HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

In CodePen, whatever you write in the HTML editor is what goes within the `<body>`

tags in a basic HTML5 template. So you don't have access to higher-up elements like the `<html>`

tag. If you want to add classes there that can affect the whole document, this is the place to do it.

In CodePen, whatever you write in the HTML editor is what goes within the `<body>`

tags in a basic HTML5 template. If you need things in the `<head>`

of the document, put that code here.

!
##### Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose **Neither** and nothing will be applied.

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance `-webkit-`

or `-moz-`

.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

Any URLs added here will be added as `<link>`

s in order, and before the CSS in the editor. You can use the CSS from another Pen by using its URL and the proper URL extension.

You can apply CSS to your Pen from any stylesheet on the web. Just put a URL to it here and we'll apply it, in the order you have them, before the CSS in the Pen itself.

You can also link to another Pen here (use the `.css`

URL Extension) and we'll pull the CSS from that Pen and include it. If it's using a *matching* preprocessor, use the appropriate URL Extension and we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

+ add another resource

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

Babel includes JSX processing.

Any URL's added here will be added as `<script>`

s in order, and run *before* the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

You can apply a script from anywhere on the web to your Pen. Just put a URL to it here and we'll add it, in the order you have them, before the JavaScript in the Pen itself.

If the script you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

You can also link to another Pen here, and we'll pull the JavaScript from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

+ add another resource

Search for and use JavaScript packages from npm here. By selecting a package, an `import`

statement will be added to the top of the JavaScript editor for this package.

Using packages here is powered by esm.sh, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ESM usage.

All packages are different, so refer to their docs for how they work.

If you're using React / ReactDOM, make sure to turn on Babel for the JSX processing.

If active, Pens will autosave every 30 seconds after being saved once.

If enabled, the preview panel updates automatically as you code. If disabled, use the "Run" button to update.

If enabled, your code will be formatted when you actively save your Pen. **Note: your code becomes un-folded during formatting.**

Visit your global Editor Settings.

