Bug 133842

Summary: [CSS Blending] Rendering defects when using background-blend-mode on a radial-gradient background image
Product: WebKit Reporter: Ion Rosca <rosca>
Component: Layout and RenderingAssignee: Nobody <webkit-unassigned>
Status: NEW ---    
Severity: Normal CC: cabanier, dino, mihnea, simon.fraser, WebkitBugTracker
Priority: P2    
Version: 528+ (Nightly build)   
Hardware: Unspecified   
OS: Unspecified   
Bug Depends on:    
Bug Blocks: 108546    
Attachments:
Description Flags
Test
none
Test with divs
none
Screenshot blending on body
none
Test with body none

Description Ion Rosca 2014-06-13 01:20:24 PDT
Rendering defects when using background-blend-mode on a radial-gradient background image.

It looks like the initial backdrop for background blending is not initialized properly and blending is done on top of a layer with misc color information.

Other possible consequence of this bug is that when using multiple radial-gradient background layers, only the top layer will be visible.
http://codepen.io/adobe/full/8a939a42e4d0fd604790b676611a9006
Comment 1 Ion Rosca 2014-06-13 03:16:16 PDT
Created attachment 233037 [details]
Test
Comment 2 Ion Rosca 2014-06-16 08:12:52 PDT
Created attachment 233158 [details]
Test with divs
Comment 3 Ion Rosca 2014-06-16 08:14:29 PDT
Created attachment 233159 [details]
Screenshot blending on body
Comment 4 Ion Rosca 2014-06-16 08:22:44 PDT
Created attachment 233160 [details]
Test with body
Comment 5 Ion Rosca 2014-06-16 09:20:06 PDT
I did a bit of investigation and it might be a CG issue. I'll describe the attached tests and what happens.

"Test with body"
* html: the body element has a background color (aqua), a radial-gradient image (a yellow oval) and background-blend-mode.
* result: the yellow oval drawn over a background being random content from other tabs (see "Screenshot blending on body")

"Test with divs"
* html: div elements have background color (aqua), radial-gradient image (yellow circle) and background-blend-mode.
* result: almost all divs (except those having normal and screen blend modes) contain a yellow circle on a white background.

Debugging:
* Everything up to CGContextDrawRadialGradient paints correctly (the aqua background color). 
* calling any CGContextFillRect function right before CGContextDrawRadialGradient will have no effect,  because the latter replaces everything on the viewport.
* calling CGContextFillRect with a red color right after CGContextDrawRadialGradient will draw the red rectangle, blending it correctly with the yellow gradient and other content in the viewport.

Exceptions:
* I could not reproduce "Test with body" on Safari 8, Mac OS 10.10, but "Test with divs" is still there.
* Screen blend mode works everywhere.
* Adding a css border on divs having background-blend-mode in the "Test with divs" file fixes the issue for this test case.
* It works correctly in minibrowser.
Comment 6 Simon Fraser (smfr) 2014-06-16 11:11:53 PDT
Seems like a bug in Core Graphics accelerated drawing code.
Comment 7 Simon Fraser (smfr) 2014-06-16 11:13:57 PDT
Does this only happen with gradients, or other background images too?
Comment 8 Ion Rosca 2014-06-16 12:13:41 PDT
This happens only with *radial* gradients.
Comment 9 Ion Rosca 2014-09-18 06:23:24 PDT
I still can see this bug on Safari 8. I reported it at bugreport.apple.com as Problem #18379644.