> Really, I'd like to see an official POV.

Officially you have to lock bitmaps, but not if the main thread is blocked :-)


The explanation is simple: you have to lock bitmaps unless you make sure 
FreeDeviceContexts is never called, which happens in TWinControl.DestroyWnd and 
TWinControl.MainWndProc. But these methods are only executed while the main 
(GUI) thread is active (processes messages etc.). I'd also avoid accessing any 
canvases or calling methods which could change a bitmap's state (resizing a 
bitmap is a no-no).


So you can: start a number of treads processing the pixel data (avoid using 
other methods or properties of TBitmap) if you make sure the main thread is 
blocked (use WaitForMultipleObjects with the handles of all threads or some 
events used by your threads signaling they have finished their work).


But you cannot: start one or more threads working with bitmaps and let the main 
thread continue (processing messages etc.) which could possibly free the device 
contexts while your threads are executed.


So basically you can parallelize your image processing tasks and make use of 
multi core CPUs, if you make sure it works like a normal method, returning when 
all the work is done. I'm doing this all the time. If you're interested I can 
post some code I'm using.


-- 
Jens Gruschel
http://www.pegtop.net