[Qt-interest] how to build a grayscale QImage

[Boudewijn Rempt]

On Mon, 22 Feb 2010, Dan Milburn wrote:

> > But no QImage constructor take a data pointer and a color table together.
> > Besides, how can i create a grayscale color table? 
> > should i set a for loop from 1 to 256 and set the current entry of color 
> > table with the current index of the loop?
> > I get confused with the Format enum. It just supports mono(0-1) and 8bit 
> > indexed; why not a "grayscale" option to make things easier? 
> > 
> Hi,
> 
> Not that I speak for Nokia, but they can't cover every possible image 
> format, and there probably isn't a great deal of demand for grayscale 
> images these days.

You'd be surprised -- grayscale images are used a lot, for instance
for masking. When working on krita, we ran into this issue only last week:
we wanted to use QImage, but since you cannot paint on an indexed image,
we had to cook up our own solution.

> 
> Besides, generating a grayscale color table is really not difficult to do:
> 
> QVector<QRgb> table( 256 );
> for( int i = 0; i < 256; ++i )
> {
>    table[i] = qRgb( i, i, i );
> }
> 
> None of the QImage constructors take this, but you can set it on the 
> image immediately afterwards.

The big problem with indexed images is that you cannot use QPainter
on them.

Boudewijn