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List:       kde-devel
Subject:    Re: pixel manipulation
From:       Mosfet <mosfet () jorsm ! com>
Date:       1999-07-25 16:45:24
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In X11 pixmaps to be displayed are stored on the server and in the server's
address space - not on the client. Accessing an image's pixel data directly
requires a copy to and from the server.

In Qt, QPixmap directly represents a server resident image and is accessed
solely through a handle, and QImage is used to represent data inside the
client application. The image data needs to be transfered back to the server
before displaying (this is what converting from a QImage to a QPixmap does).

This is a limitation of X11 and was designed to be this way in order to
efficently handle network display of images. You can get around it by using
MITSHM, which places the image data in shared memory shared between the server
and client - but be aware, it is not easy to do implement this in a general
manner and there are many limitations (segment size, etc...).

On Sun, 25 Jul 1999, Uwe Thiem wrote:
> It seems there is no way to manipulate the colour of single pixels of
> a QPixmap efficiently. Yes, I know that QImage provides this, if not
> for the HSV model. But I can't bitBlt() a QImage to a QWidget and I
> need that as well. :-(
> 
> Of course, I can store the colour values somewhere else and draw the
> QPixmap anew everytime. But that is a waste of memory and dog slow.
> 
> Does anyone know an efficient way to do this?
> 
> Uwe
> 
> -------------------------------------------------------------------------
> Uwe Thiem                                        Tel: +264 - 061 - 244511
> P.O.Box 30955                                    Fax: +264 - 061 - 244511
> Windhoek                                         Email:   uwe@uwix.alt.na
> Republic of Namibia                                           uwe@kde.org
>                                                        http://www.kde.org
>                   **********************************
>          You can still escape from the GATES of hell: Use KDE!
> -------------------------------------------------------------------------
-- 
Daniel M. Duley - Unix developer & sys admin.
mosfet@kde.org
mosfet@jorsm.com

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