12705 2011-05-19 15:10:37 +0000 setTransform spec silent on what to do with singular matrices 2011-12-10 00:05:36 +0000 1 1 1 Unclassified HTML WG LC1 HTML Canvas 2D Context unspecified PC Linux CLOSED FIXED canvas P2 trivial --- 1 benjamin contributor ayg excors ian mike public-html-admin public-html-wg-issue-tracking public-html-bugzilla oldest_to_newest 48637 0 benjamin 2011-05-19 15:10:37 +0000 The 2dcontext.setTransform method specification doesn't say how to react when passed a singular matrix. 48639 1 ayg 2011-05-19 17:21:04 +0000 I'd expect that if the transformation matrix is singular, nothing that's affected by the transformation matrix should be displayed, since it would make everything zero pixels thick. What do browsers do? 48641 2 benjamin 2011-05-19 17:44:26 +0000 WebKit apparently notes that it has a singular matrix and ignores every operation. Firefox doesn't handle it at all. Personally, I'd like to see it throw an error, since using a singular matrix as a transform doesn't make much sense. 48642 3 ayg 2011-05-19 17:56:06 +0000 Yeah, it probably does make the most sense to ignore it or throw an exception or something. Problem: how do you define "singular"? The arguments are doubles. Do you require that the determinant of {{a, c}, {b, d}} be within epsilon of 0 for some specific epsilon? What happens if you call transform() when the existing matrix and the matrix you passed to transform() both have determinants greater than epsilon in absolute value, but their product has a determinant smaller than epsilon? 48643 4 benjamin 2011-05-19 18:04:20 +0000 Let a b c d be a matrix where a, b, c, d are some floating point data type. We'll say it's singular iff a*d == c*b. This mostly defeats mistakes like passing the zero matrix or a simple non-invertible one. I'm not sure why we'd have to worry about them being close to equal. It should only arise with pathological matrices. 48649 5 ayg 2011-05-19 22:16:38 +0000 You can *never* rely on equality for floating-point. Rounding errors mean that basic mathematical identities don't hold, like (a*b)*c == a*(b*c) or (a/b)*b == a. Having a determinant almost exactly zero wouldn't have to be pathological at all, it could be any matrix that went through some numeric operations. For instance: data:text/html,<!doctype html><script>alert((10/3000 - 2 + 2)*3000 == 10)</script> alerts false in Firefox 4.0, Chrome 12 dev, and Opera 11.10. 48650 6 benjamin 2011-05-19 23:49:58 +0000 I realize floating comparisons are bad, but it's what browsers currently use afaict. 48665 7 ayg 2011-05-20 18:48:46 +0000 Do you have a specific test-case? I don't know enough about canvas to easily make one, but I'd be interested in fiddling and seeing how browsers behave here. Your description in comment 2 makes it sound like WebKit doesn't handle singular matrices specially at all, it just draws everything with zero width. If that's correct, presumably it behaves the same if the eigenvalues are both very small, not just if they're zero (since the width will round down to zero). 