[OpenJDK 2D-Dev] 6603887: Where are transparent areas filled with bgColor?

[Dmitri Trembovetski]

   Hi Clemens,

   thanks for getting at the bottom of this.

   I think you have correctly identified the cause of the
   problem.

Clemens Eisserer wrote:
> Hello again,
> 
> Seems I've identified the bug:
> _X11SDOps.bgPixel is used as a hint wether the background was already
> filled with the bgColor specified in drawImage or not - if
> (xsdo->bgPixel != pixel + 1) it thinks that the transparent pixels of
> the image have already been painted to the bgColor which works except
> in the case when pixel = -1, bgColor is always initialized with 0 so
> for this case the transparent pixels get never ever their color.
> 
> I now have a few ideas how this could be changed:
> 1.) Add a jboolean-field to the _X11SDOps struct, indicating wether
> the bgColor is valid or not. Currently bgColor=0 servers this purpose.
> It seems to be in my eyes the best approach, however I don't know
> wether this could break any API/ABI?
> 2.) Choose a value for the case bgColor==invalid which cannot be taken
> by a pixel+1 value. As far as I guess the pixel+1 should serve this
> purpose - never let the pixel become 0 ... but it seems wrong-way
> thought.
> Is pixel always a negative value? If yes couldn't the check be

   In general, not necessarily - for an 8-bit visual a pixel is
   an index into a Colormap.

> replaced with xsdo->bgPixel != pixel)?  pixel will never become 0
> anyway?

   It may - if the transparent pixel is 0th entry in the colormap.

   I think we'll have to resort to another boolean which tells
   whether bgPixel is valid - like you suggested.

   I have tried to find the code review for this stuff but couldn't -
   I seem to recall that there was an explanation offered why
   the +1 trick should work, but I can't remember it. May be Jim can?

   Thanks,
     Dmitri


> 
> Looking foreward to hear your suggestions, lg Clemens