Alpha fixed to float (0-1), was: is alpha fixed to 8bit unsigned

[Boudewijn Rempt]

I just stumbled into this myself, in the context of the fill painter. You want 
to be able to fill a rect with a certain color and a certain opacity. The 
color, if it's a kiscolor, can have an alpha channel, of course (and we 
should add an opacity slider to all color selector palettes to support that), 
but despite that, it may still be necessary to set an opacity explicity. 
Especially when we're talking about patterns, too.

So, what I did was:

void KisFillPainter::fillRect(Q_INT32 x1, Q_INT32 y1, Q_INT32 w, Q_INT32 h, 
const KisColor& kc, QUANTUM opacity)
{

        Q_INT32 y;
        Q_UINT32 depth = m_device->pixelSize();
        m_device->colorStrategy()->setAlpha(kc.data(), &opacity, 1);

        for (y = y1; y < y1 + h; y++)
        {
                KisHLineIterator hiter = m_device->createHLineIterator(x1, y, 
w, true);
                while( ! hiter.isDone())
                {
                        memcpy(hiter.rawData(), src, depth);
                        ++hiter;
                }
        }
}

And setAlpha is defined as:

void KisStrategyColorSpace::setAlpha(Q_UINT8 * pixels, Q_UINT8 * alpha, 
Q_INT32 nPixels) {

	// Decide size of pixel
	// Determine startpoint of alpha in pixel
	// Determine size in bytes of alpha

        Q_UINT8 * ptr = pixels;
        for (Q_INT32 i = 0; i < nPixels; ++i) {
                memcpy(pixels + (i * psize) + alphaPos, alpha, alphaSize);
                ptr += psize;
        }


}

Or something like this -- this is untested code since I'm at work now and 
cannot do more than think a little while my work code is compiling. This 
falls through, of course, at the Q_UINT8 * alpha because nothing in Krita can 
now how to initialize this value. 

What I propose as a solution is to define alpha in Krita outside 
colorstrategies as a float with a range from 0 to 1, with 0 transparent and 1 
opaque. The colorstrategies can then convert the float value to whatever 
floats their own boat.

so:

void KisStrategyColorSpace::setAlpha(Q_UINT8 * pixels, float alpha, Q_INT32 
nPixels) {
	
	// Convert alpha to whatever fits the range of this, maybe justa Q_UINT8
	// memcpy alpha to the right place in all the given pixels 

}

Oh, and by the way: what about premultiplied alpha?

-- 
Boudewijn Rempt 
http://www.valdyas.org/fading/index.cgi
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