Filters
NEShim has three independent filter axes, all configurable at runtime via the in-game pause menu and the main menu under Settings. Each persists to config.json.
| Axis | Config key | Menu location | Description |
|---|---|---|---|
| Audio Filter | audioFilter | Settings → Sound → Audio Filter | DSP processing applied to the NES mono audio output |
| Video Filter | videoFilter | Settings → Video → Video Filter | Structural transform applied to the NES pixel buffer |
| Color Effect | videoColorFilter | Settings → Video → Color Effect | Color-grade applied after the structural video filter (D3D11 only) |
Audio Filter
Seven audio processors ship with NEShim, all operating on the 44.1 kHz mono output of the NES APU. The processor runs before the NAudio ring buffer and produces stereo output.
| Filter | audioFilter value | Signal chain |
|---|---|---|
| Default | "Default" | HP@37 Hz → HP@39 Hz → LP@14 kHz. Matches the NES hardware output filter. |
| Warm | "Warm" | HP@80 Hz → HP@80 Hz → LP@14 kHz → LP@8 kHz. Raised HP cutoffs tighten bass transients; the extra LP stage rolls off harsh square-wave harmonics above 8 kHz. |
| Pseudo Stereo | "PseudoStereo" | Standard NES chain + Haas-effect widening. L = direct × 0.6, R = 20 ms delayed × 0.4. |
| Warm Stereo | "WarmStereo" | Pseudo Stereo + independent LP@8 kHz on each channel after the Haas split. |
| Compression | "Compression" | Standard NES chain + look-ahead RMS compressor (220-sample window, −6 dBFS threshold, 3:1 ratio, +2 dB makeup). Evens out DPCM channel level spikes. |
| Bass Boost | "BassBoost" | Standard NES chain + additive low-shelf boost at 150 Hz (+4 dB at DC, ≈+2 dB at 150 Hz). For fuller sound on bass-light speakers or headphones. |
| Saturation | "Saturation" | Standard NES chain + tanh soft-clip (drive = 1.5, normalised). Super-linear below full scale (mild mid-level boost) with smooth limiting at peaks. |
Switching the audio filter takes effect immediately. The new processor’s state is reset before it activates to prevent pops from accumulated DC offset.
Default value: "Default"
Adding a new audio filter: see the Architecture guide — Adding a new audio processor.
Video Filter (structural)
Controls how the 256×240 NES pixel buffer is scaled and stylised before display. The available options depend on the active rendering path.
| Filter | videoFilter value | GDI+ | D3D11 | Description |
|---|---|---|---|---|
| Pixel Perfect | "PixelPerfect" | Yes | Yes | Nearest-neighbour scaling with 8:7 pixel aspect ratio correction. NES pixels were never square; this ratio (≈1.143) gives the correct display geometry on modern widescreen monitors. "NearestNeighbour" is a legacy alias. |
| Smooth | "Bilinear" | Yes | Yes | Bilinear interpolation for a softer, anti-aliased look at arbitrary window sizes. In D3D11 mode a linear-clamp sampler is used; there is no structural shader. |
| CRT Scanlines | "CrtScanlines" | — | D3D11 only | Nearest-neighbour with a Gaussian scanline brightness profile. Each NES scanline peaks at full brightness at its centre and fades toward the row edges, recreating the electron-beam spot shape of a CRT phosphor screen. The gap between scanlines emerges from the falloff of adjacent rows rather than binary alternation, so the effect scales naturally — invisible at 1× zoom, increasingly visible as the display size grows. |
| CRT Phosphor | "CrtPhosphor" | — | D3D11 only | CRT scanlines plus an aperture-grille phosphor mask. Each NES pixel is subdivided into three sub-columns (R/G/B dominant), mimicking the continuous vertical phosphor stripes of an aperture-grille CRT (e.g., Sony Trinitron). Stacks with any color effect. |
| NTSC Composite | "NtscComposite" | — | D3D11 only | YIQ colour-space NTSC simulation running entirely on the GPU. A 5-tap chroma Gaussian blurs IQ components while keeping Y (luma) sharp, producing authentic chroma smearing and luma/chroma cross-talk at colour boundaries. Adds an animated analogue noise layer that shifts each frame, recreating the grain shimmer of a real composite signal. Output is 256 pixels wide (standard NES resolution). |
Default value: "PixelPerfect"
The active rendering path is detected at startup and shown in neshim.log when enableLogging is true. It can be forced to GDI+ for debugging via "forceRenderer": "gdi" in config.json. The Video Filter sub-menu shows only the options supported by the current renderer — D3D11-only filters do not appear in GDI+ mode.
