Clarification of colour alpha + alpha channel blending semantics

----- Original Message ----- 
From: "Carsten Haitzler (The Rasterman)"
<rasterrasterman.com>
> really? evas doesn't use premul-alpha in its software
or gl engines, and 
> the
> xrender engine converts to premul to keep xrender
happy. (nb - this is on 
> the
> todo list to change to premul internally).
Here is the relevent code segment from
evas_engine_directfb_image_draw

{
int r, g, b, a;
int mulr, mulg, mulb, mula;
int flags;
    if (im->flags & RGBA_IMAGE_HAS_ALPHA)
        flags = DSBLIT_BLEND_COLORALPHA |
DSBLIT_BLEND_ALPHACHANNEL;
    else
        flags = DSBLIT_NOFX;
    evas_engine_directfb_context_color_get(data, context,
&r, &g, &b, &a);
    if (evas_engine_directfb_context_multiplier_get
        (data, context, &mulr, &mulg, &mulb,
&mula))
    {
        re->backbuf->SetColor(re->backbuf, mulr,
mulg, mulb, mula);
        flags |= DSBLIT_COLORIZE;
    }
    else
    {
        re->backbuf->SetColor(re->backbuf, r, g, b,
a);
    }
    re->backbuf->SetSrcBlendFunction(re->backbuf,
DSBF_SRCALPHA);
    img->SetSrcBlendFunction(img, DSBF_INVSRCALPHA);
    re->backbuf->SetBlittingFlags(re->backbuf,
flags);
    re->backbuf->StretchBlit(re->backbuf, img,
&inrect, &outrect);
}


As the code doesn't specify DSBLIT_SRC_PREMULTIPLY then
the expectation is that the source image is already
premultiplied. Evas
images are always 32bit ARGB. Images loaded from sources
without an
alpha channel get converted from the image provider and
therefore get
an alpha of 1.0f (i.e. 255). So these are technically
correctly 
pre-multiplied.
For images with alpha then I guess it is up to the image
creater to get
this right if they want correct blending.

The question of exactly what the combination of color alpha
and alpha 
channel
does still stands, the question of what Evas is trying to
achieve is another
matter and I guess not of much interest to this list.

Regards,
-stephen


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