Bug 42012

Summary: Alpha value obtained by getComputedStyle is noticeably less than the actual value.
Product: WebKit Reporter: Ryosuke Niwa <rniwa>
Component: CSSAssignee: Nobody <webkit-unassigned>
Status: RESOLVED DUPLICATE    
Severity: Normal CC: ap, patrice, simon.fraser
Priority: P2    
Version: 528+ (Nightly build)   
Hardware: All   
OS: All   
Attachments:
Description Flags
demonstrates the bug
none
demonstrates issue with getComputedStyle for opacity none

Ryosuke Niwa
Reported 2010-07-09 17:47:51 PDT
Created attachment 61124 [details] demonstrates the bug When the background color is set to rgba(255, 255, 0.5), the computed style's background color indicates the actual alpha value to be 0.496094. Firefox returns 0.5 correctly. The bug is demonstrated in the attachment.
Attachments
demonstrates the bug (276 bytes, text/html)
2010-07-09 17:47 PDT, Ryosuke Niwa 		no flags
Details
demonstrates issue with getComputedStyle for opacity (651 bytes, text/html)
2010-12-13 01:44 PST, Daniel Wagner 		no flags
Details
View All Add attachment proposed patch, testcase, etc.
patrice
Comment 1 2010-09-09 17:04:03 PDT
Also, sometimes, assigning the generated rgba color append to generate a new value : document.body.style.backgroundColor="rgba(0,0,0,0.3)"; "rgba(0,0,0,0.3)" document.body.style.backgroundColor = document.body.style.backgroundColor; "rgba(0, 0, 0, 0.296875)" document.body.style.backgroundColor = document.body.style.backgroundColor; "rgba(0, 0, 0, 0.292969)" document.body.style.backgroundColor = document.body.style.backgroundColor; "rgba(0, 0, 0, 0.292969)"
Daniel Wagner
Comment 2 2010-12-13 01:44:08 PST
Created attachment 76360 [details] demonstrates issue with getComputedStyle for opacity Similarly, if an element's opacity is set to 0.7, getComputedStyle returns 0.699999988079071.
Simon Fraser (smfr)
Comment 3 2010-12-13 10:37:47 PST
I think this is a divide-by-255 vs. 256 issue.
Simon Fraser (smfr)
Comment 4 2010-12-13 14:05:28 PST
Internally rgba(255, 255, 0, 0.5) is stored as 0x7fffff00 (ARGB), and 127/255 is 0.49803921568627. (In reply to comment #2) > Created an attachment (id=76360) [details] > demonstrates issue with getComputedStyle for opacity > > Similarly, if an element's opacity is set to 0.7, getComputedStyle returns 0.699999988079071. This is just floating point rounding error.
Ryosuke Niwa
Comment 5 2012-07-15 20:39:54 PDT
*** This bug has been marked as a duplicate of bug 91355 ***
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