48666 8 989 benjamin 2011-05-20 20:29:25 +0000 Created attachment 989 singular and almost singular matrix example Here is an example of a almost singular matrix and a singular matrix being used to transform. 48667 9 benjamin 2011-05-20 20:31:41 +0000 WebKit "handles" singular matrices by detecting when there are in effect and ignoring every operation on the canvas while they are. 48682 10 ayg 2011-05-22 22:38:28 +0000 Your "almost singular" matrix isn't very close to singular at all. It has a determinant of .00002. That's quite a lot, since your lineWidth is 1999999. Try something like this instead: http://software.hixie.ch/utilities/js/live-dom-viewer/saved/1002 If I set e to 0.0000001, Firefox draws the red square successfully. If I add one more zero, so it's 0.00000001, Firefox doesn't draw anything, even though w(1 - e == 1) logs "false". WebKit seems to have the same magic threshold, after which it just ignores the operation (even though the width variable is large enough that it should really draw the big black line). So it doesn't seem to be just testing whether it's actually singular, it looks like they're testing that it's just "close" to singular. 49966 11 ian 2011-06-21 06:14:13 +0000 Why can't implementations just do the maths as suggested by the spec? I don't understand why a special case is needed here. 50116 12 ayg 2011-06-22 20:51:52 +0000 I don't understand why either, but it seems like implementations do have the special case. 52790 13 mike 2011-08-04 05:04:14 +0000 mass-move component to LC1 54671 14 ian 2011-08-11 05:49:15 +0000 This was also discussed on the WHATWG list: http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2011-June/thread.html#32229 Philip, any advice? 61361 15 ian 2011-12-09 23:26:24 +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 recent diffs (sorry I don't have a specific revision to point to at this time) Rationale: Concurred with reporter's comments. 989 2011-05-20 20:29:25 +0000 2011-05-20 20:29:25 +0000 singular and almost singular matrix example singular.html text/html 858 benjamin PCFET0NUWVBFIGh0bWw+CjxodG1sPgo8aGVhZD4KPHRpdGxlPlNpbmd1bGFyIGFuZCBhbG1vc3Qg c2luZ3VsYXIgdHJhbnNmb3JtczwvdGl0bGU+CjxzY3JpcHQgdHlwZT0idGV4dC9qYXZhc2NyaXB0 Ij4KZnVuY3Rpb24gdHJhbnNmb3JtKGN0eCwgYSwgYiwgYywgZCwgZSwgZikgewogICAgY3R4Lmxp bmVXaWR0aCA9IDE5OTk5OTk7CiAgICBjdHguc3Ryb2tlU3R5bGUgPSAiYmxhY2siOwogICAgY3R4 LnNhdmUoKQogICAgY3R4LnNldFRyYW5zZm9ybShhLCBiLCBjLCBkLCBlLCBmKQogICAgY3R4LnN0 cm9rZVJlY3QoNTAwLCA1MDAsIDEwMCwgMTAwKTsKICAgIGN0eC5yZXN0b3JlKCkKICAgIGN0eC5m aWxsU3R5bGUgPSAicmVkIjsKICAgIGN0eC5maWxsUmVjdCgxMCwgMTAsIDUwLCA1MCk7Cn0KZnVu Y3Rpb24gdGVzdCgpIHsKICAgIGMxID0gZG9jdW1lbnQuZ2V0RWxlbWVudEJ5SWQoImMxIik7CiAg ICB0cmFuc2Zvcm0oYzEuZ2V0Q29udGV4dCgiMmQiKSwgMSwgLjk5OTk4LCAxLCAxLCAwLCAwKTsK ICAgIGMyID0gZG9jdW1lbnQuZ2V0RWxlbWVudEJ5SWQoImMyIik7CiAgICB0cmFuc2Zvcm0oYzIu Z2V0Q29udGV4dCgiMmQiKSwgMSwgMSwgMSwgMSwgMSwgMCwgMCk7Cn0Kd2luZG93Lm9ubG9hZCA9 IHRlc3QKPC9zY3JpcHQ+CjxzdHlsZSB0eXBlPSJ0ZXh0L2NzcyI+CmNhbnZhcyB7CiAgICBib3Jk ZXI6IDJweCBzb2xpZCBibGFjazsKfQo8L3N0eWxlPgo8L2hlYWQ+Cjxib2R5Pgo8cD5BbG1vc3Qg c2luZ3VsYXIgdHJhbnNmb3JtPC9wPgo8Y2FudmFzIGlkPSJjMSI+PC9jYW52YXM+CjxwPlNpbmd1 bGFyIHRyYW5zZm9ybTwvcD4KPGNhbnZhcyBpZD0iYzIiPjwvY2FudmFzPgo8L2JvZHk+CjwvaHRt bD4K