467518 - gfxAlphaBlur tries to blur outside the radius

Status: RESOLVED FIXED

(Core :: Graphics, defect, P3)

Description

[Robert O'Callahan (:roc) (email my personal email if necessary)]

gfxAlphaBlur does three box blurs each with a left/right lobe size of approximately mBlurRadius/2. This means the blur results actually extend about 1.5*mBlurRadius beyond the edge of the object. This seems unintuitive and conflicts with CSS3 text-shadow's description of the blur radius as "a length value that indicates the boundaries of the blur effect".
http://www.w3.org/TR/css3-text/#text-shadow

Since the temporary surface outsets the object by just one mBlurRadius, this creates visible shadow clipping:
http://www.howtocreate.co.uk/textshadow.html
> Mozilla/Firefox blurs shadows slightly more than the requested blur value, then
> clips the shadows to the required distance, making them end too abruptly and
> appear too faint.

I really think we should fix this before we ship shadow support in Firefox 3.1. We shouldn't be just setting the box-blur size to the blur radius; it needs to be about 2/3 of the blur radius. We'd also need to change CalculateBlurRadius to match.

Comment 1

[Michael Ventnor]

So what would be the best way to fix this, simply decreasing the size of the lobes to blurRadius/3? Also, I don't know how CalculateBlurRadius needs to be adjusted. Are you sure it needs changing? All it seems to do is convert a std deviation to a radius.

Comment 2

[Robert O'Callahan (:roc) (email my personal email if necessary)]

It's actually slightly tricky for small radii. I think I'll have a crack at it since I've already thought about it.

CalculateBlurRadius needs to change since it was assuming the blur radius equals the box-blur size, which is no longer going to be the case.

Comment 3

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Attachment: testcase

Comment 4

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Attachment: trunk screenshot

Comment 5

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Attachment: Screenshot with upcoming patch applied

You have to zoom in to really see it, but with the patch applied, the bottom right (10px) shadow looks significantly better --- less abrupt termination of the shadow.

Comment 6

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Attachment: fix

Passes canvas mochitests.

Comment 7

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Pushed ce9f05b57b95.

Comment 8

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Pushed to 1.9.1 as 75113d9d8bc2