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Report/uml/canvas-viewport-algorithm.puml

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+@startuml
+title Canvas/ViewPort: алгоритм вычисления видимой области и редроу
+
+start
+:Get zoomed document rect\n(getZoomedImageSize);
+:Read viewport rect + scroll offset;
+:Compute visible width/height\nmin(viewport, image size);
+:Compute raw visible x/y\nfrom scroll - image offset;
+:Clamp x/y into image bounds;
+:Store Rect_i{x,y,w,h} as _visible_rect;
+
+if (visible rect changed\nOR texture is null?) then (yes)
+  :request redraw;
+else (no)
+  :no redraw needed;
+endif
+
+if (redraw pending?) then (yes)
+  :Read render quality %;
+  :Convert area-quality to side-scale\nscale = sqrt(q/100);
+  :Scale full canvas size;
+  :Scale visible rect;
+  :Clamp scaled rect to scaled canvas;
+  if (scaled visible rect valid?) then (yes)
+    :Render only scaled visible area\nthrough RenderEngine;
+    :Update texture + stats + throttle;
+  else (no)
+    :Drop texture / skip rendering;
+  endif
+else (no)
+  :Keep previous frame;
+endif
+stop
+
+@enduml