simple scanline shader

fullscreen-quad-libgdx.glsl

/* post-fx pass by blending a solid white fullscreen quad, no texture required */

/*uniform*/ float fmin = 0.7;

void main(void)
{
    float fmod = mod(gl_FragCoord.y, 2.0);
    float fstep = fmin + (1.0 - fmin) * fmod;
    gl_FragColor = vec4(1.0, 1.0, 1.0, fstep);
}

shadertoy.glsl

/* shadertoy test - blends with a texture */

float fmin = 0.7;

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    float fmod = mod(fragCoord.y, 2.0);
    float fstep = fmin + (1.0 - fmin) * fmod;
    
    vec2 uv = fragCoord.xy / iResolution.xy;   
    fragColor = vec4(texture2D(iChannel0, uv).rgb * fstep, 1.0);
}