[graphics/krita] libs/ui/tool: Thread-safe zoom preview

Git commit 2e7505044d162dd9d91ed4b8391710911fd5c871 by Emmet O'Neill, on behalf of Dat Le.
Committed on 13/08/2026 at 23:16.
Pushed by emmetoneill into branch 'master'.

Thread-safe zoom preview

M  +12   -12   libs/ui/tool/KisAsyncColorSamplerHelper.cpp
M  +2    -1    libs/ui/tool/KisAsyncColorSamplerHelper.h

https://invent.kde.org/graphics/krita/-/commit/2e7505044d162dd9d91ed4b8391710911fd5c871

diff --git a/libs/ui/tool/KisAsyncColorSamplerHelper.cpp b/libs/ui/tool/KisAsyncColorSamplerHelper.cpp
index 7a250fd107f..520d1fd608b 100644
--- a/libs/ui/tool/KisAsyncColorSamplerHelper.cpp
+++ b/libs/ui/tool/KisAsyncColorSamplerHelper.cpp
@@ -727,28 +727,28 @@ QImage KisAsyncColorSamplerHelper::cacheCanvasImage(QRect &canvasPixelRect) {
 
     if (m_d->cacheCanvasPreviewRect.isEmpty() || !m_d->cacheCanvasPreviewRect.contains(canvasPixelRect)) {
         // Cache an area larger than the needed preview area to avoid rapid small dynamic allocations
-        // Attempt to reduce cache size when sample size is big to reduce delay
-        qreal cacheScale = 1000;
-        // if (canvasPixelRect.width() < 250) cacheScale = 4;
-        // else if (canvasPixelRect.width() < 500) cacheScale = 2;
+        // And also avoid frequent preview delay from fetching canvas image asynchronously repeatedly
+        qreal cacheScale = 4;
 
         QRect cacheCanvasRect = canvasPixelRect;
         cacheCanvasRect.setSize(canvasPixelRect.size() * cacheScale);
         cacheCanvasRect.moveCenter(canvasPixelRect.center());
 
-        // if cache larger than canvas, then just copy whole canvas
+        // if cache requirement is larger than canvas, then just copy whole canvas
         if (cacheCanvasRect.width() >= canvasImage->width() || cacheCanvasRect.height() >= canvasImage->height()) {
             cacheCanvasRect = canvasImage->bounds();
         }
 
-        // If already fetching, then do nothing
-        if (m_d->canvasPreviewFetchingStarted) return QImage();
-
-        m_d->canvasPreviewFetchingStarted = true;
-        m_d->strokesFacade()->addJob(m_d->strokeId,
-            new KisColorSamplerStrokeStrategy::GenerateCanvasZoomPreviewData(canvasImage, cacheCanvasRect, canvasImage->profile()));
+        // If not already have a job fetching canvas image, then do it
+        // TODO: There's probably a better way to do this
+        if (!m_d->canvasPreviewFetchingStarted) {
+            m_d->canvasPreviewFetchingStarted = true;
+            m_d->strokesFacade()->addJob(m_d->strokeId,
+                new KisColorSamplerStrokeStrategy::GenerateCanvasZoomPreviewData(canvasImage, cacheCanvasRect, canvasImage->profile()));
+        }
 
-        return QImage();
+        // Instead of returning an empty QImage and cause painful flickering
+        // Just return old cache, it's not really noticeable normally. In extreme cases, we have a cool lazy loading effect :D
     }
     canvasPixelRect.translate(-m_d->cacheCanvasPreviewRect.topLeft());
 
diff --git a/libs/ui/tool/KisAsyncColorSamplerHelper.h b/libs/ui/tool/KisAsyncColorSamplerHelper.h
index 4b0513f8b32..13a955f24c3 100644
--- a/libs/ui/tool/KisAsyncColorSamplerHelper.h
+++ b/libs/ui/tool/KisAsyncColorSamplerHelper.h
@@ -83,8 +83,9 @@ private:
     void paintCircle(QPainter &gc, const QRectF &viewRectF, const QColor &currentColor, const QColor &baseColor);
 
     void paintCircleCrosshair(QPainter &gc, const QRectF &viewRectF, const QColor &currentColor);
+    // Return true if cache miss, will repaint later when canvas image is fetched
+    // Return false if cache hit, and painted
     bool paintCircleCanvasPreview(QPainter &gc, const QRectF &viewRectF, const QRectF &zoomDocRectF, const QPainterPath &clip);
-    // Return true if at least one reference image preview was drawn
     void paintCircleReferenceImagePreview(QPainter &gc, const QRectF &viewRectF, const QRectF &zoomDocRectF, const QPainterPath &clip);
 
     // Returns a QImage that should either contain the canvasPixelRectF, or the whole canvas if canvasPixelRectF > canvas size.