Solution for the grayscale selections

[Boudewijn Rempt]

I think you need to take a step back from the proposal you came up with a 
year ago and take a look at the big picture, because since then we have 
learned a number of things.

The problem itself should be redefined as "we need to be able to handle
colorspaces without an alpha channel", not we need to figure out a trick 
to make painting on selections work.

There are two other points:

* Selections should not be an alpha mask, but a grayscale mask without an 
alpha channel.
* We want to be able to apply vectorization to the composite ops, and for 
that to work we need to take the alpha mask out of band.
* Limiting the brush mask to 8 bits integer precision makes painting on 
higher bit depth colorspace paint devices lose precision.

If we take the alpha channel out of band, then pass the brush mask as an 
alpha channel as a separate parameter to KoColorSpace::bitBlt, we'll 
actually have solved the problem in a generic way, without adding hacks to 
KisPaintDevice.

If we make sure the alpha mask is the size of the active colorspace, then 
HDR painting will be much improved.

I think that that's the way forward.

On Tue, 25 Sep 2012, Dmitry Kazakov wrote:

> 
> On Tue, Sep 25, 2012 at 12:58 PM, Boudewijn Rempt <boud at valdyas.org> wrote:
>
>       As discussed before, there are a number of places where we need to check whether we get into trouble:
>
>       * brush mask generation without alpha channel
> 
> 
> Currently, the paint ops generate the mask exactly into the alpha channel. So the data about the current color is simply lost. You will have to rewrite
> the paintops to mix this value to the resulting singlebyte pixel. More than that, the resulting composite op will never work like COMPOSITE_OVER. It'll
> only be able to work like COMPOSITE_ADD, which is quite different.
>  
>       * (possibly) the conversion between colorspaces with and without alpha channel in KoColorSpace::bitBlt
> 
> 
> To make this conversion you need to have two colorspaces first. And to have two colorspaces you need to tell the paint ops to use some other colorspace.
> 
>  
>       * specific composite ops for the gray without alpha channel colorspace
> 
> 
> They will never work like usual COMPOSITE_OVER. And to generate a dab in such colorspace we need to make changes in all the paint ops.
>  
> 
> 
> 
> 
> 
> --
> Dmitry Kazakov
> 
>