How this function from "Book of Shaders" does what it does?

Questions
1

My question relates to the first example of “Book of Shaders” (see here https://thebookofshaders.com/05/?lan=en):

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

// Plot a line on Y using a value between 0.0-1.0
float plot(vec2 st, float pct){
return  smoothstep( pct-0.02, pct, st.y) -
        smoothstep( pct, pct+0.02, st.y);
}

void main() {
    vec2 st = gl_FragCoord.xy/u_resolution;

    float y = st.x;

    vec3 color = vec3(y);

    // Plot a line
    float pct = plot(st,y);
    color = (1.0-pct)*color+pct*vec3(0.0,1.0,0.0);

    gl_FragColor = vec4(color,1.0);
}

I have difficulties in understanding the combination of the “float pct” variable and the “float plot” function. I can see it works but I do not understand why.

While saving the results of the plot function,why “float pct” can be part of the plot funtion itself as parameter?

I am also stumpling upon the “y” of the “float pct” – does not “st” itself has “st.x” and “st.y” and therefore “float pct” could simply be “plot(st)”?

2

The code is not super clear. Rewriting it like so makes it maybe less confusing:

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

float plot(vec2 st){
  return  smoothstep( st.x-0.02, st.x, st.y) -
          smoothstep( st.x, st.x+0.02, st.y);
}

void main() {
	vec2 st = gl_FragCoord.xy/u_resolution;
    vec3 color = vec3(st.x);

    // Plot a line
    float pct = plot(st);
    color = (1.0-pct)*color+pct*vec3(0.0,1.0,0.0);

	gl_FragColor = vec4(color,1.0);
}

However, this simplification does not work in the subsequent examples since y will be different than st.x.

3

Awesome, @Mugen87!
Is there anything, you have no answer to? :slight_smile:

Now, I understand every single character.
(So, the original code contains preparations for the following examples which are not necessary for the one above.)

Thank you so much.

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