` ````
<canvas id="c"></canvas>
```

` ````
```

` ````
// Tiny Raytracer (C) Gabriel Gambetta 2013
// ----------------------------------------
//
// Configuration and scene
//
// Size of the canvas. w is also reused as a "big constant" / "+infinity"
var w = 600;
// Sphere: radius, [cx, cy, cz], R, G, B, specular exponent, reflectiveness
// R, G, B in [0, 9], reflectiveness in [0..9].
var spheres = [
w, [ 0, -w, 0], 9, 9, 0, w, 2, // Yellow sphere
1, [ 0, 0, 3], 9, 0, 0, w, 3, // Red sphere
1, [-2, 1, 4], 0, 9, 0, 9, 4, // Green sphere
1, [ 2, 1, 4], 0, 0, 9, w, 5 // Blue sphere
];
// Ambient light.
var ambient_light = 2;
// Point lights: intensity, [x, y, z]
// Intensities should add to 10, including ambient.
var lights = [
8, [2, 2, 0]
];
// -----------------------------------------------------------------------------
// Shorten some names.
var math = Math;
var sqrt = math.sqrt;
var max = math.max;
// Global variables.
var out_idx = 0;
// Closure doesn't rename vars unless they're declared with "var", which takes
// space. So most vars are 1-letter and global:
//
// C: sphere center
// L: light vector
// N: surface normal at intersection
// X: intersection point
// a: quadratic equation constant
// b: quadratic equation constant
// c: color channel
// d: quadratic equation discriminant
// e: loop variable
// f: candidate parameter t
// h: half-width of the canvas
// i: illumination
// j: (ray origin) - (sphere center)
// k: <N, L>
// l: light index in loop
// n: <N, N>
// q: sphere index in loop
// r: sphere radius
// s: closest intersection sphere index
// t: closest intersection t
// u: intensity of lights[l]
// v: closest sphere found in loop
//
// The exceptions are vars that need to be initialized here (we still pay the
// "a=", so we pay a single "var" above, and use nice names) and some vars in
// trace_ray, which is recursive, so some of it vars can't be global.
// Get to the raw pixel data.
var canvas = document.getElementById("c");
var context2d = canvas.getContext("2d");
var image_data = context2d.getImageData(0, 0, w, w);
var raw_data = image_data.data;
canvas.width = canvas.height = w;
// Dot product.
function dot(A, B) {
return A[0]*B[0] + A[1]*B[1] + A[2]*B[2];
}
// Helper: A_minus_Bk(A, B, k) = A - B*k. Since it's used more with k < 0,
// using - here saves a couple of bytes later.
function A_minus_Bk (A, B, k) {
return [A[0] - B[0]*k, A[1] - B[1]*k, A[2] - B[2]*k];
}
// Find nearest intersection of the ray from B in direction D with any sphere.
// "Interesting" parameter values must be in the range [t_min, t_max].
// Returns the index within spheres of the center of the hit sphere, 0 if none.
// The parameter value for the intersection is in the global variable t.
function closest_intersection(B, D, t_min, t_max) {
t = w; // Min distance found.
// Quadratic equation coefficients are K1, K2, K3. K1 is constant for the ray.
a = 2*dot(D, D); // 2*K1
// For each sphere.
// Get the radius and test for end of array at the same time;
// spheres[n] == undefined ends the loop.
// q points to the 2nd element of the sphere because of q++; +6 skips to next
// sphere.
for (v = q = 0; r = spheres[q++]; q += 6) {
b = -2*dot(j = A_minus_Bk(B, spheres[q], 1), D); // -K2; also j = origin - center
// Compute sqrt(Discriminant) = sqrt(K2*K2 - 4*K1*K3), go ahead if there are
// solutions.
if ( d = sqrt(b*b - 2*a*(dot(j, j) - r*r)) ) {
// Compute the two solutions.
for (e = 2; e--; d = -d) {
f = (b - d)/a; // f = (-K2 - d) / 2*K1
if (t_min < f && f < t_max && f < t) {
v = q;
t = f;
}
}
}
}
// Return index of closest sphere in range; t is global
return v;
}
// Trace the ray from B with direction D considering hits in [t_min, t_max].
// If depth > 0, trace recursive reflection rays.
// Returns the value of the current color channel as "seen" through the ray.
function trace_ray(B, D, t_min, t_max, depth) {
// Find nearest hit; if no hit, return black.
if (!(s = closest_intersection(B, D, t_min, t_max)))
return 0;
// Compute "normal" at intersection: N = X - spheres[s]
N = A_minus_Bk(X = A_minus_Bk(B, D, -t), // intersection: X = B + D*t = B - D*(-t)
spheres[s], 1);
// Instead of normalizing N, we divide by its length when appropriate. Most of
// the time N appears twice, so we precompute its squared length.
n = dot(N, N);
// Start with ambient light only
i = ambient_light;
// For each light
for (l = 0; u = lights[l++]; ) { // Get intensity and check for end of array
// Compute vector from intersection to light (L = lights[l++] - X) and
// k = <N,L> (reused below)
k = dot(N, L = A_minus_Bk(lights[l++], X, 1));
// Add to lighting
i += u *
// If the pont isn't in shadow
// [t_min, t_max] = [epsilon, 1] - epsilon avoids self-shadow, 1
// doesn't look farther than the light itself.
!closest_intersection(X, L, 1/w, 1) * (
// Diffuse lighting, only if it's facing the point
// <N,L> / (|N|*|L|) = cos(alpha)
// Also, |N|*|L| = sqrt(<N,N>)*sqrt(<L,L>) = sqrt(<N,N>*<L,L>)
max(0, k / sqrt(dot(L, L)*n))
// Specular highlights
//
// specular = (<R,V> / (|R|*|V|)) ^ exponent
// = (<-R,-V> / (|-R|*|-V|)) ^ exponent
// = (<-R,D> / (|-R|*|D|)) ^ exponent
//
// R = 2*N*<N,L> - L
// M = -R = -2*N*<N,L> + L = L + N*(-2*<N,L>)
//
// If the resultant intensity is negative, treat it as 0 (ignore it).
+ max(0, math.pow( dot(M = A_minus_Bk(L, N, 2*k/n), D)
/ sqrt(dot(M, M)*dot(D, D)), spheres[s+4]))
);
}
// Compute the color channel multiplied by the light intensity. 2.8 maps
// the color range from [0, 9] to [0, 255] and the intensity from [0, 10]
// to [0, 1], because 2.8 ~ (255/9)/10
//
// spheres[s] = sphere center, so spheres[s+c] = color channel
// (c = [1..3] because ++c below)
var local_color = spheres[s+c]*i*2.8;
// If the recursion limit hasn't been hit yet, trace reflection rays.
// N = normal (non-normalized - two divs by |N| = div by <N,N>
// D = -view
// R = 2*N*<N,V>/<N,N> - V = 2*N*<N,-D>/<N,N> + D = D - N*(2*<N,D>/<N,N>)
var ref = spheres[s+5]/9;
return depth-- ? trace_ray(X,
A_minus_Bk(D, N, 2*dot(N, D)/n), // R
1/w, w, depth)*ref
+ local_color*(1 - ref)
: local_color;
}
// For each y; also compute h=w/2 without paying an extra ";"
for (y = h=w/2; y-- > -h;) {
// For each x
for (x = -h; x++ < h;) {
// One pass per color channel (!). This way we don't have to deal with
// "colors".
for (c = 0; ++c < 4;) {
// Camera is at (0, 1, 0)
//
// Ray direction is (x*vw/cw, y*vh/ch, 1) where vw = viewport width,
// cw = canvas width (vh and ch are the same for height). vw is fixed
// at 1 so (x/w, y/w, 1)
//
// [t_min, t_max] = [1, w], 1 starts at the projection plane, w is +inf
//
// 2 is a good recursion depth to appreciate the reflections without
// slowing things down too much
//
raw_data[out_idx++] = trace_ray([0, 1, 0], [x/w, y/w, 1], 1, w, 4);
}
raw_data[out_idx++] = 255; // Opaque alpha
}
}
context2d.putImageData(image_data, 0, 0);
```

999px

Also see: Tab Triggers