Trouble plotting a particular equation

I’m trying to plot the following equation : y = random( floor(uv.x) ):

figure 1

I’ve tried other equations and they work just fine:

The problem starts when I try equations with steep angles like y = random(floor(uv.x)), they get their horizontal lines cut off :frowning:

The color gets cut off at a certain angle.

I’m not sure what I’m doing wrong, but this is what I’ve tried so far:

out vec4 color;

float Random(in float x) {
    return fract(sin(x) * 1e4);

void main () {

    vec2 uv     = gl_FragCoord.xy/ screen.xy;
    vec2 ndc    = vec2(uv.x * 2.0 - 1.0, uv.y * 2.0 - 1.0);
    uv.x        *= screen.x / screen.y;

    uv -= 0.5;  // -0.5 <-> 0.5
    uv *= 5.0;  // Zoom Out, if not we don't see much

    vec4 final = vec4(0.0);
    final.a = 1.0;

    float i = floor(uv.x); // Integer Coords
    float f = fract(uv.x); // Fractional Coords

    // Calculate Graph

    // This Works
    // float u = f * f * (3.0 - 2.0 * f );
    // float x = mix(Random(i), Random(i + 1.0), u);

    float x = Random(i); // No horizontal Lines :(
    float y = uv.y;

    // Plot
    float blur = 0.01;
    final.r = smoothstep(x - blur, x, y); // bottom
    final.r -= smoothstep(x - blur, x, y - blur);

    color = final;  


I’m clearly not understanding something :confused: I re-edited the entire question with new and shorter code. As always, thanks!

[QUOTE=hashbrown;1289292]I’m trying to plot the following equation : y = random( floor(uv.x) ):

figure 1 [/QUOTE]

that is not a y=f(x) equation, simply because there are x-values which lead to different y-values. besides that, why are you are using the fragment shader ? a better way is to compute the xyz value of a vertex in the vertex shader, and let the fragment shader just compute the color for a pixel.

for example:
you call glDrawArrays(GL_LINE_STRIP, 0, 100); to invoke the vertex shader 100 times. let z be const, just determine x dependent on gl_VertexID, that ranges from 0 to 99. 0 means most left, 99 means most right. dependent on x, you call a separate function in the vertex shader:

#version 450 core

/* no input !! */

float f(float x)
    return sin(6.28f * x); /* or whatever function you want to plot ... */

void main()
    /* there are 100 invocations, hence 0.01f ... */
    float x = -1.0f + 2.0f * float(gl_VertexID) * 0.01f; 
    float y = f(x);
    float z = 0.5f; /* or any constant ... */
    gl_Position = vec4(x, y, z, 1.0f);

that should work, the fragment shader just gives the fragment its color, for example:

#version 450 core

/* no input !! but output color */
layout (location = 0) out vec4 fragment0;

vec4 color(float y)
    vec4 green = vec4(0, 1, 0, 1);
    vec4 red= vec4(1, 0, 0, 1);
    return mix(red, green, y);

void main()
    float y = gl_FragCoord.y;
    fragment0 = color(y);

the problem with that the f(x) that it is “hardcoded” in the program, to change it you have to re-compile + re-link the shaders again with different source code

Thanks a lot for the reply John!

I agree, nonetheless I’m using the frag shader in order to use sites like ShaderToy. As far as I know, users can only edit the fragment shader on that platform. For a game I would have used the vertex shader though. If you’re curious, I’m learning most of my glsl skills at The Book of Shaders, where they too use the Fragment shader for everything. I’m aware that this is not the most optimal approach though.

I’m also compiling my shaders on WebGL 1, so I might have to make my own gl_VertexID implementation via an attribute I guess.

Since I’m doing a lot of generative art, I’d love to keep working from the frag shader instead of compiling all the time, and perhaps later refactor my code a little to use the vertex shader and GL_LINES like you recommend once I’m satisfied with my design.

Oh before I end the post, I found some online shader editors similar to ShaderToy. You CAN use the vertex shader on these sites.

- Shdr Editor
- Vertex Shader Art

Again thanks a lot for the help :slight_smile:

Hi hashbrown,

I was peeking over your post you had made months ago here on the forums (

I wanted to ask If you can send me / post your program code (draw loop) and full vertex shader? The reason is that I am desperate to move the vertices individually in my OGL application but cannot do so :tired:

Please, please help! Uniforms, updating buffers on CPU …tried everything, no luck! I need to get this working through the shader, without TF or instancing for now)!

Thanks a lot!

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