Simple scanline effect for RealWorld Paint, usable as a layer style or effect via the JavaScript layer style or custom operation effect. The effect can be horizontal, vertical or both using the scanline_x and scanline_y variables and the alpha value of the effect can be adjusted with scanline_alpha.

rwpaint_scanlines.js

var img = Document.RasterImage;
var sizeX = img.sizeX;
var sizeY = img.sizeY;
var scanline_x = 2;
var scanline_y = 0;
var scanline_alpha = 0;

for (var x = 0; x < sizeX; x += 1) {
  for (var y = 0; y < sizeY; y += 1) {
		if (x%scanline_x === 0) {
			img.SetPixelAlpha(x, y, 0, 0, scanline_alpha);
		}
		if (y%scanline_y === 0) {
			img.SetPixelAlpha(x, y, 0, 0, scanline_alpha);
		}
	}
}