Performance Tuning Guide for Alpha Piscium

Documentation Index

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Alpha Piscium pushes cutting-edge rendering techniques — ReSTIR-based global illumination, epipolar volumetric scattering, RTWSM adaptive shadows, and volumetric clouds — all of which are computationally demanding by design. The shader ships six quality profiles that scale these techniques up and down as a unit, and every individual setting can be adjusted independently for fine-grained control. This guide walks through the settings with the highest performance impact so you can find the right balance for your hardware.

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Quality Profiles

The profile selector appears at the top of the shader options screen. Each profile is a preset:

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Global Illumination

ReSTIR SSGI is the most GPU-intensive system in Alpha Piscium. The following settings have the largest individual impact.

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Initial Sampling Screen Space Tracing Steps

SETTING_GI_INITIAL_SST_STEPS controls how many steps the screen-space ray marcher takes when gathering initial GI samples. The default is 128. Reducing this is one of the most effective single-setting changes you can make.

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Spatial Reuse

SETTING_GI_SPATIAL_REUSE (toggle) and SETTING_GI_SPATIAL_REUSE_COUNT (sample count, default 6) control ReSTIR spatial resampling, which borrows GI samples from neighboring pixels to improve quality. Spatial reuse is expensive because it multiplies per-pixel work by the reuse count.

• To disable spatial reuse entirely, turn off SETTING_GI_SPATIAL_REUSE.
• To reduce its cost while keeping it active, lower SETTING_GI_SPATIAL_REUSE_COUNT from 6 to 2 or 3.

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Temporal Reuse Limit

SETTING_GI_TEMPORAL_REUSE_LIMIT (default 24) caps how many previous frames can contribute to temporal GI accumulation. Higher values improve quality but increase lighting response latency. Reducing it does not dramatically affect GPU cost, but can help with perceived lighting accuracy.

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Denoiser

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Temporal History Length

SETTING_DENOISER_HISTORY_LENGTH (default 64) sets how many frames are accumulated for the ReBLUR temporal denoiser. A longer history produces smoother results but consumes more VRAM for the accumulation buffers. Reducing this value lowers both VRAM usage and the latency before lighting changes settle.

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Shadows

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Shadow Map Resolution

SETTING_SHADOW_MAP_RESOLUTION (default 2048) determines the texel density of the shadow map. RTWSM warping concentrates shadow detail near the player, so the effective quality is higher than a naïve shadow map of the same resolution — but a larger map still improves quality at shadow edges and in complex scenes.

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PCSS Sample Count

SETTING_PCSS_SAMPLE_COUNT (default 16) controls how many samples are used for Percentage Closer Soft Shadow filtering, which produces realistically soft shadow edges based on distance from the shadow caster. Reducing this lowers GPU cost at the expense of softer shadow edge smoothness. Available values: 4, 6, 8, 12, 16, 24, 32, 48, 64.

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Volumetric Lighting (Atmosphere)

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Epipolar Slices

SETTING_EPIPOLAR_SLICES (default 1024) sets the number of epipolar slices used for volumetric light scattering. This is the dominant cost factor for atmospheric rays.

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Slice Samples

SETTING_SLICE_SAMPLES (default 512) sets the number of samples per epipolar slice. Reducing it saves GPU time proportionally.

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Sky Samples

SETTING_SKY_SAMPLES (default 32) controls sample count for sky lighting. The range is 16–64. Reducing to 16 or 24 saves GPU time with minimal visible impact in most scenes.

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Clouds

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Disable Low Clouds

SETTING_CLOUDS_CU toggles the volumetric cumulus cloud layer. Volumetric clouds involve ray marching through a noise-based density field — disabling them is one of the largest single performance wins available.

• To disable: turn off Enable Low Clouds under Shader Options → Clouds → Low Clouds.
• High clouds (SETTING_CLOUDS_CI) are much cheaper and can remain enabled independently.

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Cloud Upscale Factor

If you want to keep volumetric clouds but reduce their cost, SETTING_CLOUDS_LOW_UPSCALE_FACTOR renders clouds at a lower internal resolution and upscales the result. The default is 2. Higher upscale factor values trade cloud detail for performance.

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Post Processing

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Depth of Field

SETTING_DOF is disabled by default (//#define SETTING_DOF). Enabling it adds a physically based bokeh blur pass. If you enable DOF and experience frame rate drops, this is one of the first things to disable. The blur quality (SETTING_DOF_QUALITY) and maximum blur radius (SETTING_DOF_MAX_SAMPLE_RADIUS) are the cost levers within DOF.

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Bloom

SETTING_BLOOM is enabled by default. Bloom adds a glow pass for overbright pixels. Disabling it saves a small amount of GPU time per frame. The number of bloom passes (SETTING_BLOOM_PASS, default 8) can also be reduced to save additional time.

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Water Caustics

SETTING_WATER_CAUSTICS is enabled by default and shows light caustic patterns on surfaces beneath water. Disabling it saves GPU time whenever water is visible in the scene.

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Mid-Range GPU Recommended Settings

The following combination is a reasonable starting point for mid-range GPUs (e.g., RX 6700, RTX 3060, RTX 4060):

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VRAM Reduction Summary

If you are running out of VRAM rather than GPU compute time, these settings reduce buffer sizes:

• Reduce SETTING_SHADOW_MAP_RESOLUTION — the shadow map is one of the largest single allocations.
• Reduce SETTING_DENOISER_HISTORY_LENGTH — accumulation buffers scale with history length.
• Reduce SETTING_DENOISER_FAST_HISTORY_LENGTH — the fast history buffer is a secondary accumulation buffer.
• Disable SETTING_CLOUDS_CU — volumetric cloud temporal accumulation buffers are freed.