[graphics/krita] libs/ui/tool/strokes: Fix off center preview in instant preview mode

Git commit b0bcdb8c0e9c0aa485f07f841b216a899c8a8be4 by Emmet O'Neill, on behalf of Dat Le.
Committed on 13/08/2026 at 23:16.
Pushed by emmetoneill into branch 'master'.

Fix off center preview in instant preview mode

M  +4    -4    libs/ui/tool/strokes/kis_color_sampler_stroke_strategy.cpp
M  +22   -2    libs/ui/tool/strokes/kis_color_sampler_stroke_strategy.h

https://invent.kde.org/graphics/krita/-/commit/b0bcdb8c0e9c0aa485f07f841b216a899c8a8be4

diff --git a/libs/ui/tool/strokes/kis_color_sampler_stroke_strategy.cpp b/libs/ui/tool/strokes/kis_color_sampler_stroke_strategy.cpp
index d60f92eaf10..095b25b0708 100644
--- a/libs/ui/tool/strokes/kis_color_sampler_stroke_strategy.cpp
+++ b/libs/ui/tool/strokes/kis_color_sampler_stroke_strategy.cpp
@@ -69,13 +69,13 @@ void KisColorSamplerStrokeStrategy::doStrokeCallback(KisStrokeJobData *data)
 
         QImage image = previewData->colorConverter->convertImageToDisplayColorSpace(tmpDev, effectiveRect, true);
 
+        // In case of instant preview, scale the sample back up to fit the original requested rect
         if (previewData->levelOfDetail > 0) {
-            KisLodTransform transform(previewData->levelOfDetail);
-            qreal scale = transform.lodToInvScale(previewData->levelOfDetail);
+            if (!previewData->originalRect.isValid()) return;
 
-            image = image.scaled(QSize(scale * image.width(), scale * image.height()));
+            image = image.scaled(previewData->originalRect.size());
 
-            effectiveRect = transform.mapInverted(effectiveRect);
+            effectiveRect = QRect(QPoint(0,0), previewData->originalRect.size());
         }
 
         Q_EMIT sigCanvasZoomPreviewUpdated(image, effectiveRect);
diff --git a/libs/ui/tool/strokes/kis_color_sampler_stroke_strategy.h b/libs/ui/tool/strokes/kis_color_sampler_stroke_strategy.h
index 00011c71604..4990ed8c0e9 100644
--- a/libs/ui/tool/strokes/kis_color_sampler_stroke_strategy.h
+++ b/libs/ui/tool/strokes/kis_color_sampler_stroke_strategy.h
@@ -51,17 +51,32 @@ public:
     public:
         GenerateCanvasZoomPreviewData(KisPaintDeviceSP _canvasDev, const QRect &_canvasPixelRect, KisDisplayColorConverter *_colorConverter,
                                       int _levelOfDetail = 0, QSharedPointer<boost::none_t> _cookie = nullptr)
-            : canvasDev(_canvasDev), canvasPixelRect(_canvasPixelRect), colorConverter(_colorConverter), levelOfDetail(_levelOfDetail), strokeCookie(_cookie)
+            : canvasDev(_canvasDev), canvasPixelRect(_canvasPixelRect), colorConverter(_colorConverter),
+            levelOfDetail(_levelOfDetail), strokeCookie(_cookie), originalRect()
         {}
 
         KisStrokeJobData* createLodClone(int levelOfDetail) override {
             KisLodTransform transform(levelOfDetail);
+
             QRect lodPixelRect = transform.map(canvasPixelRect);
+
+            // The LOD pixel rect size should be odd so that the pixel sampled is in the center
+            // Otherwise, even after scaling back, it will be wrong
+            if (lodPixelRect.width() % 2 == 0) {
+                int oddSize = lodPixelRect.width() - 1;
+                if (oddSize < 3) oddSize = 3;
+                lodPixelRect.setSize(QSize(oddSize, oddSize));
+            }
+            lodPixelRect.moveCenter(transform.map(canvasPixelRect.center()));
+
             GenerateCanvasZoomPreviewData *newData =
                 new GenerateCanvasZoomPreviewData(canvasDev, lodPixelRect, colorConverter, levelOfDetail);
+
             // When Lod is involved, swap the cookie to the new Lod clone to track execution
             // The original object seems to leaks
-            newData->strokeCookie.swap(strokeCookie);
+            newData->swapCookie(strokeCookie);
+
+            newData->originalRect = canvasPixelRect;
 
             return newData;
         }
@@ -71,11 +86,16 @@ public:
             return strokeCookie;
         }
 
+        void swapCookie(QSharedPointer<boost::none_t> &newCookie) {
+            strokeCookie.swap(newCookie);
+        }
+
         KisPaintDeviceSP canvasDev;
         QRect canvasPixelRect;
         KisDisplayColorConverter *colorConverter;
         int levelOfDetail;
         QSharedPointer<boost::none_t> strokeCookie;
+        QRect originalRect;
     };
 public:
     KisColorSamplerStrokeStrategy(int radius, int blend, int lod = 0);