feat: add screen-space ambient occlusion (SSAO)

[hubbardp]

Summary

Screen-space ambient occlusion (SSAO) simulates shadows on 3D mesh surfaces by darkening crevices and concavities where ambient light would be occluded. It adds depth cues that help us perceive shapes, and it makes the display more appealing. SSAO is an efficient post-processing effect applied to the perspective view after opaque geometry is drawn. See src/ssao/README.md for more details and example images.

Screenshots

Usage

• Press q to toggle SSAO on and off.
• Use sliders in the "Settings" panel to adjust intensity and radius (softness).

Known limitations

• SSAO is disabled in any perspective view that contains a volume-rendering layer; a one-time status banner notifies the user.
• Translucent annotations covering a mesh suppress SSAO at the covered pixels.

Performance

• The first use of SSAO triggers the creation of the NORMAL attachment, retained thereafter.
• The first use of SSAO triggers mesh shader recompilation.
• All uses of SSAO add three additional full-screen passes.
• No user-perceptible change in performance, even when tested on older laptops with only integrated graphics.

Algorithm

GTAO (Ground Truth Ambient Occlusion): Jimenez et al., "Realtime Strategies for Accurate Indirect Occlusion", SIGGRAPH 2016

Testing

• Manually verified with various datasets from Janelia, OpenOrganelle.
• New browser tests in src/ssao/shaders.browser_test.ts cover composite math, GTAO and composite sentinel paths, blurring pass.

[chrisj]

@hubbardp this is very exciting! Looks great from a quick look so far

flyem_fib-25 DEMO

[chrisj]

In the link I posted, I can't see any visible difference when changing the radius value, I'm wondering if the value is always larger than MAX_KERNEL_FRACTION

[hubbardp]

For those who want to try the new functionality without building the code, it's running here:
http://neuroglancer-ssao.janelia.org/

[chrisj]

@hubbardp neuroglancer github actions are set up to create a deployment for every PR, available if you click "view details", it's definitely not obvious:

That's what I used to create the link I posted.

[jbms]

What is the reason for disabling the effect on highlighted segments? It makes the highlighting rather jarring, though I haven't tested with the alternative behavior.

[hubbardp]

What is the reason for disabling the effect on highlighted segments? It makes the highlighting rather jarring, though I haven't tested with the alternative behavior.

I tried not disabling the SSAO darkening on highlighted segments, and they were too dark to see when highlighted. Personally, I like the brighter highlighting. I found that before SSAO, sometimes the randomly-chosen segment color was bright enough that the highlighting seemed barely distinguishable.

[fcollman]

this look really cool! i'm excited by it.

I immediately went to a very busy microns dataset to see what it would look like

link

and I see thing looks reasonable when viewed from the +/-xz, +/-yz view, but rotating it +/- xy everything looks dim and muddy.

YZ looks good here

XY all the cells in the middle look dulled, when i was expecting some to look brighter.