Step 3 GLSL

step_03.glsl

// x, y position of the light
uniform vec2 lightPosition;
// Size of light in pixels
uniform float lightSize;

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Distance in pixels to the light
    float distanceToLight = length(lightPosition - fragCoord);

    // Normalize the fragment coordinate from (0.0, 0.0) to (1.0, 1.0)
    vec2 normalizedFragCoord = fragCoord/iResolution.xy;

    // Start our mixing variable at 1.0
    float lightAmount = 1.0;

    // Find out how much light we have based on the distance to our light
    lightAmount *= 1.0 - smoothstep(0.0, lightSize, distanceToLight);

    // We'll alternate our display between black and whatever is in channel 1
    vec4 blackColor = vec4(0.0, 0.0, 0.0, 1.0);

    // Our fragment color will be somewhere between black and channel 1
    // dependent on the value of b.
    fragColor = mix(blackColor, texture(iChannel1, normalizedFragCoord), lightAmount);
}