Virtual shadowmap

[Try]

In general, I'm looking for a new, more sophisticated, solution to replace projective shadowmap.

Goals

• scaling with resolution (faster on low-res rendering)
• single solution for any GPU (unlike ray-tracing)
• high (pixel-perfect) quality

Known solutions

Not a lot actually...
nanite (page 119): https://advances.realtimerendering.com/s2021/Karis_Nanite_SIGGRAPH_Advances_2021_final.pdf
assassins creed (page 55): https://advances.realtimerendering.com/s2015/aaltonenhaar_siggraph2015_combined_final_footer_220dpi.pdf
https://ktstephano.github.io/rendering/stratusgfx/svsm
https://www.cse.chalmers.se/~uffe/ClusteredWithShadows.pdf
https://www.gamedevs.org/uploads/efficient-shadows-from-many-lights.pdf

not vsm, but close enough in concept:
http://lukaskalbertodt.github.io/2023/11/18/tiled-soft-shadow-volumes.html

Very first WIP

Some page-data to illustrate

Initial implementation

Decide to start with common parameters for now:

• only sun-light for now
• 4k*4k page atlas
• 128x128 page size: up to 1024 pages
• Clipmaps + page table 64x64x32
• Meshlets are duplicated, as many time as many pages they overlap

Current considerations:

1. Cluster culling: cull all clusters versus all pages is expensive
   1.1 Coarse culling not possible - need to output exact pageId, for visible meshlets
   1.2 Output size is not deterministic and no good way to react to out-of-memory
2. Cull versus clip-map (HiZ like) is easier, than versus each page individually
   2.1 Won't be able to use hw-rasterizer to output data (need to use image-atomics + image-less rendering)
   2.2 Image-less rendering limited by maxViewportDimensions = 4k
3. Software renderer?!
   3.1 Immediate one still requires atomics, and wont be better than render-pass based one
   3.2 Tile-base can be an interesting take, but not valid without bindless
4. Requires some complementary solution to work with volumetrics

[YALdysse]

@Try, Does this mean OpenGothic will work on weaker graphics cards ?

[Try]

@YALdysse by 'weaker' graphics card you mean weaker than what?)

[YALdysse]

@YALdysse by 'weaker' graphics card you mean weaker than what?)

OpenGothic consumes an average of 95% of my graphics card - AMD Radeon RX Vega 7 (CPU - AMD Ryzen 5500U).

[YALdysse]

OpenGothic consumes an average of 95% of my graphics card - AMD Radeon RX Vega 7 (CPU - AMD Ryzen 5500U).

Unfortunately, we are not talking about stable 60 FPS, but I can play.

[Try]

OK, I would like to avoid setting any expectations, as virtual-shadow is pretty-much experimental tech.

AMD Radeon RX Vega 7

With Vega there are 2 major issues:

• It's way to different to any other gpu and hard to reason about what is going on in this black box
• it have no mesh-shader. So engine have to fallback to non-indexed draws, what about 3x more expesive

[Try]

Some numbers [RTX3070]:

Protective shadowmap (current solution):

• 7836 meshlets on tested scene (see starting post)
• 0.16ms

Virtual shadow

• 408 pages
• 43392 meshlets (shorten shadow range; meshlets are duplicated for each page)
• Meshlets in page: 111 = 27 of paladin with gear + landscape
• Render time of single page: 0.12ms
  • most of rendering time is PROP (pre-rasterization work in HW)

Numbers for one of smaller pages, close to the camera:

[Try]

clip-distance helps with fragment workload, bringing adequate FPS (still slower than regular SM)

[Try]

Some rendering examples:

[Try]

City:

Ship:

[Try]

Even with large pages look is not good:

vsm.header.pageCount	604
vsm.header.meshletCount	91656 // total to draw for VSM
vsm.header.counterM	10787 // total amount of unique meshlets for VSM, meaning duplication factor is ~x9
vsm.header.counterV	 8733 // meshlets that a drawn in case of good-old shadowmap

non-empty shadow mips:  8

Testing area:

Non related, just cool screens:

[Try]

Testing is now enabled by command line: -vsm 1

[Try]

some data on culling:

// baseline
vsm.header.pageCount	546
vsm.header.meshletCount	38316
vsm.header.counterM	13753
vsm.header.counterV	8733

// cull dummy tiles in larger page
vsm.header.pageCount	560
vsm.header.meshletCount	34514
vsm.header.counterM	0
vsm.header.counterV	8733

// hiz
vsm.header.pageCount	558
vsm.header.meshletCount	29547
vsm.header.counterM	0
vsm.header.counterV	8733

About 23% reduction in meshlet count. Now runtime is about 1ms for rendering.

vsm.header.counterV 8733

Amount of meshlets for protective shadow-map. So we still using about 4x meshlets, relative to projective sm

[Try]

fog + vsm doesn't quite work (non-resident pages):

[Try]

experimenting with vsm-fog:

[Try]

Some frame case-study on draw-amount.
(note: I do prefer to measure amount of meshlets, instead of draw-time as it doesn't depend on gpu-model/temperature/etc)

projectiveSM: 8.7k
virtualSM: 20.6k

Now breakdown per clipmap:

mip:  all, land, obj
 0  1041   590   455
 1  1516   784   732
 2  1263   603   660
 -
 3  2152   996  1156
 4  2516  1238  1282
 5  4731  1016  3715
 6  3900   865  3035
 -
 7  3148   740  2408
 8  156     82    78

Fog adds roughly another 5k to mip5 (and a bit to others)

[Try]

vsm: epipolar fog in progress

fog is back to reasonable timings: 1.51ms -> 0.57ms. However comes at cost of some flicker, that need to be worked on

[Try]

L'Hiver. Nice details on individual rocks, ~30% gpu load - similar to vanilla game.