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Nikita Lisitsa

So I'm using alpha cutoff transparency for foliage, and mipmaps were always a nuisance. Simple averaging can make the model disappear at a distance if the alpha channel isn't dense enough.

Experimenting with max alpha (1st row) instead of average alpha (2nd row) for mipmap generation, seems to preserve the overall shape better (3rd vs 4th rows).

Jan 04, 2025, 01:23 PM·Public
0boosts·7favorites
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Scherzog von Beast Oil

@lisyarus I've had very nice results with hashed alpha (although that is a bit noisy)

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Nikita Lisitsa

@scherzog You mean smth like dithered alpha?

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Scherzog von Beast Oil

@lisyarus Not quite, it's pseudorandom, unlike dither: research.nvidia.com/labs/rtr/p

NVIDIA Real-Time Graphics Research · Hashed Alpha Testing | NVIDIA Real-Time Graphics ResearchRenderers apply alpha testing to mask out complex silhouettes using alpha textures on simple proxy geometry. While widely used, alpha testing has a long-standing problem that is underreported in the literature, but observable in commercial games: geometry can entirely disappear as alpha mapped polygons recede with distance. As foveated rendering for virtual reality spreads, this problem worsens as peripheral minification and prefilitering also causes this problem for nearby objects. <p> We introduce two algorithms, stochastic alpha testing and hashed alpha testing, that avoid this issue but add some noise. Instead of using a fixed alpha threshold, stochastic alpha testing discards fragments with alpha below a randomly chosen threshold in (0...1]. Hashed alpha testing uses a hash function to choose the threshold procedurally per-pixel, producing stable noise that reduces temporal flicker. <p> With a good hash function and inputs, hashed alpha testing maintains distant geometry without introducing more temporal flicker than traditional alpha testing. We describe how hashed and stochastic alpha testing apply to alpha-to-coverage and screen-door transparency, and how they simplify stochastic transparency.
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Scherzog von Beast Oil

@lisyarus But still consistent over time unlike pure stochastic alpha testing

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Kornel

@lisyarus It seems like this could be done better by trying to preserve highest possible frequencies. Maybe try flipping between min/max alpha on each column?
Maybe downsample in frequency domain?
Or brute-force/genetic algorithm optimizing for number of blades after thresholding.

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