12824 2011-05-30 22:11:46 +0000 This doesn't say how to serialize non-zero alpha values: how many digits should be used? The correct answer is the minimum number necessary so that the resulting string evaluates to the same floating-point number as you started with. 2011-08-10 22:34:12 +0000 1 1 1 Unclassified HTML WG LC1 HTML Canvas 2D Context unspecified Other other RESOLVED FIXED http://www.whatwg.org/specs/web-apps/current-work/#colors-and-styles P3 normal --- 1 contributor ian ayg bzbarsky ian mike public-html-admin public-html-wg-issue-tracking public-html-bugzilla oldest_to_newest 48936 0 contributor 2011-05-30 22:11:46 +0000 Specification: http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html Section: http://www.whatwg.org/specs/web-apps/current-work/complete.html#colors-and-styles Comment: This doesn't say how to serialize non-zero alpha values: how many digits should be used? The correct answer is the minimum number necessary so that the resulting string evaluates to the same floating-point number as you started with. Posted from: 68.175.61.233 User agent: Mozilla/5.0 (X11; Linux i686) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/13.0.772.0 Safari/535.1 48941 1 bzbarsky 2011-05-31 02:09:56 +0000 Actually, the correct answer may be to use however many are needed to ensure that your round-trip back to the same value you have now if you parse the result. For example, consider a UA that stores alpha internally as an integer in the range [0, 255] (a very common thing to do; Presto, WebKit nightly, and Gecko all do it). It most certainly cannot produce the original floating-point number when asked. What it _can_ produce is a floating-point number which when parsed and converted to an integer in that range will produce the integer you start with. In practice that means either 2 or 3 digits of precision. I agree that this should be specified. I'll attach a simple testcase that tests what browsers do right now for alpha values of 0 to 1 in steps of 0.001; it shows that Presto, Gecko, and WebKit all step up their serialized value only once every 0.004 or so, which means they're storing 250-ish distinct values, as I said above. Note that of the three Gecko is the only one that even tries to make the serialized value look "nice" (in the sense of using the smallest number of digits that will round-trip to the same integer; this means that 0.1 ends up serialized as "0.1" and not "0.09804" (Chrome) or "0.10196" (Presto and Safari)). 48942 2 991 bzbarsky 2011-05-31 02:10:21 +0000 Created attachment 991 Testcase 48956 3 ayg 2011-05-31 14:20:49 +0000 (In reply to comment #1) > Actually, the correct answer may be to use however many are needed to ensure > that your round-trip back to the same value you have now if you parse the > result. Good catch. > For example, consider a UA that stores alpha internally as an integer in the > range [0, 255] (a very common thing to do; Presto, WebKit nightly, and Gecko > all do it). Should this be standardized? It has some author-visible implications. To start with, if alpha values are serialized as you suggest, setting and then reading back strokeStyle or fillStyle will give you a different number of digits in different UAs for the alpha, depending on how much internal precision they have. More than that, presumably it will also be detectable by drawing some non-opaque things on top of each other and measuring the exact color of the result. If everything gets clamped to 8-bit integers, the rounding will be different from if you use actual floats. > Note that of the three Gecko is the only one that even tries to make the > serialized value look "nice" (in the sense of using the smallest number of > digits that will round-trip to the same integer; this means that 0.1 ends up > serialized as "0.1" and not "0.09804" (Chrome) or "0.10196" (Presto and > Safari)). Actually, this bug was prompted by the fact that WebKit just changed to more Gecko-like behavior here: http://trac.webkit.org/changeset/87512 The commit summary said "The number of decimals for alpha values between 0 and 1 is not specified, so match Gecko and Presto by returning the lowest number of decimals needed to represent the number", so my immediate reaction was to get the number of decimal places standardized. (I don't know why it says "and Presto" -- I get the same results in Opera as you do.) 48961 4 bzbarsky 2011-05-31 15:58:36 +0000 > setting and then reading back strokeStyle or fillStyle will give you a > different number of digits in different UAs for the alpha, depending on how > much internal precision they have. Yep. And if you required the same number of digits, then you could still detect the limited internal precision. If you're asking whether the fact that alpha is stored as an integer in the range 0-255 should be standardized, the answer is "maybe". Right now the way floats are converted to such integers is not the same across UAs either, last I checked... Standardizing makes some sense, but we need to be careful to not overconstrain future implementations that may want to allow a wider range of alpha values, assuming anyone would want that. Worth a mailing list thread. 52635 5 mike 2011-08-04 05:03:18 +0000 mass-move component to LC1 54643 6 ian 2011-08-10 22:34:01 +0000 EDITOR'S RESPONSE: This is an Editor's Response to your comment. If you are satisfied with this response, please change the state of this bug to CLOSED. If you have additional information and would like the editor to reconsider, please reopen this bug. If you would like to escalate the issue to the full HTML Working Group, please add the TrackerRequest keyword to this bug, and suggest title and text for the tracker issue; or you may create a tracker issue yourself, if you are able to do so. For more details, see this document: http://dev.w3.org/html5/decision-policy/decision-policy.html Status: Accepted Change Description: see diff given below Rationale: Concurred with comments above. I tried. Not sure I like the result. 54644 7 contributor 2011-08-10 22:34:12 +0000 Checked in as WHATWG revision r6411. Check-in comment: Try to tighten up the definition of how to serialise rgba() in canvas. http://html5.org/tools/web-apps-tracker?from=6410&to=6411 991 2011-05-31 02:10:21 +0000 2011-05-31 02:10:21 +0000 Testcase test.html text/html 485 bzbarsky PCFET0NUWVBFIGh0bWw+CjxodG1sPgogIDxjYW52YXMgaWQ9ImMiIHdpZHRoPSIxIiBoZWlnaHQ9 IjEiPjwvY2FudmFzPgogIDx0YWJsZT4KICAgIDx0cj48dGg+U2V0PC90aD48dGg+U2VyaWFsaXpl ZDwvdGg+PC90cj4KCiAgICA8c2NyaXB0PgogICAgICB2YXIgYyA9IGRvY3VtZW50LmdldEVsZW1l bnRCeUlkKCJjIikuZ2V0Q29udGV4dCgiMmQiKTsKICAgICAgZm9yICh2YXIgaSA9IDA7IGkgPCAx OyBpICs9IDAuMDAxKSB7CiAgICAgICAgYy5maWxsU3R5bGU9InJnYmEoMCwgMCwgMCwgIiArIGkg KyAiKSIKICAgICAgICBkb2N1bWVudC53cml0ZSgiPHRyPjx0ZD4iICsgaSArICI8L3RkPjx0ZD4i ICsgCiAgICAgICAgICAgICAgICAgICAgICAgYy5maWxsU3R5bGUucmVwbGFjZSgvLiosLywgIiIp LnJlcGxhY2UoIikiLCAiIikgKwogICAgICAgICAgICAgICAgICAgICAgICI8L3RkPjwvdHI+Iik7 CiAgICAgIH0KICA8L3NjcmlwdD4KPC9odG1sPgo=