From adbaeaab9065a2b7b1aaa239179fa7076e47839d Mon Sep 17 00:00:00 2001
From: Jonathan Brown <jonathanbr30@gmail.com>
Date: Tue, 3 Jun 2025 15:48:59 +0100
Subject: [PATCH 1/3] Add bool

---
 lib/extras/enc/jpegli.cc | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/extras/enc/jpegli.cc b/lib/extras/enc/jpegli.cc
index 14a7a398..630d6f9e 100644
--- a/lib/extras/enc/jpegli.cc
+++ b/lib/extras/enc/jpegli.cc
@@ -34,6 +34,7 @@
 #include "lib/extras/xyb_transform.h"
 #include "lib/jpegli/common.h"
 #include "lib/jpegli/encode.h"
+#include "lib/jpegli/encode_internal.h"
 #include "lib/jpegli/types.h"
 
 namespace jxl {
@@ -435,6 +436,7 @@ Status EncodeJpeg(const PackedPixelFile& ppf, const JpegSettings& jpeg_settings,
     jpegli_set_cicp_transfer_function(&cinfo, cicp_tf);
     jpegli_set_defaults(&cinfo);
     if (!jpeg_settings.chroma_subsampling.empty()) {
+      cinfo.master->chroma_subsampling_set_by_cli = true;
       if (jpeg_settings.chroma_subsampling == "444") {
         cinfo.comp_info[0].h_samp_factor = 1;
         cinfo.comp_info[0].v_samp_factor = 1;

From c8ecec633485e84d484c3c86fb2a317432f01138 Mon Sep 17 00:00:00 2001
From: Jonathan Brown <jonathanbr30@gmail.com>
Date: Tue, 3 Jun 2025 15:54:13 +0100
Subject: [PATCH 2/3] Set parameters based on distance

---
 lib/jpegli/encode.cc | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/lib/jpegli/encode.cc b/lib/jpegli/encode.cc
index ac4917d2..5d261553 100644
--- a/lib/jpegli/encode.cc
+++ b/lib/jpegli/encode.cc
@@ -837,6 +837,22 @@ void jpegli_set_distance(j_compress_ptr cinfo, float distance,
                          boolean force_baseline) {
   CheckState(cinfo, jpegli::kEncStart);
   cinfo->master->force_baseline = FROM_JXL_BOOL(force_baseline);
+  if (distance >= 1.9f && !(cinfo->master->xyb_mode) &&
+      !cinfo->master->chroma_subsampling_set_by_cli) {
+    // At medium qualities, 420 subsampling begins to outperform 444.
+    cinfo->comp_info[0].h_samp_factor = cinfo->comp_info[0].v_samp_factor = 2;
+    if (cinfo->jpeg_color_space == JCS_YCCK) {
+      cinfo->comp_info[3].h_samp_factor = cinfo->comp_info[3].v_samp_factor = 2;
+    }
+  }
+  // Disable adaptive quantization at high qualities.
+  if (distance <= 1.0f && !(cinfo->master->xyb_mode)) {
+    cinfo->master->use_adaptive_quantization = false;
+  }
+  // At quality 100 (distance 0) auto select RGB colorspace.
+  if (distance <= 0.01f && cinfo->in_color_space == JCS_RGB) {
+    jpegli_set_colorspace(cinfo, JCS_RGB);
+  }
   float distances[NUM_QUANT_TBLS] = {distance, distance, distance};
   jpegli::SetQuantMatrices(cinfo, distances, /*add_two_chroma_tables=*/true);
 }
@@ -862,6 +878,22 @@ void jpegli_set_quality(j_compress_ptr cinfo, int quality,
   CheckState(cinfo, jpegli::kEncStart);
   cinfo->master->force_baseline = FROM_JXL_BOOL(force_baseline);
   float distance = jpegli_quality_to_distance(quality);
+  if (distance >= 1.9f && !(cinfo->master->xyb_mode) &&
+      !cinfo->master->chroma_subsampling_set_by_cli) {
+    // At medium qualities, 420 subsampling begins to outperform 444.
+    cinfo->comp_info[0].h_samp_factor = cinfo->comp_info[0].v_samp_factor = 2;
+    if (cinfo->jpeg_color_space == JCS_YCCK) {
+      cinfo->comp_info[3].h_samp_factor = cinfo->comp_info[3].v_samp_factor = 2;
+    }
+  }
+  // Disable adaptive quantization at high qualities.
+  if (distance <= 1.0f && !(cinfo->master->xyb_mode)) {
+    cinfo->master->use_adaptive_quantization = false;
+  }
+  // At quality 100 (distance 0) auto select RGB colorspace.
+  if (distance <= 0.01f && cinfo->in_color_space == JCS_RGB) {
+    jpegli_set_colorspace(cinfo, JCS_RGB);
+  }
   float distances[NUM_QUANT_TBLS] = {distance, distance, distance};
   jpegli::SetQuantMatrices(cinfo, distances, /*add_two_chroma_tables=*/false);
 }

From 491a12b4de7f4d19db969ecf35631d53f1bf625e Mon Sep 17 00:00:00 2001
From: Jonathan Brown <jonathanbr30@gmail.com>
Date: Tue, 3 Jun 2025 15:55:04 +0100
Subject: [PATCH 3/3] Add bool

---
 lib/jpegli/encode_internal.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/jpegli/encode_internal.h b/lib/jpegli/encode_internal.h
index 3f5d1a76..b0236a6a 100644
--- a/lib/jpegli/encode_internal.h
+++ b/lib/jpegli/encode_internal.h
@@ -75,6 +75,7 @@ struct jpeg_comp_master {
   uint8_t cicp_transfer_function;
   bool use_std_tables;
   bool use_adaptive_quantization;
+  bool chroma_subsampling_set_by_cli = false;
   int progressive_level;
   size_t xsize_blocks;
   size_t ysize_blocks;