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Scott

  • For these uniforms:
uniform vec2 u_resolution; // Canvas size (width,height)
uniform vec2 u_mouse;      // mouse position in screen pixels
uniform float u_time;      // Time in seconds since load

…what determines these variable names? Are these set by you, so I could change them, or are these determined by the graphics card, GLSL, or something else?

  • This list of GLSL-supported functions is super. Maybe you could link to a comprehensive list here, too.

• In "Slow down the frequency until the color change becomes almost imperceptible" took me a minute to figure out the right place to do that. I ended up writing:

    gl_FragColor = vec4( abs(sin(u_time * 0.4)) ,0.0,0.0,1.0);

I mention it because I felt this is the first place where you might lose some people. Consider giving a little more guidance with this instruction.

  • So gl_FragCoord is a varying, so it doesn’t have to be declared. But uniforms do have to be declared before you can use them? Is that right?

  • Starting in this example, I need to be told what the components of a vec4() are. It looks like you are using vec4.x and vec4.y. But you didn’t introduce these. What other options do I have?

  • Also I would want the fields of gl_FragCoord explained. You used gl_FragCoord.xy but didn’t explain it. I assume it’s a vec2 with the x and y values, of course.

Come to think of it, it is crazy that you can write:

gl_FragCoord.xy / u_resolution

and have that return a vec2! In Processing you have to use something like vector.divide(). Maybe it would be worth explaining a bit what your line of code above does? Does it really divide both the X and Y values by u_resolution?

NOTES:

uniform vec2 u_resolution; // vec2(500.0,500.0) uniform vec2 u_mouse; uniform float u_time;

void main() { // X Y // gl_FragCoord.xy = vec2(0.0,0.0); // / / // vec2(500.0,500.0);

vec2 st = gl_FragCoord.xy/u_resolution;

// ST -> vec2(0.0,0.0) to vec2(1.0,1.0)

//                  RED   GREEN   BLUE  ALPHA
gl_FragColor = vec4(st.x, st.y,   0.0,  1.0);

}

Nicolas

  • You can picture the uniforms like little bridges between the CPU and the GPU

    a 'one-way'' bridges that is :) I translated it to:

    We can picture the uniforms as small one-way bridges from the CPU (our main program) to the GPU (where the shader will be executed).

  • In the same way GLSL gives us a default output, vec4 gl_FragColor, it also gives us a default input, vec4 gl_FragCoord, which holds the screen coordinates of the pixel or screen fragment that the active thread is working on.

    I developped a bit more to disambiguate pixel & fragment, I think it's an important step, the first time we meet the word fragment for real

  • I've stressed the fact that gl_FragCoord is implicitly declared so that people don't panic when they don't find it in the new code sample :) also those are reserved names, so I stressed the fact that you can't use them as var names in your custom code.