Simulate Film Grain with Poisson Noise in GIMP

Simulating film grain adds an authentic, analog texture to digital images by recreating the random distribution of silver halide crystals found in traditional film stocks. Unlike standard Gaussian noise, Poisson noise varies dynamically according to the underlying luminance of the photograph, generating more realistic grain in midtones and highlights while behaving naturally in shadows. This step-by-step guide explains how to generate, blend, and refine Poisson noise in GIMP to achieve realistic film grain without third-party plugins.

Step 1: Create a Grain Layer

  1. Open your target image in GIMP.
  2. In the Layers panel, click Create a new layer (or press Shift + Ctrl + N).
  3. Set the layer name to "Film Grain".
  4. Set the Fill with option to 50% Gray (#808080) and click OK.

Step 2: Apply the Poisson Noise Filter

GIMP includes built-in GEGL operations that process Poisson distribution directly:

  1. Select the new "Film Grain" layer.
  2. Go to Tools > GEGL Operation... (or Filters > Generic > GEGL Operation...).
  3. In the Operation dropdown menu, select noise-poisson.
  4. Adjust the Alpha or scale parameter to control the density of the noise. A lower value produces fine 35mm-style grain, while a higher value yields rougher, high-ISO grain.
  5. Click OK to apply the effect to the gray layer.

Step 3: Blend the Grain with Your Image

  1. Ensure the "Film Grain" layer is selected in the Layers panel.
  2. Change the Mode (Blend Mode) dropdown from Normal to Overlay or Soft Light.
    • Overlay creates higher contrast and more prominent grain.
    • Soft Light produces a subtle, gentle grain texture.
  3. Lower the layer Opacity (typically between 40% and 75%) until the intensity matches your desired film aesthetic.

Step 4: Soften and Adjust Grain Sharpness

Real optical film grain is slightly soft due to the physical emulsion and scanning process:

  1. With the "Film Grain" layer selected, go to Filters > Blur > Gaussian Blur....
  2. Set both the Size X and Size Y to a low value between 0.5 and 1.2 pixels.
  3. Click OK to smooth out the digital pixel sharpness into a more organic texture.

Step 5: Restrict Grain in Pure Highlights and Shadows (Optional)

To mimic photographic film behavior where extreme shadows and blown highlights carry less visible grain:

  1. Right-click the "Film Grain" layer and select Add Layer Mask.
  2. Choose Grayscale copy of layer and select your original base image layer, then check Invert mask if targeting specific tonal ranges, or use Layer Effects to blend the grain specifically out of the deep blacks and pure whites using the Curves tool on the mask.