Simple fragment shader that allows you to use any RGB image as a fading mask

fader.glsl

const float RATE = 5.0;

/**
 * Converts RGB value from our texture channel into an HSV value.
 * By using the V we can perform a transition using any color image
 */
vec3 rgb2hsv(vec3 c)
{
    vec4 K = vec4(0.0, -1.0 / 3.0, 2.0 / 3.0, -1.0);
    vec4 p = mix(vec4(c.bg, K.wz), vec4(c.gb, K.xy), step(c.b, c.g));
    vec4 q = mix(vec4(p.xyw, c.r), vec4(c.r, p.yzx), step(p.x, c.r));

    float d = q.x - min(q.w, q.y);
    float e = 1.0e-10;
    return vec3(abs(q.z + (q.w - q.y) / (6.0 * d + e)), d / (q.x + e), q.x);
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    vec2 uv = fragCoord.xy / iResolution.xy;
    //get current clear value
    float clear = clamp(iGlobalTime / RATE, 0.0, 1.0);
    
    //find the normal value to render to the screen if it's not masked
    fragColor = texture2D(iChannel0, uv).rgba;
    
    //find the masking value at the pixel
    vec4 clearColor = texture2D(iChannel1, uv);
    float clearV = rgb2hsv(clearColor.rgb).z;
    
    //if the masking value is less than our current clear time, erase the pixel
    if (clearV < clear) {
    	fragColor = vec4(0,0,0,0);
    }
}

gistfile1.txt

Currently the shader uses variables that are suitable for ShaderToy only.  I'll be looking into making it work for LibGDX