I guess it's caused by the recent changes for high resolution support, but need to confirm with @oka-san.

Targeted 

I fixed a bug in the algorithm to restore the pixels on the border of the intersection, to always use the neighbor cell nearest to the center.
Sent CL: https://chrome-internal-review.googlesource.com/#/c/301635/

The is the screenshot: https://drive.google.com/file/d/0B7EYjkGAjRAhanV5U09nRzZuTzQ/view

As seen in the screenshot, ink-bleeding-like effect is observed if we move brush slowly.
It's not intentional, but it's something stems from technical ambiguity, and apparently there is no clear solution to fix this (technical details below).

@zalcorn @elizabethchiu
What do you think about this (unintentional) ink-bleeding-like effect (see above screenshot)? Should we try to fix this?


Technical details:
Single brush stroke is composed from multiple unit lines which has transparency. If we naively draw those unit lines, the intersection of two lines (say A and B) is painted twice and it looks less transparent. To fix this, we restore the pixels in the intersection after B is drawn with what they were before B is drawn. The ambiguous thing is, which color to use to restore a pixel on the edge of the intersection. Since lines drawn on canvas are anti-aliased, after A is drawn, the pixel would have color lighter than we want (so that the pixel doesn't stand out). However, after the line B is drawn, the color would be denser than we want. Therefore, what color to use on restoring the pixel is not apparent. With #4, we use the heuristic to use the color of the neighbor pixel which is the nearest to the center of the intersection, but it could cause the ink-bleeding effect if an edge of a pre-drawn line and the pixel on the edge of the intersection is adjecent (a color of pre-drawn line can be propagated to that pixel).

I think it's looking good and I don't mind the additional texture. The attached is what I proposed before for the brush strokes. I think for now it's good to keep the pen tool in smooth and solid, and only apply texture to the paintbrush (more like watercolor/ acrylic paint). 

Deleted: Screen Shot 2016-11-02 at 10.56.07 AM.png 54.4 KB

	Screen Shot 2016-11-02 at 10.56.07 AM.png 54.4 KB View Download

Thank you.
As I told before, it's not clear how to implement the screenshot #6.
If the screenshot https://drive.google.com/file/d/0B7EYjkGAjRAhanV5U09nRzZuTzQ/view
is good enough, I'll keep it as is.

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-assets/+/4798df853362b495e6acdc47772217be6c345000

commit 4798df853362b495e6acdc47772217be6c345000
Author: Keigo Oka <oka@google.com>
Date: Tue Nov 01 06:34:48 2016

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-assets/+/a38504e6aecec274f8110270a24a9028e34f3946

commit a38504e6aecec274f8110270a24a9028e34f3946
Author: Keigo Oka <oka@google.com>
Date: Tue Nov 01 06:34:48 2016

Change was cherry-picked in M55.

Not able to reproduce the image in comment#6.  I attached the image I got from Kevin.
M	ChromeOS	Chrome	ARC	Type	Channel
56	8989.0.0	56.0.2919.0	3473122	release	dev

Deleted: bug660311.jpeg 137 KB

	bug660311.jpeg 137 KB View Download

#6 is not requirement.