On Fri, 27 Mar 2009, Dirk Schulze wrote:
why does the spec want an INDEX_SIZE_ERR exception on arc's with a
negative radius?
Because it is indicative of a fundamental error. Calculations involving
radii should never be able to go negative.
Another example is:
http://blahbleh.com/molecools.php?name=1,2%20dimethylcyclopropane
If you turn the molecule a bit, the circles disappear with a
INDEX_SIZE_ERR exception.
Why is this sometimes calculating negative radii?
Isn't it better to just ignore the arc and go on with the drawing, like
Firefox does? And perhaps the same for arcTo?
There are three places where there are radii: gradients, arc(), and
arcTo().
On Fri, 27 Mar 2009, Martin Atkins wrote:
Assuming that we desire a do what I mean approach, it seems sane to me
for a negative radius to be normalized to a positive radius with the
same magnitude.
It goes from being the distance from the center to the edge to being the
distance from the center to the opposite edge.
You might also like to visualize this as drawing the back of the
circle.
That's one option.
On Sun, 29 Mar 2009, Dirk Schulze wrote:
A negative value doesn't mean, that you want an arc. They can happen
during the automatic calculation process. There is no reason, that a
developers takes negative values and wants to see an arc otherwise he
could cut of the sign.
Well then why would there be a negative value?
I'm just speaking of ignoring negative arcs instead of giving an
exception to continue further drawings.
It's unclear why this would be better.
On Sun, 29 Mar 2009, Oliver Hunt wrote:
I commented to the list on this issue some time ago (and a number of the
APIs were updated base don my comments); in general developers do not
expect graphics libraries to throw exceptions, especially for values
that are frequently computed. We've had numerous cases where webkit has
had to relax exception logic in canvas (having tightened it to match
spec) where the result has been broken sites and widgets (alas the
webkit and mozilla canvas implementations do have some edge case where
one throws and the other does not), i believe the most consistent
approach will be to fail silently when given invalid, but finite
arguments to any given API. The alternative is that certain APIs will
relax (regardless of spec), whereas others will not, leading to a more
or less random combination of throw vs. no throw on invalid arguments.
Generally most exceptions in the canvas API have been dropped; the only
ones remaining are for cases where there really doesn't seem to be a good
reason why you'd ever pass that value.
When faced with negative radii: Gecko throws an exception for arcTo(),
ignores the call for arc(), and uses the magnitude of the radii for
createRadialGradient(); Webkit throws for arcTo() and arc() and does
something I couldn't understand for createRadialGradient(); and Opera
throws an exception for all three.
I haven't changed the spec, which says to throw an exception for all
three, since that's the change that results in the fewest changes to
browser implementations.
--
Ian Hickson U+1047E)\._.,--,'``.fL
http://ln.hixie.ch/ U+263A/, _.. \ _\ ;`._ ,.
Things that are impossible just take longer. `._.-(,_..'--(,_..'`-.;.'
