Step 4 GLSL

step_04.glsl

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

float terrain(vec2 samplePoint)
{
    float samplePointAlpha = texture(iChannel0, samplePoint).a;
    float sampleStepped = step(0.1, samplePointAlpha);
    float returnValue = 1.0 - sampleStepped;

    return returnValue;
}

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;

    float shadowAmount = terrain(normalizedFragCoord);
    lightAmount *= shadowAmount;

    // 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);
}