How to fill the area below a curve in a glsl fragment shader? - glsl

Question

I'm using the following code to plot a sine-wave curve:
#ifdef GL_ES
precision mediump float;
#endif
uniform vec2 u_resolution;
uniform float u_time;
const float AMPLITUDE = 0.125;
const float PERIOD = 1.0;
const float VELOCITY = 8.0;
const vec3 COLOR1 = vec3(1.0, 0.5, 0.0);
const vec3 COLOR2 = vec3(1.0, 0.0, 0.0);
#define PI 3.141592653589793
#define TWO_PI 6.283185307179586
void main() {
vec2 uv = gl_FragCoord.xy / u_resolution;
float phase = u_time * VELOCITY / PI;
float curve = AMPLITUDE * sin(uv.x * TWO_PI / PERIOD - phase);
float shape = step(distance(curve + uv.y, 0.5), 1.0 / u_resolution.x);
vec3 color = (1.0 - shape) * COLOR2 + shape * COLOR1;
gl_FragColor = vec4(color, 1.0);
}
Which produces this image:
I would like to fill the area below the curve with COLOR1, as in this image:

Answer 1

You only need to test if curve is greater than uv.y - 0.5. Use the step for this:
step(edge, x)
step generates a step function by comparing x to edge.
For element i of the return value, 0.0 is returned if x[i] < edge[i], and 1.0 is returned otherwise.
e.g:
float shape = step(distance(curve + uv.y, 0.5), 1.0 / u_resolution.x);
float shape = step(uv.y - 0.5, curve);
(function loadscene() {
var canvas, gl, vp_size, prog, bufObj = {};
function initScene() {
canvas = document.getElementById( "ogl-canvas");
gl = canvas.getContext( "experimental-webgl" );
if ( !gl )
return;
progDraw = gl.createProgram();
for (let i = 0; i < 2; ++i) {
let source = document.getElementById(i==0 ? "draw-shader-vs" : "draw-shader-fs").text;
let shaderObj = gl.createShader(i==0 ? gl.VERTEX_SHADER : gl.FRAGMENT_SHADER);
gl.shaderSource(shaderObj, source);
gl.compileShader(shaderObj);
let status = gl.getShaderParameter(shaderObj, gl.COMPILE_STATUS);
if (!status) alert(gl.getShaderInfoLog(shaderObj));
gl.attachShader(progDraw, shaderObj);
gl.linkProgram(progDraw);
}
status = gl.getProgramParameter(progDraw, gl.LINK_STATUS);
if ( !status ) alert(gl.getProgramInfoLog(progDraw));
progDraw.inPos = gl.getAttribLocation(progDraw, "inPos");
progDraw.u_time = gl.getUniformLocation(progDraw, "u_time");
progDraw.u_resolution = gl.getUniformLocation(progDraw, "u_resolution");
gl.useProgram(progDraw);
var pos = [ -1, -1, 1, -1, 1, 1, -1, 1 ];
var inx = [ 0, 1, 2, 0, 2, 3 ];
bufObj.pos = gl.createBuffer();
gl.bindBuffer( gl.ARRAY_BUFFER, bufObj.pos );
gl.bufferData( gl.ARRAY_BUFFER, new Float32Array( pos ), gl.STATIC_DRAW );
bufObj.inx = gl.createBuffer();
bufObj.inx.len = inx.length;
gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, bufObj.inx );
gl.bufferData( gl.ELEMENT_ARRAY_BUFFER, new Uint16Array( inx ), gl.STATIC_DRAW );
gl.enableVertexAttribArray( progDraw.inPos );
gl.vertexAttribPointer( progDraw.inPos, 2, gl.FLOAT, false, 0, 0 );
gl.enable( gl.DEPTH_TEST );
gl.clearColor( 0.0, 0.0, 0.0, 1.0 );
window.onresize = resize;
resize();
requestAnimationFrame(render);
}
function resize() {
//vp_size = [gl.drawingBufferWidth, gl.drawingBufferHeight];
vp_size = [window.innerWidth, window.innerHeight];
//vp_size = [256, 256]
canvas.width = vp_size[0];
canvas.height = vp_size[1];
}
function render(deltaMS) {
gl.viewport( 0, 0, canvas.width, canvas.height );
gl.clear( gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT );
gl.uniform1f(progDraw.u_time, deltaMS/1000.0);
gl.uniform2f(progDraw.u_resolution, canvas.width, canvas.height);
gl.drawElements( gl.TRIANGLES, bufObj.inx.len, gl.UNSIGNED_SHORT, 0 );
requestAnimationFrame(render);
}
initScene();
})();
<script id="draw-shader-vs" type="x-shader/x-vertex">
#version 100
attribute vec2 inPos;
void main()
{
//ndcPos = inPos;
gl_Position = vec4( inPos.xy, 0.0, 1.0 );
}
</script>
<script id="draw-shader-fs" type="x-shader/x-fragment">
#ifdef GL_ES
precision mediump float;
#endif
uniform vec2 u_resolution;
uniform float u_time;
const float AMPLITUDE = 0.125;
const float PERIOD = 1.0;
const float VELOCITY = 8.0;
const vec3 COLOR1 = vec3(1.0, 0.5, 0.0);
const vec3 COLOR2 = vec3(1.0, 0.0, 0.0);
#define PI 3.141592653589793
#define TWO_PI 6.283185307179586
void main() {
vec2 uv = gl_FragCoord.xy / u_resolution;
float phase = u_time * VELOCITY / PI;
float curve = AMPLITUDE * sin(uv.x * TWO_PI / PERIOD - phase);
//float shape = step(distance(curve + uv.y, 0.5), 1.0 / u_resolution.x);
float shape = step(uv.y - 0.5, curve);
vec3 color = mix(COLOR2, COLOR1, shape);
gl_FragColor = vec4(color, 1.0);
}
</script>
<canvas id="ogl-canvas" style="border: none"></canvas>