Max Vlasov schrieb:
Ok, another example. Imagine that two alpha png is merged and drawed on the form on the fly.
Is that on-the-fly merging really necessary?
And you drag a small window over this form. My experience tells me that win32 will do great automatic job of narrowing the final drawing of the bitmap to smallest rectangle possible, but my internal procedures of alpha-blending can get this information only from GetUpdateRect/GetUpdateRgn, otherwise every single pixel moving of this window will lead to full alpha-blending of these bitmaps. ClipRect won't provide this information
I'm not sure what you mean. When the dragged window also uses alpha-blending, then Windows (GDI) must do that job. Using GetUpdateRect IMO should not work, because BeginPaint restricts output to the ClipRect.
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