Bug 20258 – HTML5 canvas: fillRect() and friends do not support negative heights

• Status: RESOLVED
• Resolution: WORKSFORME
• Component: Layout and Rendering

[David Jones]

PROBLEM
fillRect(x, y, w, h)
will fail with "INDEX_SIZE_ERR: INDEX_SIZE_ERR: DOM Exception 1" (or something like that) when h is negative.

There's nothing in the spec to prohibit fillRect() from taking a negative h.  The specification requires that the path be formed from the four coordinates: "(x, y), (x+w, y), (x+w, y+h), (x, y+h)".  These four coodinates still have a perfectly sensible interpretation when either h or w is negative, and there's nothing I can see in the spec from prohibiting h and w from being negative.

It is sometimes even quite natural to use a negative h, for example if one has flipped the Y-axis by issuing a scale(1,-1) then one still might want (x,y) to be the top-left of a rectangle, in which case h will be negative.

[Dirk Schulze]

Created attachment 22929 [details]
Negative width/height

This test has a negative with and height for fillRect() and strokeRect(). Can't confirm it on cairo but can not test Safari.

[David Jones]

Ah thanks for the test (odd, I had one lying around when I reported the bug, I wonder why I didn't add it).

Running the test on Safari results in no visible output and "INDEX_SIZE_ERR: DOM Exception 1" in the error console.

[Dirk Schulze]

Can't confirm it on Mac with the latest trunk. Possibly fixed in some earlier versions. Do you still have problems with it? Please try one of the http://nightly.webkit.org/ builds to check.

[Oliver Hunt]

I agree with dirk, the code obviously supports this behaviour, and the testcase passes.  I assume this has been fixed at some point.