[Gambas-user] Pb with transparency?

[Benoît Minisini]

> Benoît Minisini a écrit :
> >> Hi,
> >>
> >> Gambas 2.12 with QT
> >>
> >> I had a prog that worked well in Gambas 2.11 but now it
> >> doesn't work correctly for all the images with transparent
> >> areas.
> >> Here is a very short illustration of the problem
> >>
> >> DIM PicSac AS Picture
> >> DIM PicCase AS Object[]
> >> Dim i AS Integer
> >>
> >> PicCase.Resize(2)
> >> FOR i = 0 TO 1
> >>     PicCase[i] = NEW Picture(14, 14, TRUE)
> >> NEXT
> >> PicCase[0] = Picture.Load(image with some transparent part)
> >> PicCase[1] = Picture.Load(image without transparent area)
> >>
> >> PicSac = NEW Picture(14, 14, TRUE)
> >> i = 0
> >> Draw.Begin(PicSac)
> >> Draw.Picture(PicCase[i], 0, 0, 14, 14, 0, 0, 14, 14)
> >> Draw.End
> >>
> >> PictureBox1.Picture = PicSac
> >>
> >> i = 0 => nothing displayed
> >> i = 1 => correct display
> >> (I know that Gambas 2.12 changed the Draw routine)
> >> Is it a bug?
> >>
> >> regards,
> >> Dominique Simonart
> >
> > I will look at that.
> >
> > Beware that the Picture class is not really transparent. It has no alpha
> > channel but a bitmap mask.
> >
> > Creating a "transparent" Picture internally creates an X11 pixmap and a
> > X11 bitmap mask. Drawing on it the must update both the pixmap and the
> > mask. I think that the mask update is buggy for the Draw.Picture()
> > method.
> >
> > Anyway, you should use the Image class and its Draw method to draw an
> > image on top another one with real alpha transparency.
> >
> > To add to the complexity, converting an Image to a Picture keeps the
> > alpha information with gb.qt, but not with gb.gtk. This is a Qt feature,
> > that uses the XRender extension for that. But GTK+ don't, and just
> > removes all pixels whose alpha component is lower than 128. But you
> > should not take that into account as it is really toolkit specific.
> >
> > Regards,
>
> Thanks Benoit,
> I replaced PicCase[] and PicSac by Image and used Draw
> method instead of Draw.Begin(...).
> The last instruction is now:
> PictureBox1.Picture = PicSac.Picture
>
> Here are the results:
> 1) i=0 now display correctly the image with transparent part
> 2) If I use 2 buttons to draw with i=0 (button1) and i=1
> (button2)
> *   button1 then button2 display correctly the two images
> successively
> * button2 then button1 display only the image without
> transparent part (button1 didn't change the display)
> * if I replace Image1 with another image without transparent
> part I could see the images associated with their buttons
>
> In conclusion, as soon as there is a transparent part in my
> image, it could never override another image
>
> Hope this helps
> Dominique Simonart
>

I fixed many bugs in Draw.Picture() and Draw.Image() in revision #1938, both 
for gb.qt and gb.gtk. Can you try it to check if you have the same problems?

Regards,

-- 
Benoît