Simulate Vintage TV Interlace Lines in GIMP

Simulating vintage television interlace lines adds an authentic retro CRT aesthetic to digital images and video stills. This effect is achieved in GIMP by rendering a custom horizontal grid over your image, fine-tuning the spacing and line thickness, and adjusting the layer blend modes to mimic the physical scanlines of old cathode-ray tube displays.

Step 1: Prepare Your Workspace

1. Open your target image in GIMP.
2. Go to Layer > New Layer... (or click the New Layer icon in the Layers panel).
3. Name the layer "Interlace Lines", set the Fill with option to Transparency, and click OK.

Step 2: Render the Custom Grid

1. Select the newly created "Interlace Lines" layer.
2. Navigate to Filters > Render > Pattern > Grid....
3. In the Grid dialogue, unlink the horizontal and vertical parameters (click the chain link icon next to Width, Spacing, and Offset so they are broken).
4. Configure the Width (line thickness):
   • Set Horizontal to 1 or 2 pixels (depending on your image resolution).
   • Set Vertical to 0 pixels to eliminate vertical lines.
5. Configure the Spacing (distance between lines):
   • Set Horizontal to 0 pixels.
   • Set Vertical to 2 to 4 pixels.
6. Configure the Color:
   • Set the line color to solid black (#000000) or a dark charcoal grey.
7. Click OK to render the horizontal scanlines across the layer.

Step 3: Blend and Refine the Effect

1. With the "Interlace Lines" layer selected, change the Mode dropdown at the top of the Layers panel from Normal to Overlay, Multiply, or Soft Light.
2. Reduce the Opacity slider to between 30% and 60% to prevent the scanlines from overpowering the underlying image.
3. For a softer, analog glow, go to Filters > Blur > Gaussian Blur... and apply a subtle blur (around 0.5 to 1.0 pixels) to the interlace layer.