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From: mrh@camcon.co.uk (Mark Hughes)
Newsgroups: comp.sys.ibm.pc
Subject: Why are bitmap save/restores so slow?
Message-ID: <3776@titan.camcon.co.uk>
Date: 12 Sep 89 09:11:33 GMT
Organization: Cambridge Consultants Ltd., Cambridge, UK
Lines: 31


I am fiddling around with graphic objects in C++ on a PC (EGA mode)
and am frustrated by the time taken to save/restore areas of the
screen bit-map.

On my 20Mhz machine, typical screen menu sizes can be saved acceptably fast,
but larger areas (say 20,000 pixels) take a noticable period to save. On
a slower machine things would get really painful.

I have had a peek at the assembly code doing the pixel copying and it
seems to take around ten instructions per pixel.

The code is supplied with an otherwise speedy graphics library,
and is not using BIOS calls. (The library is the "flash graphics" library
supplied with Zortech C++ v1.07).

***** Can this process be speeded up? ********

I'm interested to know what features of PC graphics hardware make this
process so tortuous, and if there are any ways of speeding things up.
If there are faster ways of doing this, do they have disadvantages - like
reduced portability/compatibility?

All comments appreciated.


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|   Mark Hughes  | Tel: +44 (0) 223 420024  Cambridge Consultants Ltd. 
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------------------ BT Gold: 72:MAG70076     Cambridge, UK. (Own opinions etc.)