[webkit-dev] White PNG image with alpha problem

Mike Fischer mf_software at mac.com
Sat Nov 4 23:40:19 PST 2006

Sorry, I forgot to add a subject!

Am 05.11.2006 um 08:35 schrieb Mike Fischer:

> Hi!
> I am seeing a strange problem with transparent PNG files in the  
> release version of Safari/WebKit on Max OS X 10.4.8 PPC:
> I created a 1 pixel PNG image with color white and an alpha value  
> of approx. 60%.
> When I display this in an IMG tag (or as a CSS background image)  
> over other content the other content does not appear lighter as  
> expected but more gray (not really darker).
> When I use a CANVAS element to draw a white fill color with 60%  
> alpha the background is lightened as expected.
> The current nightly build does not show this problem. By searching  
> for bugs related to PNG on <http://bugs.webkit.org> I couldn't find  
> anything relevant. Also Firefox, Opera 9.02 and OmniWeb  
> 5.5.1 show the expected color effect, i.e. lightening the background.
> OTOH using a 1 pixel PNG with color black and an alpha of 50% works  
> exactly as expected, i.e. darkening the background for a shadow  
> effect.
> My questions:
> a) Was there a bug in WebKit causing this that got fixed for the  
> current nightlies? Or is this some unintended side effect and I  
> need to file a new bug to ensure that this stays fixed in the future?
> b) Is there a workaround for the released version of WebKit  
> (419.3)? (Other than using a CANVAS element ;-)
> A test page demonstrating the effect is here (the top transparent  
> area shows the problem):
> <http://home.fi-works.de/test/test-png-alpha.html>
> Note that the testing area is built using JavaScript DOM methods in  
> this example. It essentially builds three layers: At the bottom  
> some text is created. Then (DIV id:"bgc1" z.index:1) some color  
> areas are created and in the top layer (DIV id:"bgc2" z-index:2)  
> the transparent testing areas are created.
> Thanks!
> Mike
> -- 
> Mike Fischer         Softwareentwicklung, EDV-Beratung
>                                     Schulung, Vertrieb
> Web: <http://homepage.mac.com/mike_fischer/index.html>

Mike Fischer         Softwareentwicklung, EDV-Beratung
                                     Schulung, Vertrieb
Web: <http://homepage.mac.com/mike_fischer/index.html>

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