

While being way less frequent than for classical JPEG images, JPEG XL
images can use the YCbCr color space. Supporting it makes us properly
decode the "The Smoke Machine" image on https://jpegxl.info/jxl-art.html
---
 .../LibGfx/ImageFormats/JPEGXLLoader.cpp      | 40 +++++++++++++++++--
 1 file changed, 37 insertions(+), 3 deletions(-)

diff --git a/Userland/Libraries/LibGfx/ImageFormats/JPEGXLLoader.cpp b/Userland/Libraries/LibGfx/ImageFormats/JPEGXLLoader.cpp
index 042bb222ca..3aa262b175 100644
--- a/Userland/Libraries/LibGfx/ImageFormats/JPEGXLLoader.cpp
+++ b/Userland/Libraries/LibGfx/ImageFormats/JPEGXLLoader.cpp
@@ -2453,6 +2453,42 @@ static ErrorOr<void> apply_image_features(Image& image, ImageMetadata const& met
 }
 ///
 
+/// L.2 - XYB + L.3 - YCbCr
+static void ycbcr_to_rgb(Image& image, u8 bits_per_sample)
+{
+    auto& channels = image.channels();
+    VERIFY(channels.size() >= 3);
+
+    VERIFY(channels[0].width() == channels[1].width() && channels[1].width() == channels[2].width());
+    VERIFY(channels[0].height() == channels[1].height() && channels[1].height() == channels[2].height());
+
+    auto const half_range_offset = (1 << bits_per_sample) / 2;
+    for (u32 y = 0; y < channels[0].height(); ++y) {
+        for (u32 x = 0; x < channels[0].width(); ++x) {
+            auto const cb = channels[0].get(x, y);
+            auto const luma = channels[1].get(x, y);
+            auto const cr = channels[2].get(x, y);
+
+            channels[0].set(x, y, luma + half_range_offset + 1.402 * cr);
+            channels[1].set(x, y, luma + half_range_offset - 0.344136 * cb - 0.714136 * cr);
+            channels[2].set(x, y, luma + half_range_offset + 1.772 * cb);
+        }
+    }
+}
+
+static void apply_colour_transformation(Image& image, ImageMetadata const& metadata, Frame const& frame)
+{
+    if (frame.frame_header.do_YCbCr)
+        ycbcr_to_rgb(image, metadata.bit_depth.bits_per_sample);
+
+    if (metadata.xyb_encoded) {
+        TODO();
+    } else {
+        // FIXME: Do a proper color transformation with metadata.colour_encoding
+    }
+}
+///
+
 /// L.4 - Extra channel rendering
 static ErrorOr<void> render_extra_channels(Image&, ImageMetadata const& metadata)
 {
@@ -2500,9 +2536,7 @@ public:
 
         TRY(apply_image_features(image, m_metadata, frame));
 
-        // FIXME: Do a proper color transformation with metadata.colour_encoding
-        if (m_metadata.xyb_encoded || frame.frame_header.do_YCbCr)
-            TODO();
+        apply_colour_transformation(image, m_metadata, frame);
 
         TRY(render_extra_channels(image, m_metadata));