Fallback behaviour: if config.json specifies a D3D11-only filter but D3D11 is unavailable, NEShim logs a warning, falls back to PixelPerfect, and saves the fallback value back to config.json.
Adding a new structural filter: see the Architecture guide — Adding a new D3D11 video filter.
Color Effect
A per-pixel color-grade transform applied after the structural video filter. D3D11 only — the selection is stored in config.json in GDI+ mode but has no visual effect until D3D11 is available.
| Effect | videoColorFilter value | Description |
|---|---|---|
| None | "None" | No color transform. Output is the raw NES palette. |
| Warm | "Warm" | Slight amber tint. Reds and greens lifted gently; blues desaturated. Recreates the warm cast of a CRT with an aging phosphor coating. |
| Greyscale | "Greyscale" | Full desaturation using BT.601 luma coefficients (0.299 R + 0.587 G + 0.114 B). Preserves perceived brightness across the NES palette. |
| NES Colors | "NesColorCorrection" | Small color-correction matrix shifting from the raw 2C02 composite palette toward a more accurate sRGB representation. Removes the slight pink/purple cast in uncorrected NES output. |
| Cool | "Cool" | Blue-green tint approximating the D93 9300K white point used by CRT displays in consumer televisions. Reds slightly reduced, blues boosted. The cold counterpart to Warm. |
Default value: "None"
The Color Effect sub-menu is D3D11 only — it is hidden entirely in GDI+ mode and does not appear in the Video settings screen. The selection is not shown or persisted when GDI+ is active.
Adding a new color effect: see the Architecture guide — Adding a new color effect.
Combining video filters
Any structural filter can be combined with any color effect. Some examples:
| Visual goal | Video Filter | Color Effect |
|---|---|---|
| Classic accurate look | Pixel Perfect | None |
| Retro CRT with aging monitor | CRT Scanlines | Warm |
| Black-and-white film look | Pixel Perfect | Greyscale |
| Full composite TV simulation | NTSC Composite | NES Colors |
| Softer look with color correction | Pixel Perfect | NES Colors |
| Authentic cold CRT (slot-mask + D93 white point) | CRT Phosphor | Cool |
| 1980s arcade monitor look | CRT Phosphor | Warm |
D3D11 shader architecture
CRT Scanlines, CRT Phosphor, and NTSC Composite are implemented as DXBC pixel shaders compiled to .cso files and embedded as assembly resources. All color effects also run as shaders via a shared include. Smooth (Bilinear) has no structural shader — it switches the renderer to a linear-clamp sampler while reusing the passthrough shader.
Uniform constant buffer
All pixel shaders share the same 4-float constant buffer (b0):
cbuffer FilterParams : register(b0)
{
float param0; // structural param 0 (nesWidth for CRT, invWidth for NTSC, 0 for PP)
float param1; // structural param 1 (nesHeight for CRT, frameParity for NTSC, 0 for PP)
float param2; // structural param 2 (scanlineIntensity / chromaStrength / 0)
float colorMode; // 0=none 1=warm 2=greyscale 3=nes_colors 4=cool — written by renderer
}
The renderer always writes the active VideoColorFilterMode into param[3]. Each structural filter writes its own params into param[0..2] via ID3D11Filter.WriteBaseParams(). For Pixel Perfect all three structural params are zero.
The buffer is intentionally fixed at 4 floats. No filter may use a second constant buffer; if a future filter genuinely needs more than 3 config floats, update the design rule in CLAUDE.md and D3D11Renderer deliberately.
Shared color grade include
All shaders #include "ColorGrade.hlsli" and call ApplyColorGrade(color, colorMode) as their final step. This means any structural filter + color effect combination works without shader permutations — the grade is baked into every shader via the shared include rather than compiled as separate variants.
Passthrough shader
Passthrough.ps.cso applies only the color grade, with no structural effect. It is bound in place of the active structural shader for two draw calls each frame:
- Sidebar quads — so letterbox bar artwork is not distorted by scanlines or NTSC simulation.
- Overlay quad — so the GDI+-rendered overlay (menus, frozen frame background, HUD elements) is not affected by structural filters. The color grade still applies via
colorMode, keeping the overlay tonally consistent with the NES frame.
DXVK / Proton
DXBC shaders are compiled to SPIR-V by DXVK on first launch and cached in Steam’s shader cache directory. The shaders are simple (< 30 instructions each); compilation is near-instant.