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List:       koffice-devel
Subject:    Re: 32-bit bimaps and QImage
From:       Nicolas Goutte <nicolasg () snafu ! de>
Date:       2003-02-20 19:17:52
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On Thursday 20 February 2003 15:41, David Faure wrote:
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> On Thursday 20 February 2003 13:39, Nicolas Goutte wrote:
> > The difference of the depth is just due to padding.
> >
> > In true color, pixels are normally 3 bytes for red, gren and blue (RGB).
> > This makes 24 bits. But processors (be it a CPU or a graphic ship) can
> > only handle 8, 16 or 32 bits (and more). So 24 bits must be handled as 3
> > times 8 bits. So for handling (especially moving) a pixel you need three
> > 8 bit operations.
> >
> > With 32 bits, you have 8 bits that are used as padding.
>
> Alpha channel, not padding.

Well, it depends. As I have written at the bottom of the email, without alpha 
this is padding. (And BMP does not know alpha)

Perhaps I have more described the video memory of a true color card than I 
have described QImage. But I wanted to explain where these 32 bits came from. 
(The alpha channel came later.)

>
> (see docu for QImage::setAlphaBuffer)

It also tells that by default alpha is disabled. So it is still padding. ;-)

>

Have a nice day/evening/night!

> - --
> David Faure -- faure@kde.org, dfaure@klaralvdalens-datakonsult.se
> Qt/KDE/KOffice developer
> Klarälvdalens Datakonsult AB, Platform-independent software solutions
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