From a2a7da3a028120785438f5440230241a3b0e6c19 Mon Sep 17 00:00:00 2001
From: Joni Poikelin <joni.poikelin@qt.io>
Date: Mon, 16 Oct 2023 12:44:03 +0300
Subject: [PATCH] Fix smooth scaling of Format_Mono and Format_MonoLSB

Fixes: QTBUG-117713
Pick-to: 6.5 6.2
Change-Id: I2fb071a4d2229da50dfacb0a92c51c3e4ea57a74
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
(cherry picked from commit 799bfe94e8c17c0ff890c45b22d670d3bc8778c9)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
---
 src/gui/image/qimage.cpp                   | 18 ++++++++++++++----
 tests/auto/gui/image/qimage/tst_qimage.cpp | 20 ++++++++++++++++++++
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/src/gui/image/qimage.cpp b/src/gui/image/qimage.cpp
index 91292d01d1f..4e948ed854b 100644
--- a/src/gui/image/qimage.cpp
+++ b/src/gui/image/qimage.cpp
@@ -4862,14 +4862,24 @@ QImage Q_TRACE_INSTRUMENT(qtgui) QImage::transformed(const QTransform &matrix, Q
             || (ws * hs) >= (1<<20)
 #endif
             ) {
+            QImage scaledImage;
             if (mat.m11() < 0.0F && mat.m22() < 0.0F) { // horizontal/vertical flip
-                return smoothScaled(wd, hd).mirrored(true, true).convertToFormat(format());
+                scaledImage = smoothScaled(wd, hd).mirrored(true, true);
             } else if (mat.m11() < 0.0F) { // horizontal flip
-                return smoothScaled(wd, hd).mirrored(true, false).convertToFormat(format());
+                scaledImage = smoothScaled(wd, hd).mirrored(true, false);
             } else if (mat.m22() < 0.0F) { // vertical flip
-                return smoothScaled(wd, hd).mirrored(false, true).convertToFormat(format());
+                scaledImage = smoothScaled(wd, hd).mirrored(false, true);
             } else { // no flipping
-                return smoothScaled(wd, hd).convertToFormat(format());
+                scaledImage = smoothScaled(wd, hd);
+            }
+
+            switch (format()) {
+            case QImage::Format_Mono:
+            case QImage::Format_MonoLSB:
+            case QImage::Format_Indexed8:
+                return scaledImage;
+            default:
+                return scaledImage.convertToFormat(format());
             }
         }
     }
diff --git a/tests/auto/gui/image/qimage/tst_qimage.cpp b/tests/auto/gui/image/qimage/tst_qimage.cpp
index 2d1137bd09f..fd155fbf1c2 100644
--- a/tests/auto/gui/image/qimage/tst_qimage.cpp
+++ b/tests/auto/gui/image/qimage/tst_qimage.cpp
@@ -108,6 +108,8 @@ private slots:
     void smoothScaleAlpha();
     void smoothScaleFormats_data();
     void smoothScaleFormats();
+    void smoothScaleNoConversion_data();
+    void smoothScaleNoConversion();
 
     void transformed_data();
     void transformed();
@@ -2058,6 +2060,24 @@ void tst_QImage::smoothScaleFormats()
     QVERIFY(rotated.hasAlphaChannel());
 }
 
+void tst_QImage::smoothScaleNoConversion_data()
+{
+    QTest::addColumn<QImage::Format>("format");
+    QTest::addRow("Mono") <<  QImage::Format_Mono;
+    QTest::addRow("MonoLSB") <<  QImage::Format_MonoLSB;
+    QTest::addRow("Indexed8") <<  QImage::Format_Indexed8;
+}
+
+void tst_QImage::smoothScaleNoConversion()
+{
+    QFETCH(QImage::Format, format);
+    QImage img(128, 128, format);
+    img.fill(1);
+    img.setColorTable(QList<QRgb>() << qRgba(255,0,0,255) << qRgba(0,0,0,0));
+    img = img.scaled(QSize(48, 48), Qt::IgnoreAspectRatio, Qt::SmoothTransformation);
+    QVERIFY(img.hasAlphaChannel());
+}
+
 static int count(const QImage &img, int x, int y, int dx, int dy, QRgb pixel)
 {
     int i = 0;